A method for coding or decoding an audio signal combines the advantages of TNS processing and noise substitution. A time-discrete audio signal is initially transformed to the frequency domain in order to obtain spectral values of the temporal audio signal. Subsequently, a prediction of the spectral values in relation to frequency is carried out in order to obtain spectral residual values. Within the spectral residual values, areas are detected encompassing spectral residual values with noise properties. The spectral residual values in the noise areas are noise-substituted, whereupon information concerning the noise areas and noise substitution is incorporated into side information pertaining to a coded audio signal. Thus, considerable bit savings in case of transient signals can be achieved.

The present invention relates to a method for coding audio signals, and in particular to a method for coding an audio signal for the standard MPEG-2 AAC being just developed.
The standardization organization ISO/IEC JTC1/SC29/WG11, which is also known as Moving Pictures Expert Group (MPEG), was founded in 1988 in order to specify digital video and audio coding schemes for low data rates. The first specification phase was completed in November 1992 by the standard MPEG-1. The audio coding system according to MPEG-1, which is specified in ISO 11172-3, operates in a single-channel or two-channel stereo mode at sampling frequencies of 32 kHz, 44.1 kHz and 48 kHz. The standard MPEG-1 Layer II delivers broadcasting quality, as specified by the International Telecommunication Union, at a data rate of 128 kb/s per channel.
In its second development phase, the aims of MPEG consisted in defining multi-channel extension for MPEG-1-Audio, which was supposed to be backward-compatible with respect to existing MPEG-1 systems, and in defining also an audio coding standard at lower sampling frequencies (16 kHz, 22.5 kHz, 24 kHz) than with MPEG-1. The backward-compatible standard (MPEG-2 BC) as well as the standard with lower sampling frequencies (MPEG-2 LSF) were completed in November 1994. MPEG-2 BC delivers a good audio quality at data rates from 640 to 896 kb/s for 5 channels with full bandwidth. Since 1994, further endeavors of the MPEG-2 audio standardization committee consist in defining a multi-channel standard of higher quality than that obtainable when backward compatibility to MPEG-1 is demanded. This not backward-compatible audio standard according to MPEG-2 is designated MPEG-2 NBC. The aim of this development consists in obtaining broadcasting qualities according to the ITU-R requirements at data rates of 384 kb/s or lower rates for 5-channel audio signals in which each channel has the full bandwidth. The audio coding standard MPEG-2 NBC was completed in April 1997. The scheme MPEG-2 NBC will constitute the core for the already planned audio standard MPEG-4 that will have higher data rates (above 40 kb/s per data channel). The NBC or not backward-compatible standard combines the coding efficiency of a high-resolution filter bank, prediction techniques and redundancy-reducing Huffman coding in order to obtain audio coding of broadcasting quality at very low data rates. The standard MPEG-2 NBC is also referred to as MPEG-2 NBC AAC (AAC=Advanced Audio Coding). A detailed representation of the technical contents of MPEG-2 AAC can be found in M. Bosi, K. Brandenburg, S. Quackenbush, L. Fielder, K. Akagiri, H. Fuchs, M. Dietz, J. Herre, G. Davidson, Yoshiaki Oikawa: xe2x80x9cISO/IEC MPEG-2 Advanced Audio Codingxe2x80x9d, 101st AES Convention, Los Angeles 1996, Preprint 4382.
Efficient audio coding methods remove both redundancies and irrelevancies from audio signals. Correlations between audio sampling values and statistics of sample value representation are exploited for removing redundancies. Frequency range and time range masking properties of the human hearing system are exploited in order to remove non-perceptible signal components (irrelevancies). The frequency content of the audio signal is subdivided into partial bands by means of a filter bank. The data rate reduction is achieved by quantizing the spectrum of the time signal in accordance with psycho-acoustic models, and this reduction may comprise a loss-free coding method.
Generally speaking, a time-continuous audio signal is sampled in order to obtain a time-discrete audio signal. The time-discrete audio signal is subjected to windowing by means of a window function in order to obtain successive blocks or frames of a specific number of, e.g. 1024, windowed time-discrete sampling values. Each block of windowed time-discrete sampling values of the audio signal is successively transformed to the frequency range, for which a modified discrete cosine transformation (MDCT) can be employed for example. The thus obtained spectral values are not yet quantized and thus need to be quantized. The main object in doing so consists in quantizing the spectral data in such a manner that the quantization noise is masked or covered by the quantized signals themselves. For doing so, a psycho-acoustic model described in the MPEG AAC standard is employed which, in consideration of the specific properties of the human ear, calculates masking thresholds depending on the audio signal present. The spectral values then are quantized in such a manner that the quantization noise introduced is hidden and thus becomes inaudible. Quantization thus does not result in audible noise.
The NBC standard makes use of a so-called non-uniform quantizer. In addition thereto, a method is used for shaping the quantization noise. The NBC method uses, just as preceding standards, the individual amplification of groups of spectral coefficients that are known as scale factor bands. In order to operate in as efficient manner as possible, it is desirable to be able to shape the quantization noise in units that are as far as possible matched to the frequency groups of the human hearing system. It is thus possible to group spectral values that very closely reflect the bandwidth of the frequency groups. Individual scale factor bands can be amplified by means of scale factors in steps of 1.5 dB. Noise shaping is achieved as amplified coefficients have higher amplitudes. Therefore, they will in general display a higher signal-to-noise ratio after quantization. On the other hand, higher amplitudes require more bits for coding, i.e. the bit distribution between the scale factor bands is changed implicitly. The amplification by the scale factors, of course, has to be corrected in the decoder. For this reason, the amplification information stored in the scale factors in units of 1.5 dB steps must be transmitted to the decoder as side information.
After quantization of the spectral values present in scale factor bands and optionally amplified by scale factors, the spectral values are to be coded. The input signal to a noise-free coding module thus is the set of, for example, 1024 quantized spectral coefficients. By means of the noise-free coding module, the sets of 1024 quantized spectral coefficients are divided into sections in such a manner that a single Huffman coding table (xe2x80x9ccodebookxe2x80x9d) is employed for coding each section. For reasons of coding efficiency, section limits can be present only on scale factor band limits, such that for each section of the spectrum both the length of the section in scale factor bands and the Huffman coding table number used for the section must be transmitted as side information.
The formation of the sections is dynamic and varies typically from block to block, such that the number of bits necessary for representing the full set of quantized spectral coefficients is reduced to a minimum. Huffman coding is used for representing n-tuples of quantized coefficients, with the Huffman Code being derived from one of 12 coding tables. The maximum absolute value of the quantized coefficients that can be represented by each Huffman coding table as well as the number of coefficients in each n-tuple for each coding table are defined from the very beginning.
The reason for forming the sections thus consists in grouping regions having the same signal statistics, in order to obtain an as high as possible coding gain for a section by means of one single Huffman coding table, with the coding gain being defined in general by the quotient of bits prior to coding and bits after coding. By means of a coding table number (xe2x80x9ccodebook numberxe2x80x9d) defined in the bit flow syntax used for the NBC method, reference is made to one of the 12 Huffman coding tables which permits the highest coding gain for a specific section. The expression xe2x80x9ccoding table numberxe2x80x9d in the present application thus is to refer to the location in the bit flow syntax reserved for the coding table number. For coding 11 different coding table numbers in binary manner, there are required 4 bits. These 4 bits have to be transmitted as side information for each section, i.e. for each group of spectral values, so that the decoder is capable of selecting the corresponding correct coding table for decoding.
FIG. 2 shows a general survey of a coder and a decoder operating in accordance with the basic principle described. An audio signal, which preferably is already present in time-discrete manner, is input via an audio input 200. The time-discrete audio signal then is subjected to windowing within a block 202 designated analysis filter bank and having a windowing function, in order to obtain blocks of time-discrete windowed audio signals that are also referred to as xe2x80x9cframesxe2x80x9d. In analysis filter bank 202, the blocks of windowed values are transformed to the frequency range. Thus, there are spectral values appearing at the output of analysis filter bank 202 which are first quantized in a block xe2x80x9cquantizing and codingxe2x80x9d 204 and then are redundancy-coded, for example, by means of Huffman coding. Furthermore, from the time-discrete audio input signal at audio input 200, masking information used in quantizing is calculated by means of a psychoacoustic model 206, so that the quantization noise introduced for quantizing the spectral values is psychoacoustically masked or covered by the same. The quantized and coded spectral values, in case of the coder, a fed into a bit flow multiplexer 208 forming of the quantized and redundancy-coded spectral values a bit flow, with the bit flow containing furthermore side information necessary for decoding, as is known to experts. The finished, coded bit flow is present at a bit flow output 210, which now constitutes a coded version of the audio signal at audio input 200. This bit flow is transmitted to a decoder and fed into a bit flow input 212 of the same. In bit flow demultiplexer 214, the bit flow is decomposed into side information and quantized and coded spectral values, which are redundancy-coded and requantized in block xe2x80x9cinverse quantizationxe2x80x9d 216, in order to be supplied to a synthesis filter bank 218 which transforms the spectral values present at the input thereof back to the time range, whereupon a coded and decoded audio signal is present at an audio output 220 of the decoder. The coded and decoded audio signal present at audio output 220, with the exception of introduced coding errors, corresponds to the original time-discrete audio signal present at audio input 200.
The so-called xe2x80x9cTemporal Noise Shapingxe2x80x9d technique also is already known and described in the art (J. Herre, J. D. Johnston, xe2x80x9cEnhancing the Performance of Perceptual Audio Coders by Using Temporal Noise Shaping (TNS)xe2x80x9d, 101st AES Convention, Los Angeles 1996, Preprint 4384). The TNS technique (TNS=Temporal Noise Shaping), generally speaking, permits temporal shaping of the fine structure of quantization noise by means of predictive coding of the spectral values. The TNS technique is based on consequent application of the dualism between time and frequency range. It is known from technology that the auto-correlation function of a time signal, when transformed to the frequency range, indicates the spectral power density of exactly this time signal. The dual case with respect thereto arises when the auto-correlation function of the spectrum of a signal is formed and transformed to the time range. The auto-correlation function transformed to the time range or transformed back therefrom is also referred to as square of the Hilbert envelope curve of the time signal. The Hilbert envelope curve of a signal thus is directly related to the auto-correlation function of its spectrum. The squared
Hilbert envelope curve of a signal and the spectral power density thereof thus constitute dual aspects in the time range and in the frequency range. When the Hilbert envelope curve of a signal remains constant for each partial band-pass signal over a range of frequencies, the auto-correlation between adjacent spectral values will be constant as well. This means in fact that the series of spectral coefficients in relation to frequency is stationary, and this is why predictive coding techniques can be used in efficient manner for representing this signal by utilizing a common set of prediction coefficients.
For illustrating these facts, reference is made to FIGS. 5a and 5b. FIG. 5a shows a short section of a xe2x80x9ccastanet-typexe2x80x9d signal of a duration of about 40 ms, which is highly transient in time. This signal was divided into several partial band-pass signals, with each partial band-pass signal having a bandwidth of 500 Hz. FIG. 5 shows the Hilbert envelope curves for these band-pass signals having center frequencies in the range from 1500 Hz to 4000 Hz. For reasons of clarity, all envelope curves have been normalized to their maximum amplitude. The shapes of all partial envelope curves obviously are highly related with each other, and therefore a common predictor can be used within this frequency range for efficiently coding the signal. Similar observations can be made with voice signals in which the effect of the glottal excitation pulses is present over the entire frequency range due to the nature of the human speech forming mechanism.
FIG. 5 thus shows that the correlation of adjacent values, for example, at a frequency of 2000 Hz is similar to that present, for example, at a frequency of 3000 Hz or 1000 Hz, respectively.
An alternative way of understanding the property of spectral predictability of transient signals can be obtained from the table shown in FIG. 4. At the upper left of the table, there is depicted a time-continuous signal u(t) having a sinusoidal path. Opposite thereto, there is shown the spectrum U(f) of this signal, which consists of a single Dirac pulse. Optimum coding for this signal consists in coding spectral data or spectral values since, for the entire time signal, only the amount as well as the phase of the Fourier coefficient need to be transmitted for being able to completely reconstruct the time signal. Coding of spectral data at the same time corresponds to prediction in the time range. Thus, predictive coding would have to take place in the time range here. The sinusoidal time signal thus has a flat temporal envelope curve, which corresponds to an envelope curve in the frequency range that is not flat at its maximum.
In the following, the opposite case is to be considered, in which the time signal u(t) is a maximum transient signal in the firm of a Dirac pulse in the time range. A Dirac pulse in the time range corresponds to a xe2x80x9cflatxe2x80x9d power spectrum, while the phase spectrum is rotating in accordance with the temporal position of the pulse. This signal obviously constitutes a problem for the afore-mention traditional methods, such as e.g. transformation coding or coding of spectral data or linear prediction coding of the time range data. This signal can be coded best and in the most efficient manner in the time range, since only the temporal position as well as the power of the Dirac pulse need to be transmitted, which by consequent application of dualism leads to the result that predictive coding in the frequency range also constitutes a suitable method for efficient coding.
It is very important not to confuse predictive coding of spectral coefficients in relation to frequency with the known dual concept of prediction of spectral coefficients from one block to the next one, which is already implemented and also described in the afore-mentioned article (M. Bosi, K. Brandenburg, S. Quakenbush, L. Fielder, K. Akagiri, H. Fuchs, M. Diets, J. Herre, G. Davidson, Yoshiaki Oikawa: xe2x80x9cISO/IEC MPEG-2 Advanced Audio Codingxe2x80x9d, 101st AES Convention, Los Angeles 1996, Preprint 4382). In the prediction of spectral coefficients from one block to the next one, which corresponds to a prediction in relation to time, the spectral resolution is increased, whereas a prediction of spectral values in relation to frequency enhances the temporal resolution. A spectral coefficient at 1000 Hz, for example, can be determined by the spectral coefficient, for example, at 900 Hz in the same block or frame.
The considerations outlined thus led to provision of an efficient coding method for transient signals. Predictive coding techniques, in consideration of the duality between time and frequency range, can be treated substantially analogous to the already known prediction from one spectral coefficient to the spectral coefficient of the same frequency in the next block. Due to the fact that the spectral power density and the squared Hilbert envelope curve of a signal are of dual character with respect to each other, a reduction of a residual signal energy or a prediction gain is obtained in accordance with a degree of flatness of the squared envelope curve of the signal in contrast to a degree of spectral flatness in case of the conventional prediction method. The potential coding gain increases with increasingly transient signals.
Possible prediction schemes are both the prediction scheme with closed loop, which is also referred to as backward prediction, as well as the prediction scheme with open loop, which is also referred to as forward prediction. In case of the spectral prediction scheme with closed loop (backward prediction), the envelope curve of the error is flat. In other words, the error signal energy is distributed uniformly in relation to time.
However, in case of forward prediction, as shown in FIG. 6, temporal shaping of the noise introduced by quantization takes place. A spectral coefficient x(f) to be predicted is fed to a summing point 600. The same spectral coefficient, furthermore, is supplied to a predictor 610 whose output signal with negative sign is supplied to summing point 600 as well. The input signal to a quantizer 620 thus represents the difference of the spectral value x(f) and the spectral value xp(f) calculated by prediction. In forward prediction, the overall error energy in the decoded spectral coefficient data will remain the same. The temporal shape of the quantization error signal, however, will appear to be temporally shaped at the output of the decoder, since the prediction with respect to the spectral coefficients was employed, whereby the quantization noise will be placed temporally under the actual signal and thus can be masked. In this manner, problems of time masking e.g. with transient signals or voice signals, are avoided.
This type of predictive coding of spectral values thus is referred to as TNS or temporal noise shaping technique. To illustrate this technique, reference is made to FIG. 7a. To the upper left of FIG. 7a, there is shown a time curve of a highly transient time signal. Confronted with the time curve is the section of a DCT spectrum at the upper right in FIG. 7a. The lower left representation in FIG. 7 shows the resulting frequency response of a TNS synthesis filter that was calculated by LPC operation (LPC=Linear Prediction Coding). It is to be noted that the (normalized) frequency coordinates in this diagram correspond to the time coordinates due to the time range and frequency range duality. The LPC calculation obviously leads to a xe2x80x9csource modelxe2x80x9d of the input signal since the frequency response of the LPC-calculated synthesis filter is similar to the envelope curve of the highly transient time signal. FIG. 7a at the lower right shows a representation of the spectral residual values, i.e. of the input signals of quantizer 620 in FIG. 6, in relation to frequency. A comparison between the spectral residual values after prediction and the spectral values with direct time-frequency transformation shows that the spectral residual values have considerably less energy than the original spectral values. In the embodiment shown, the energy reduction of the spectral residual values corresponds to an overall prediction gain of about 12 dB.
The following is to be noted with respect to the representation at the lower left in FIG. 7a. In case of classical application of prediction for time range signals, the frequency response of the synthesis filter is an approximation of the value spectrum of the input signal. The synthesis filter (re)generates so to speak the spectral shape of the signal from a residual signal having an approximately xe2x80x9cwhitexe2x80x9d spectrum. When applying the prediction to spectral signals, as in case of the TNS technique, the frequency response of the synthesis filter is an approximation of the envelope curve of the input filter. The frequency response of the synthesis filter is the Fourier transform of the pulse response, as holds for the classic case, but the inverse Fourier transform. The TNS synthesis filter so to speak (re)generates the envelope curve pattern of the signal from a residual signal having an approximately xe2x80x9cwhitexe2x80x9d (i.e. flat) envelope curve. The representation at the lower left in FIG. 7a thus shows the envelope curve of the input signal as modelled by the TNS synthesis filter. This is in the present case a logarithmic representation of the envelope curve approximation of the castanet-type signal shown in the representation thereabove.
Thereafter, a coding noise was introduced into the spectral residual values, such that in each coding band having a width of 0.5 Bark, for example, a signal-to-noise ratio of about 13 dB resulted. The error signals in the time range, resulting from introduction of the quantization noise, are shown in FIG. 7b. The left-hand representation in FIG. 7b depicts the error signal due to the quantization noise in case the TNS technique is used, whereas in the right-hand diagram the TNS technique was not used for reasons of comparison. As is expected, the error signal in the left-hand diagram is not distributed uniformly across the block, but concentrates in that portion in which there is also a high signal component present that will cover this quantization noise in optimum manner. In the case shown on the right-hand side, the introduced quantization noise, in contrast thereto, is distributed uniformly in the block, i.e. in relation to time, which has the result that in the forward portion, in which actually no or almost no signal is present, there will also be noise present that will be audible, whereas in the region containing high signal components, relatively little noise will be present through which the masking possibilities of the signal are not fully exploited.
Implementation of a TNS filter 804 in a coder is shown in FIG. 8a. The latter is arranged between an analysis filter bank 802 and a quantizer 806. The time-discrete input signal in case of the coder shown in FIG. 8a is fed to an audio input 800 whereas the quantized audio signal and, respectively, quantized spectral values or the quantized spectral residual values are output at an output 808 which may have a redundancy coder connected downstream thereof. The input signal thus is transformed into spectral values. On the basis of the spectral values calculated, a usual linear prediction calculation is carried out which takes place, for example, by forming the auto-correlation matrix of the spectral values and using a Levinson-Durbin recursion. FIG. 8b shows a more detailed view of the TNS filter 804. The spectral values x(1), . . . , x(i), . . . , x(n) are input at a filter input 810. It may happen that only a specific frequency range has transient signals, whereas another frequency range is rather of stationary nature. This fact is taken into consideration in TNS filter 804 by an input switch 812 as well as by an output switch 814, however, with the switches taking care first of parallel to serial and serial to parallel conversion, respectively, of the data to be processed. Depending on whether a specific frequency range is non-stationary and promises a specific coding gain by the TNS technique, only this spectral range will be TNS-processed, which takes place in that input switch 812 starts for example at the spectral value x(i) and runs e.g. until reaching the spectral value x(i+2). The inner portion of the filter again consists of the forward prediction structure, i.e. the predictor 610 as well as the summing point 600.
The calculation for determining the filter coefficients of the TNS filter and for determining the prediction coefficients, respectively, is carried out as follows. The formation of the auto-correlation matrix and the use of the Levinson-Durban recursion is carried out for the highest admissible order of the noise-shaping filter, e.g. 20. When the prediction gain calculated exceeds a specific threshold, TNS processing is activated.
The order of the noise-shaping filter used for the present block then is determined by subsequent removal of all coefficients with a sufficiently small absolute value from the end of the coefficient array. In this manner the orders of TNS filters usually are in the order of magnitude of 4 to 12 for a voice signal.
When, for a range of spectral values x(i), for example, a sufficiently high coding gain is ascertained, the latter is processed, and not the spectral value x(i), but the spectral residual value xR(i) is issued at the output of the TNS filter. The latter value has a considerably lower amplitude than the original spectral value x(i), as can be seen from FIG. 7a. The side information transmitted to the decoder thus contains, in addition to the usual side information, a flag indicating the use of TNS and, if necessary, information on the target frequency range and also on the TNS filter used for coding. The filter data can be represented as quantized filter coefficients.
In the decoder outlined in FIG. 9a, TNS coding is reversed for each channel. Spectral residual values XR(i) are requantized in inverse quantizer 216 and fed to an inverse TNS filter 900 the structure of which is shown in more detail in FIG. 9b. The inverse TNS filter 900 again delivers as output signal spectral values which are transformed to the time range in a synthesis filter bank 218. The TNS filter 900 again comprises an input switch 902 as well as an output switch 908 which firstly serve again for parallel to serial conversion and serial to parallel conversion, respectively, of the data processed. Input switch 902 furthermore takes into account a possibly employed target frequency range in order to supply only spectral residual values to inverse TNS coding, whereas spectral values that are not TNS-coded are allowed to pass to an output 910 without being changed. The inverse prediction filter again comprises a predictor 906 and a summation point 904. However, differently from the TNS filter, they are connected as follows. A spectral residual value is fed via input switch 902 to summation point 904 where it is summed with the output signal of predictor 906. The predictor delivers an estimated spectral value xp(i) as output signal. The spectral value x(i) is output via the output switch to the output of the inverse TNS filter. The TNS-related side information thus is decoded in the decoder, with the side information comprising a flap indicating the use of TNS and, if necessary, information concerning the target frequency range. In addition thereto, the side information contains furthermore the filter coefficients of the prediction filter that was used for coding a block or xe2x80x9cframexe2x80x9d.
The TNS method thus can be summarized as follows. An input signal is transformed to a spectral representation by means of a high-resolution analysis filter bank. Thereafter, linear prediction in the frequency range is carried out, namely between the frequency-adjacent spectral values. This linear prediction can be interpreted as filtering process for filtering the spectral values, which is carried out in the spectral range. The original spectral values are thus replaced by the prediction error, i.e. by the spectral residual values. These spectral residual values, just as usual spectral values, are transmitted to the decoder in quantized and coded form, in which the values are decoded again and inversely quantized. Prior to application of the inverse filter bank (synthesis filter bank), a prediction inverse to the prediction carried out in the coder is performed by using the inverse prediction filter for the prediction error signal transmitted, i.e. the requantized spectral residual values.
By application of this method, it is possible to match the temporal envelope curve of the quantization noise to that of the input signal. This permits improved exploitation of the masking of the error signals in case of signals having a pronounced fine structure in terms of time or a pronounced transient nature. In case of transient signals, the TNS method avoids the so-called xe2x80x9cpre-echoesxe2x80x9d in which the quantization noise appears already prior to xe2x80x9cstrikingxe2x80x9d of such a signal.
It is known, furthermore, from the field of psychoacoustics that the impression of perception of noise signals is determined primarily by the spectral composition thereof and not by the actual signal shape thereof. This permits the use of a so-called noise substitution method in the data reduction of audio signals.
The xe2x80x9cnoise substitutionxe2x80x9d is described in Donald Schulz: xe2x80x9cImproving Audio Codecs by Noise Substitutionxe2x80x9d, Journal of the Audio Eng. Soc., Vol. 44, No. 7/8, pages 593 to 598, July/August 1996. As was already mentioned, conventional audio coding algorithms make use of masking effects of the human ear for significantly reducing the data rate or the number of bits to be transmitted. Masking thus means that one or more frequency components as spectral values make other components of lower levels inaudible. This effect can be exploited in two ways. First, audio signal components masked by other components need not be coded. Second, the introduction of noise is permitted by the afore-described quantization when such noise is masked by components of the original signal.
In case of noise-like signals, the human hearing system is not capable of ascertaining the exact path in time of such a signal. With conventional algorithms, this led to the result that even the waveform of white noise, which is nearly irrelevant for the human ear, was coded. Hearing-compensated coding of noise-containing signals thus requires high bit rates for information that is not audible when there are no specific precautions taken. If, however, noise-containing components of signals are detected and coded along with information on their noise levels, on their frequency range or on their extension in time, such superfluous coding can be reduced, which may lead to extraordinarily high bit savings. This fact is supported by psychoacoustics which says that the impression of perception of noise signals is determined primarily by the spectral composition thereof and not by the actual waveform. This permits thus the utilization of the noise substitution method for the data reduction of audio signals.
The coder thus is confronted with the task of finding or recognizing noise-like or noisy spectral values in the entire spectrum of the audio signal. A definition for noise-like spectral values reads as follows: when a signal component can be characterized by its level, by its frequency range and by its extension in time in such a manner that it can be reconstructed by a noise substitution method without audible differences for the human sense of hearing, this signal component is classified as noise. The detection of this characteristic can be carried out either in the frequency range or in the time range, as pointed out in the publication cited last. The simplest method, for example, consists in detecting tonal, i.e. not noise-like, components by making use of a time-frequency transformation and in following stationary peaks in spectrums following each other in time. These peaks are referred to as tonal, everything else is referred to as noise-like. However, this constitutes are relatively coarse system of noise detection. Another possibility of distinguishing between noise-like and tonal spectral components consists in making use of a predictor for spectral values in successive blocks. A prediction is carried out from one spectrum to the next spectrum, i.e. the spectrum associated with the next temporal block or frame. When there is no or only little difference between a predicted spectral value and a spectral value of the next block or frame in time that has actually been ascertained by transformation, it will be assumed that this spectral value is a tonal spectral component. From this, a measure of tonality can be derived the value of which constitutes a basis for decision for distinguishing tonal and noise-like spectral values from each other. However, this method of detection is suitable only for strictly stationary signals. It does not permit a detection of situations in which sinusoidal signals with slightly varying frequencies in relation to time are present. Such signals often appear in audio signals, such as e.g. vibratos, and it is obvious to the expert that these cannot be replaced by a noise-like component.
Another possibility of detecting noise-like signals consists in noise detection by prediction in the time range. To this end, a matched filter can be used as predictor which can be employed repeatedly for performing a linear prediction, as it is sufficiently known in technology. Passed audio signals are input, and the output signal is compared with the actual audio sampling value. In the case of a minor prediction error, tonality can be assumed. For determining the character of different frequency regions, i.e. for detecting the spectral range, whether a group of spectral values is a noise-like group, time-frequency transformations both of the original signal and of the predicted signal have to be carried out. A measure of tonality can then be calculated for each frequency group by comparing the original and the predicted values with each other. A major problem in this respect is the limited dynamic range of the predictor. A noise-like frequency group of high level dominates the predictor due to the resulting high error. Other frequency regions with tonal components could be interpreted as noise-like. This problem can be reduced by using an iterative algorithm in which the error signal usually is of lower level than the original signal and is again input by an additional predictor, whereupon both predicted signals are added. Further methods are listed in the publication by Schulz.
The group of spectral values that is now classified as noise-like, is not transmitted to the receiver in quantized and entropy- or redundancy-coded form (for example by means of a Huffman table) as is usual. Rather, only an identification for indicating the noise substitution as well as a measure of the energy of the noise-like group of spectral values are transmitted as side information. In the receiver, the substituted coefficients are replaced by random values (noise) along with the transmitted energy. The noise-like spectral values thus are substituted by random spectral values with the corresponding amount of energy.
By transmitting a single item of energy information instead of a group of codes, i.e. several quantized and coded spectral values, for the quantized spectral coefficients, there are considerable data savings possible. It is apparent that the data rate savings attainable are dependent upon the signal. For example, when a signal with very few noise components, i.e. very few noise-like groups, or with transient properties is involved, lower data rate savings will be possible than in case of coding of a very noisy signal having very many noise-like groups.
The initially described standard MPEG-2 Advanced Audio Coding (AAC) does not support the possibility of noise substitution. The considerable data rate savings are thus not possible so far with the presently existing standard.
FIG. 3 again shows a survey of a coder and a decoder, with the coder and decoder in FIG. 3 corresponding to the coder and decoder, respectively, described with regard to FIG. 2, while however containing noise substitution. It is to be emphasized here once more that the implementation of noise substitution as shown in FIG. 3 is not part of the prior art, as was already pointed out in the preceding paragraph. Like reference numerals refer to like functional blocks. It can thus be seen that the coder in FIG. 3 merely has one new block xe2x80x9cnoise detectionxe2x80x9d 310 in comparison with FIG. 2. The noise detection is carried out with the output signals, i.e. the spectral values, of the analysis filter bank 202. However, it is also possible to use for noise detection the temporal input signals of the analysis filter bank, as indicated by the arrow symbol connecting the audio input 200 to the block xe2x80x9cnoise detectionxe2x80x9d 310. Noise substitution requires two categories of side information, as indicated by the two arrows extending from the block xe2x80x9cnoise detectionxe2x80x9d to the block xe2x80x9cquantizing and codingxe2x80x9d 204 and, respectively, from the block xe2x80x9cnoise detectionxe2x80x9d 310 to the block xe2x80x9cbit flow multiplexerxe2x80x9d 208. In order to be able to decode again a noise-substituted coded signal, a noise substitution indication has to be transmitted as side information, which indicates in which frequency range or, in a preferred method, in which scale factor band a noise substitution has been carried out. Furthermore, the measure for the energy of the spectral values in the noise-like group or the noise-like scale factor band also has to be transmitted as side information. It is to be pointed out here that noise-substituted spectral values are not quantized and coded, i.e. the block xe2x80x9cquantizing and codingxe2x80x9d 204 is informed that noise substitution is present in a scale factor band. The bit flow multiplexer also receives the noise substitution indication and, as side information, the measure of the energy of the spectral values in a noise-like group.
The decoder also is similar to the already described decoder of FIG. 2, with the exception of the new block xe2x80x9cnoise substitutionxe2x80x9d 312. Block xe2x80x9cnoise substitutionxe2x80x9d 312 receives as input signals from bit flow demultiplexer 204 the side information containing noise substitution indication and the energies of the substituted signals, i.e. the measure of the energy of the spectral values in a noise-like group and in a noise-like scale factor band, respectively. The block xe2x80x9cnoise substitutionxe2x80x9d generates for the noise-like groups and noise-like scale factor bands, respectively, random or xe2x80x9cnoisexe2x80x9d spectral values which are fed to synthesis filter bank 218 in order to obtain again a coded and decoded time-discrete audio signal. It is obvious that it is irrelevant for the synthesis filter bank 218 whether it transforms noise spectral values or xe2x80x9cnormalxe2x80x9d requantized spectral values to the time range.
The known noise substitution method involves the problem that parts of the input signal can be substituted by noise and then can be decoded again with inaudible quality losses when the input signal exhibits a uniform noise structure, i.e. a plane or flat spectrum. This does not hold in case of transient signals or voice signals, so that either the use of noise substitution has to be dispensed with completely or, in case noise substitution is employed nevertheless, disturbing distortions of the signal are caused.
It is the object of the present invention to provide a method of coding and decoding audio signals, respectively, which renders possible a high coding efficiency while entailing, if possible, no audible signal distortions.
In accordance with a first aspect of the present invention, this object is met by a method for coding an audio signal, comprising the steps of transforming a temporal audio signal to the frequency domain in order to obtain spectral values of the temporal audio signal; performing a prediction of the spectral values in relation to frequency in order to obtain spectral residual values; detecting noise areas in the spectral residual values; noise-substituting the spectral residual values in the noise areas; and introducing information concerning the noise areas and the noise-substitution in side information of a coded audio signal.
In accordance with a second aspect of the present invention, this object is met by a method for decoding a coded audio signal, comprising the steps of receiving the coded audio signal; detecting information in the side information that relates to noise substitution and to noise areas of the spectral residual values; generating spectral noise residual values on the basis of the detected information in the noise areas; performing an inverse prediction in relation to frequency in order to obtain spectral values from the noise-substituted spectral noise residual values; and transforming the spectral values to the time domain in order to obtain a decoded audio signal.
The present invention is based on the realization that a corresponding combination of TNS technique and noise substitution yields a further increase in coding gain without audible signal distortions. The spectral residual values created by TNS processing have of their own a considerably lower energy content than the original spectral values. The signal belonging to the spectral residual values has a considerably flatter path in comparison with the original signal. By prediction of the spectral values in relation to frequency, the highly fluctuating pattern of the envelope curve of transient signals is so-to-speak extracted, whereby a signal having a flat envelope curve is left to which noise substitution can be applied in accordance with the invention, in order to be able to achieve considerable bit savings in case of transient signals as well.

Fraunhofer-Gesellschaft zur Forderung Der Angewandten Forschung E.V.


Carbonated hydroxyapatite compositions and their preparation are described. The compositions are biologically resorbable and are prepared as flowable masses which can be administered by syringe to set in situ to serve as a support structure, filler, prosthesis or the like. Optionally the compositions may include proteins or serve as a depot for compositions of pharmacological interest.

The field concerns the preparation of substantially pure compositions of carbonate-substituted forms of hydroxyapatite, and the biomedical use of such compositions.
A number of calcium phosphate minerals, such as hydroxyapatite, fluorapatite, octacalcium phosphate, whitlockite, brushite and monetite may have application as biocompatible materials. The various crystalline forms of the calcium phosphate minerals can confer different physical properties that may be more or less desirable for a specific biomedical application. For instance, octacalcium phosphate and whitlockite are less resorbable than brushite or monetite (Brown and Chow, Ann. Rev. of Materials Science (1976) 6:213-236).
Of particular interest are the apatites. Apatite is a general term for a wide range of compounds represented by the general formula M2+10(ZO43xe2x88x92)6Yxe2x88x922, where M is a metal atom, particularly an alkali or alkaline earth atom, ZO4 is an acid radical, where Z may be phosphorous, arsenic, vanadium, sulphur, silicon, or may be substituted in whole or in part by carbonate (CO32xe2x88x92), and Y is an anion, usually halide, hydroxy, or carbonate. When ZO43xe2x88x92 is partially or wholly replaced by trivalent anions (such as CO32xe2x88x92) and/or Y is partially or wholly replaced by divalent anions, then charge balance may be maintained in the overall structure by additional monovalent cations (such as Na+) and/or protonated acid radicals (such as HPO42xe2x88x92).
Hydroxyapatite (HAp), as well as its various forms, has been the focus of substantial interest because it is a major structural component of biological tissues such as bone, teeth, and some invertebrate skeletons. There are many situations where bone has been broken, surgically removed, destroyed, degraded, become too brittle, or been subject to other deteriorating effects. In many of these situations it would be desirable to be able to replace the bone structure or strengthen the bone structure. In providing materials to substitute for natural bone, teeth, or other calcified tissues, there are a number of restraints on the natural composition of the material.
Dental applications might prefer a fluoride substituted hydroxyapatite, i.e. francolite, that would reduce solubility and increase resistance to decay.
The material should ideally possess certain characteristics that facilitate the production, storage life, and biomedical application of the material. Specifically, a material which could be a material which could be fingerpacked in an open surgical procedure or percutaneously injected as a flowable composition to fill voids or completely fill-in areas deficient of hard bone is very desirable. Where the material is to be placed in the body and formed and hardened in situ, a variety of considerations come to the fore. For example, the rate at which hydroxyapatite forms as well as the extent to which the formation of hydroxyapatite is exothermic or may generate gas can also be important. Where the reaction is highly exothermic, it may cause thermal necrosis of the surrounding tissue.
As the final form of the material must be stable under physiological conditions, so must the form in which the material is introduced be stable while it is hardening in the environment to which it is introduced, as must be any intermediate products of the formation reaction.
The material should also be physiologically acceptable at all phases of curing to the final product, so as to avoid the initiation of clotting, inflammatory responses, and the like. Two different forms of apatite are particularly desirable: One being an hydroxyapatite or a fluoridated derivative thereof that is non-resorbable in vivo; the other includes forms of apatite that are substantially resorbable in vivo. In addition, both forms of apatite must usually be strong and non-friable. Furthermore, there should be a strong adhesion between the material and the remaining bone or calcified tissue. Also, the material should desirably be able to substitute some of the other functions of natural bone such as: accommodating stem cells; allowing infiltration by cells normally resident in natural bone such as osteoclasts, osteoblasts, and the like; allowing remodeling of the material by the infiltrating cells followed by new bone in-growth; and acting in metabolic calcium exchange in a manner similar to native bone.
Carbonate has been shown to inhibit crystal growth of HAp (Blumenthal, et al., Calcif. Tissue Int. (1984) 36:439-441; LeGeros, et al., xe2x80x9cPhosphate Minerals in human tissuesxe2x80x9d, in Phosphate Minerals (Berlin), J. Nriagu (eds): Springer, 1984, pp. 351-385; LeGeros, et al., J. Dent. Res. (1989) 68:1003; Nauman and Neuman, The Chemical Dynamics of Bone Mineral, University of Chicago Press, Chicago, (1958); Newesley, Arch. Oral Biol. (1961) 6:174-180; Posner, Clin. Orthop. (1985) 200:87-99). Carbonates are present in the apatites of hard tissues, and their presence alters the properties of stoichiometric apatite. Carbonate has been described as causing: 1) a reduction in crystallite size, 2) changes in the morphologies of the mineral phase from needles and rods to equi-axis crystals (spheroids), 3) contraction of the a-axis, as well as an expansion in the c-axis, 4) internal strain, and 5) chemical instability (LeGeros, et al., supra , 1984; LeGeros, et al., supra, 1989). All of these factors lead to higher solubilities of carbonate-substituted HAp. The x-ray diffraction patterns as well as the radial distribution function are changed considerably in that as the concentration of carbonate increases, the patterns become more amorphous in character (LeGeros et al., supra 1989; Glimcher, M. J., xe2x80x9cRecent studies of the mineral phase in bone and its possible linkage to the organic matrix by protein-bound phosphate bondsxe2x80x9d, Phil. Trans. R. Soc. London Ser. B., 304, 479-508). The line broadening observed in the diffraction pattern is caused by decreasing crystallite size and crystallinity. In addition to inhibiting HAp crystal growth, carbonate substitution markedly increases the solubility of HAp (Nelson, et al., Ultramicroscopy (1986) 19:253-266. Another interesting experimental finding was that, whether the carbonates are structurally bound within, or absorbed onto HAp, differences in dissolution behavior were observed. This suggests that dissolution increased in HAps containing structurally bound carbonates, while decreasing in HAps with absorbed CO32xe2x88x92. The decrease in dissolution was explained by the fact that hydronium ions had to compete for the surface of HAp, hence the deposition of the CO32xe2x88x92 layer was required.
The extent of carbonate uptake during HAp precipitation under normal physiologic conditions is approximately 1% by weight CO32xe2x88x92 (Posner, supra, 1985). Bone consists of approximately 4% by weight CO32xe2x88x92. Thus, HAp precipitation reactions in air generally contain relatively low concentrations of carbonate. Bone mineral apatite with a level of carbonate between 2% and 10% by weight has been referred to by convention as dahllite (McConnell, J. Dent. Res. (1952) 31:53-63 and McConnell, Clin. Orthopaed. (1962) 23:253-268).
Carbonates can substitute in both the Z and Y sites of the apatite structure, and it is generally accepted that carbonates substitute for PO43xe2x88x92 groups during precipitation reactions leading to HAp formation. More specifically, HAp products formed at lower temperature exhibit carbonate substitution at the phosphate sites, and due to its smaller size, a decrease in the a-axis of the apatite results (LeGeros, et al., supra, 1984 and LeGeros, et al., supra, 1989). Conversely, in most high temperature apatites, the carbonates are found in the vicinity of the six fold axis, where they replace hydroxyl ions. Since the carbonate is larger than the hydroxyl ion, an increase in the a-axis results (Brown and Chow, (1986), supra).
The skeleton is the reservoir for almost all of the body""s calcium and most of its phosphorus and magnesium (Avioli and Krane, Metabolic Bone Disease and Clinically Related Disorders, 2nd Ed., 1990, W. B. Saunders Co., Philadelphia, p. 2). The carbonate levels in human enamel have been shown to increase in concentration from the surface to the dentin. The carbonate concentration in the surface enamel has also been shown to decrease with age (Brudevold and Soremark, Chemistry of the mineral phase of enamel, Miles (ed.), In: Structural and Chemical Organizations of Teeth. Academic Press, New York, 1967, Vol. II, p. 247. The ease of ionic substitution in the lattice of apatite allows for the ionic substitution of ions from the fluids surrounding the bone, and vice versa. This implies that hard tissues act as a regulatory reservoir for certain ions by incorporating ions into its structure when ionic concentration in the serum rises too high, and dissolving ions when the body is deficient in them. Possible candidates for this form of regulation might include some of the inorganic constituents of serum such as ionized and complexed calcium, inorganic phosphates, magnesium, bicarbonate, sodium, chloride, potassium, among others (Eidleman, et al., Calcif. Tissue Int. (1 987a) 41:18-26; Eidleman, et al., Calcif. Tissue Int. (1987b) 40:71-78; Meyer and Fleisch, Miner. Electrolyte Metab. (1984) 10:249-258).
Carbonate is especially important in hard tissue in that it appears to be required for the cellular infiltration of bone by osteoclasts, osteoblasts and other bone resident cells. Since osteoclasts, osteoblasts and the like are involved in mineral replacement and bone remodeling, any synthetic apatite-associated bioimplant would preferably use a carbonated form of apatite, or dahllite. Because dahllite can be remodeled by the bodies natural processes, the dahllite component of an implant should, through the action of osteoclasts and osteoblasts, eventually be replaced by natural bone. Thus, dahllite implants should eventually gain many or all of the desirable features of natural bone such as increased strength, elasticity and durability.
Previous methods of chemically forming monolithic bodies of hydroxyapatite have not produced dahllite or hydroxyapatites with physiologically significant levels of structurally incorporated carbonate. This is primarily because the acid present in the reactions of other methods tend to react with the carbonate to produce gaseous CO2. Gaseous escape removes carbonate from the reaction which is forming apatite and can result in a product that is substantially more friable than that generally desired by virtue of the trapped gas bubbles disrupting the structural integrity of the product. Thus, a major obstacle to the production of dahllite has been devising a method to maintain carbonate in the product despite the presence of the acid required to form the apatitic structure.
Relevant Literature
Patents of interest include U.S. Pat. Nos. 3,787,900; 3,913,229; 3,679,360; 4,097,935; 4,481,175; 4,503,157; 4,612,053; 4,659,617; and 4,693,986. See also, Arends and Jongebloed, Rec. Trav. Chim. Pays-Bas (1981) 100:3-9. Use of calcium phosphate as a sealer-filler material is described in Chohayeb et al., J. Endodontics (1987) 13:384-387 and Lowenstam and Weiner, On Biomineralization, (1989), Oxford University Press, New York. See also, Ohwaki et al., 13th Ann. Mtg. of the Soc. for Biomaterials, Jun. 2-6, 1987, New York, N.Y., p209.
Compositions comprised of dahllite, analogs thereof, or otherwise carbonate-substituted forms of hydroxyapatite (dahllite-like compositions) are provided that are useful in a variety of biomedical applications. The compositions can be prepared such that they are flowable, moldable, and capable of hardening in situ. The compositions harden into monolithic polycrystalline structures that can be shaped subsequent to hardening.
Compositions are provided that are comprised of substantially pure (greater than 80% by weight) dahllite-like compositions referred to as carbonated hydroxyapatite that can be produced substantially free of the blood-borne or organic components of natural bone. The compositions can be used to substitute many of the functions of naturally occurring calcified tissues or in the repair of such tissues, in particular teeth and bone. The dahllite or francolite-like products can be readily formed by combining the wet and dry reactants to provide a substantially uniform mixture, shaping the mixture as appropriate, and allowing the mixture to harden. During hardening, the mixture crystallizes into a solid and essentially monolithic apatitic structure. The dahilite and francolite-like apatitic compositions can also be shaped subsequent to hardening. Alternatively, the dahllite or francolite-like apatitic compositions can be precursor reaction mixtures placed into an appropriate section of the body and hardened and/or shaped in situ.
The composition of the carbonated hydroxyapatite may vary, frequently being non-stoichiometric in having incorporated extra hydrogen atoms. Also, the calcium/phosphate ratio may vary, where the ratio may be as low as 1.33 (1.67 is the natural ratio), so that there is a defective lattice structure from the calcium vacancies and as high as 2.0. For a ratio of 1.33, there will be two calcium ions absent. The extra hydrogens may be up to about 2 hydrogen ions per phosphate, usually not more than about one hydrogen ion per phosphate. The ions will be uniformly distributed throughout the product and for the most part, the composition will be monophasic having a single crystal structure. As compared to sintered hydroxyapatite and hydrothermally prepared hydroxyapatite, the X-ray diffraction and Fourier transform infra-red spectra of the subject compositions are substantially different.
The reactants will generally consist of a phosphoric acid source substantially free of unbound water, an alkali earth metal, particularly calcium, source, optionally crystalline nuclei, particularly hydroxyapatite or calcium phosphate crystals, calcium carbonate, and a physiologically acceptable lubricant, such as water, which may have various solutes. The dry ingredients may be pre-prepared as a mixture and subsequently combined with the liquid ingredients under conditions where substantially uniform mixing occurs. Where gases are evolved the mixture is agitated to cause the release of large pockets of gas.
The phosphoric acid source may be any partially neutralized phosphoric acid, particularly up to and including complete neutralization of the first proton as in calcium phosphate monobasic. Alternatively or additionally, it can consist of orthophosphoric acid, possibly in a crystalline form, that is substantially free of combined water. The acid source will generally be about 15 to 35 weight percent of the dry components of the mixture, more usually 15 to 25 weight percent.
In selecting the calcium source, particularly where the calcium source serves a dual role of providing calcium and acts in a neutralizing capacity, one must consider that the desired final product will depend on the relative ratios of calcium and phosphate. Calcium sources will generally include counterions such as carbonate, phosphate or the like. Of particular interest are dual sources of calcium phosphate and phosphate such as tetracalcium phosphate (C4P) or tricalcium phosphate (C3P). Tetracalcium phosphate or tricalcium phosphate may typically be present in the mixture at from about 0 to 70 weight percent, more usually from about 0 to 40 weight percent, and preferably from about 2 to 18 weight percent of dry weight of the dry components of the mixture. With calcium carbonate present to neutralize the acid and to serve as a source of calcium and carbonate, the reaction will result in relatively little temperature rise; however, there is substantial evolution of gas which must be released during mixing. Calcium carbonate will be present in the mixture from about 2 to 70 weight percent, more usually from about 2 to 40 weight percent, and preferably from about 2 to 18 weight percent of dry weight of the dry components of the mixture Calcium hydroxide may also be present in the mixture from about 0 to 40 wt. %., more usually from about 2 to 25 wt. %, and optimally from about 2 to 20 wt. %.
Halides such as fluorine and chlorine may be added to form fluorapatite (francolite), or chlorapatite respectively. Various sources of fluoride or chloride may be employed. Generally, the sources will include either soluble salts such as calcium chloride, calcium hexafluorosilicate or sodium fluoride or, less desirably, the source may be added as a dilute acid in the aqueous lubricant, generally at concentrations of less then about 1 M. Halides, if present at all, will constitute from about 0 to 4 weight percent, more usually from about 2 to 4 weight percent, preferably from about 3 to 4 weight percent of dry weight. Usually at least about 5, more usually at least about 10% of the hydroxyl groups will be replaced, and up to 100%. Francolite is of particular interest because of the potential dental applications of this partially fluorine substituted form of dahllite.
The various dry components may be combined prior to the addition of the wet components. Mixing will be used to combine the ingredients and can be used to regulate the extent of the inter-ingredient reactions. Any or all of the dry ingredients may be added prior to the initiation of mixing or prior to the completion of mechanical mixing. Methods of mixing can include ball milling, Brabender mixing, rolling between one or more rollers and a flexible container, or the like. Preferably, mixing will be thorough and will occur for a relatively short time or until a uniform dispersal of ingredients is obtained.
By varying the proportion of liquid lubricant, particularly water, added to the subject mixtures, the fluidity of the composition can be varied with respect to flowability and viscosity. Besides or in combination with water, other water miscible pharmacologically acceptable liquids may be used, particularly alkanols, more particularly polyols, such as ethylene glycol, propylene glycol or glycerol, usually being present in less than about 10 volume percent in an appropriate medium. The liquid will generally be from about 15 to 50, more usually from about 20 to 35 weight percent of the entire composition. Various solutes may be included in the aqueous medium. Of particular interest is the use of a gel or colloid, which has as a solute alkali metal hydroxide, acetate, phosphate, or carbonate, particularly sodium, more particularly phosphate or carbonate, at a concentration in the range of about 0,01 to 2 M, particularly 0.05 to 0.5 M, and at a pH in the range of about 6-11, more usually about 7-9, particularly 7-7.5.
Implantation may be by syringe or catheter injection; particularly, the composition may be used as a paste that passes through a needle in the range of about 10-18 gauge, preferably about 14-16 gauge. Alternatively, if less lubricant is added, the composition is kneadable or moldable, being capable of forming clay-like putty that may be molded prior to setting. By varying the amount of lubricant employed, the setting time of the compositions can also be varied.
After mixing, the mixture is allowed to anneal while remaining quiescent, followed by an extended period of time during which the mixture hardens. During hardening, crystal growth occurs and the product becomes an integral mass. Hardening will take at least about 5 minutes, usually at least about 15 minutes, and not more than about 20 minutes. Compounds produced in this manner will have a wide variety of desirable properties for use in physiological applications.
The claimed compositions will contain, structurally incorporated into the apatitic structure, between about, 2% and about 10% carbonate by weight, usually between 2.5% to 7%, and optimally between about 4% to about 6% carbonate by weight.
The subject compositions are biocompatible having a pH in the range of about 5.5-8.5, usually in the range of about 6-7.5. They can be prepared so that they can be administered to an environment having a temperature in the range of about 0-45xc2x0 C., usually 20-40xc2x0 C., and optimally about normal physiological temperature, 37xc2x0 C. The compositions have low or no toxicity when prepared in accordance with described methods, are substantially inactive as to detrimental interactions with various host components in vivo, and are readily implantible. Furthermore, they are readily resorbable in vivo so as to be replaced by natural bone.
Various additional components may be included during the formation of the carbonated hydroxyapatite, dahllite. Of particular interest are pharmacologically active agents, proteins, polysaccharides, or other biocompatible polymers, or the like. Of particular interest are proteins involved in skeletal structure such as different forms of collagen, especially Type I, fibrin, fibrinogen, keratin, tubulin, elastin, and the like, or structural polysaccharides, such as chitin. Pharmacologically active agents might include drugs that enhance bone growth, serve as a variety of cell growth factors, or act as anti-inflammatory or anti-microbial agents. Examples of such proteins might include but not be limited to: bone morphogenetic protein, cartilage induction factor, platelet derived growth factor, and skeletal growth factor. Pharmacologically active or structural proteins may be added as an aqueous dispersion or solution. Usually the protein will be present in from about 1-10 wt % of the aqueous dispersion. The protein will be present in the final composition after setting in from about 0.01 to 10, usually from about 0.05 to 5 weight percent. The amount of water added to the compositions to which protein in aqueous dispersion has also been added will be adjusted accordingly. By varying the proportions of the reactants, compositions with varying and predictable rates of resorption in vivo, can be made. Thus, the subject compositions enable one of ordinary skill in the art to add drug and inorganic components both subsequent to and during, and possibly prior to, the formation of the subject compositions in order to practice an implantible or injectable time-release delivery platform for drugs, inorganic mineral supplements, or the like.
When used as cements or fillers, the subject compositions bond to other apatites when applied to an apatitic surface, such as bones or teeth which are mainly comprised of dahllite and collagen. The applicable compositions are able to strongly adhere and bond to surfaces that are wet or coated with saliva, blood or lymphatic fluid, will fill voids, and conform to irregular surfaces such as concavities and convexities. The compositions may be applied as a continuous mass without the formation of fragments or loose particles to a significant degree. Furthermore, the subject compositions are found to be structurally compatible in providing for the structural functions of replaced connective tissue.
The subject compositions can be used to form carbonated hydroxyapatite coatings on bioimplants or other formed objects.
The subject composition, as a flowable or formable product, can serve as a bone cement, or an infiltrate cement for the treatment of osteoporotic bone.
Paste or clay-like mixtures of product are provided that may be formed and hardened into a monolithic carbonated hydroxyapatite product, either externally or in A.
Of particular interest is preparation of the subject carbonated hydroxyapatite by a process whereby a calcium source, at least one component of which is calcium carbonate, and an acidic phosphate source, optionally comprised of ortho phosphoric acid crystals substantially free of uncombined water, are mechanically mixed for sufficient time for a partial reaction of said calcium source and acidic phosphate source. The partially reacted composition can be subsequently mixed with a physiologically suitable lubricant which varies the fluidity of the product, allows the substantially complete reaction of the reactants, and eventually results in a monolithic solid carbonated hydroxyapatite product. The final mixture may be subsequently shaped and hardened, hardened then shaped, or placed in the body and hardened in situ. The carbonated hydroxyapatite of the subject process will have substantially reduced or non-exothermic setting which may better provide for the stability of introduced pharmacological agents, and, when hardened in situ, is desirable for purposes of patient comfort. The compositions of this process are also applicable as bone cements or fillers, dental or endodontic filling agents, coatings for bioimplantible substrates, or formed into suitable shapes before or after hardening into a monolithic structure.
The calcium source used in the above process will typically include a mixture of tetracalcium phosphate (C4P) and calcium carbonate (CC) with C4P typically present in from about 55 to 75 wt. %, or more usually 60-70 wt. %, and CC typically present in from about 1 to 40 wt. %, or more typically 2 to 18 wt. % of the dry weight of the total reaction mixture.
The acid phosphate source will be about 15 to 35, or more usually 15 to 25 wt. % of the dry weight of the reaction mixture.
An alternative formula will typically include a mixture of tricalcium phosphate (C3P), calcium carbonate (CC), and calcium hydroxide (CH) with C3P typically present in from about 50 to 90 wt. %, or more usually 75 to 90 wt. %, CC typically present from about 1 to 40 wt. % or more usually 2 to 18 wt. %, and CH typically present from about 0 to 40 wt. % or more usually 2 to 20 wt. % of the dry weight of the total reaction mixture.
The acid phosphate source for this alternative mixture will be about 5 to 35 wt. % or more usually 5 to 25 wt. % of the dry weight of the reaction mixture.
A fluoride source may generally be added to the mixture and, if at all present, will be in an amount from about 0 to 4 wt. %, preferably 3 to 4 wt. % of dry weight.
After the dry ingredients are combined, the reactants will be placed in intimate contact by mechanical mixing or milling. Prior to the completion of mixing/milling, proteins and/or small organic molecules, especially those containing pharmacological significance as indicated earlier, may be added to the mixture to alter the physical or physiological properties of the final product. The amount of additive will generally vary from about 1 to 40 weight percent, more usually from about 1 to 25 weight percent of the inorganic materials. It may be preferred that the additive be combined with the inorganic materials before mixing/milling.
Mechanical mixing may be by any form that results in an intimate mixing of the reactants. A variety of equipment may be used for these purposes including ball mills, planetary mills, centrifugal mills, mechanofusion systems, air pulverizers, jet mills, vibratory mills, colloid mills, attrition mills, disc mills, and the like.
The course of mixing can be monitored by periodically removing samples and testing whether or not the samples result in the formation of a product with the desired properties after mixing with as aqueous medium and subsequent hardening.
During mixing or milling, the walls of the mixing vessel may be periodically scraped to better promote a more uniform product. The milling media should remain stable as inert throughout the process as would media such as alumina, zirconia, tungsten carbide, boron carbide, etc.
The product of the above process will have undergone a relatively stable partial reaction and will require less lubricant to provide a workable mixture as well as a reduced setting time.
All of the above-mentioned products or their precursors may be sterilized by gamma-irradiation or other applicable methodologies prior to bioimplantation.
The following example is offered by way of illustration and not by way of limitation.

Paste compositions capable of setting into carbonated apatite


This invention embodies an improved process for annealing integrated circuits to repair fabrication-induced damage. An integrated circuit is annealed in a pressurized, sealed chamber in which a forming gas comprising hydrogen is present. Pressurization of the chamber reduces the contribution made by the final anneal step to total thermal exposure by increasing the diffusion rate of the hydrogen into the materials from which the integrated circuit is fabricated. Ideally, the forming gas contains, in addition to hydrogen, at least one other gas such as nitrogen or argon that will not react with hydrogen and, thus, reduces the danger of explosion. However, the integrated circuit may be annealed in an ambiance containing only hydrogen gas that is maintained at a pressure greater than ambient atmospheric pressure.

1. Field of the Invention
This invention relates to annealing processes used to repair process-induced damage in integrated circuits.
2. Description of Related Art
Integrated circuit devices are typically fabricated in mass on silicon wafers. During the fabrication process, a semiconductor wafer is transformed into a multitude of integrated circuits by subjecting it to numerous ordered steps, which may include coating with photoresist resin, exposure of the photoresist to electromagnetic radiation using a reticle affixed to a highly precise stepper device, development of the photoresist to form a mask, oxidation, nitridization, silicidation, wet etching, plasma etching, chemical vapor deposition, plasma-enhanced chemical vapor deposition, sputter deposition, ion implants, and annealing steps for activating and driving implanted ions. A number of these steps create residual damage to the material being treated. For example, plasma etches typically leave dangling silicon bonds. In such a case, the silicon surface exposed to the etch comprises many silicon atoms with unfilled orbitals. Another example is the crystal damage wrought by ion implants. It is axiomatic that the larger the implanted ion, the greater the crystal damage. Arsenic, a relatively large ion, is particularly adept at damaging the silicon crystal structure when implanted. Plasma-enhanced chemical vapor deposition processes can also create damage similar to that caused by plasma etches. In addition, silicon dioxide deposited using plasma-enhanced chemical vapor deposition processes tends to trap electrons. The charge buildup resulting from trapped electrons can shift threshold voltage values in field-effect transistors to the point where they will not fully shut off. Unwanted current leakage through the transistors results.
Following fabrication, but before encapsulation or other packaging steps, the wafers are annealed at temperatures of about 400xc2x0 C. in a forming gas containing diatomic hydrogen for approximately 30 minutes in order to repair the damage caused by the various process steps. Generally, the forming gas contains both diatomic hydrogen and diatomic nitrogen gases. The partial pressure of hydrogen within the forming gas is usually about 20 percent of the total pressure. At anneal temperatures, nitrogen functions as an inert gas, as it does not react with either hydrogen or the materials from which the circuits were fabricated within this temperature range. Although other inert gases such as helium, neon or argon may be used as components of the forming gas, nitrogen is used primarily for cost considerations. In addition, although forming gases having higher concentrations of hydrogen or even pure hydrogen could be used for the final anneal ambiance, safety concerns dictate that hydrogen be diluted to a partial pressure of around 20 percent of the total pressure. At elevated temperatures, silicon crystals damaged by implanted ions tend to heal themselves. Although the healing process may be less than perfect, a significant reduction in transistor leakage is observed. During the anneal, the hydrogen gas molecules, on account of their small size, are able to diffuse throughout the circuit and react with any dangling bonds that are present. Dangling bonds may be present on silicon atoms in substrate regions which have been plasma etched, in polycrystalline silicon layers which have been plasma etched, and in silicon atoms in silicon dioxide layers which have been formed using plasma-enhanced chemical vapor deposition. The diffusion of hydrogen throughout the oxide layers dissipates trapped charges and enhances circuit performance.
During the fabrication process, an integrated circuit is subjected on numerous occasions to elevated temperature. Generally, the elevated temperature is required to effect a necessary step in the fabrication process. For example, oxidation of silicon, implant activations, and chemical vapor deposition of silicon dioxides are generally performed at temperatures in excess of 500 degrees centigrade. Metallization steps are also performed at high temperatures. Although a certain amount of exposure to elevated temperatures is required both to activate implanted ions and to cause them to diffuse within the implanted material, too much exposure to elevated temperature will effect too great an overlap and counterdoping of adjacent implants having opposite conductivity types and cause source/drain regions of field- effect transistors to diffuse too far into the channel regions. Outdiffusion of dopants into the channel regions is irreversible and will result in transistor leakage. Greater outdiffusion will, at some point, completely destroy the functionality of the circuit. The exposure of integrated circuits to heat is analogous in two respects to the exposure of living organisms to ionizing radiation. Not only is exposure cumulative, but at some exposure level, the organism will die. Each integrated circuit device has an optimum total thermal exposure limit that is generally referred to as the circuit""s thermal budget. The budget is calculated by summing all exposures with regard to both time and temperature. Although time is a linear function, the diffusion rate does not increase in direct proportion to the increase in temperature. Instead, the diffusion rate increases geometrically with increasing temperature.
As device geometries are shrunk for new generations of integrated circuits, thermal budgets must be lowered by a corresponding amount. Unless the process is modified to reflect these reduced thermal budgets, it will become increasingly difficult to stay within those budgets. It is a given that final anneals in a hydrogen-containing ambiance for extended periods will become increasingly problematic in terms of thermal budget considerations as device dimensions shrink.
Silicon nitride films are used extensively as moisture barriers in integrated circuits. Silicon nitride films are also nearly impermeable to diatomic hydrogen molecules at standard temperature and pressure. This has not posed a problem until fairly recently, as the silicon nitride film is deposited as the final layer in the fabrication process. The hydrogen anneal step is performed prior to depositing the final silicon nitride film. However, silicon nitride films are also being used with increasing frequency for sidewall spacers and capping layers on field-effect transistor gates when self-aligned contact etches are required for sub-half-micron geometries. These silicon nitride gate structures must be fabricated long before the hydrogen anneal step. The presence of these silicon nitride structures greatly hampers the diffusion of hydrogen molecules into silicon dioxide layers associated with the transistor gates.
Therefore, a new final thermal anneal process is needed that is more compatible with the limited thermal budgets and the silicon nitride gate structures associated with the new generations of integrated circuits.
This invention embodies an improved process for annealing integrated circuits to repair fabrication-induced damage. An integrated circuit is annealed in a pressurized, sealed chamber in which a forming gas containing hydrogen is present. Pressurization of the chamber reduces the contribution made by this final anneal step to total thermal exposure by increasing the diffusion rate of the hydrogen into the materials from which the integrated circuit is fabricated. Ideally, the forming gas contains, in addition to hydrogen, at least one other gas such as nitrogen or argon that will not react with hydrogen and, thus, reduces the danger of explosion. However, the integrated circuit may be annealed in an ambiance containing only hydrogen gas that is maintained at a pressure greater than the prevailing ambient atmospheric pressure. Typical anneal temperatures are within a range of 300 to 500xc2x0 C. for circuits having aluminum metallization. Higher upper temperature limits and shorter anneal times may be used for circuits employing metallization materials with higher melting points than that of aluminum. Although the efficiency of anneal operations improves by increasing the annealing pressure above one atmosphere, optimum annealing pressures are deemed to be in the 10 to 25 atmosphere range.

High-pressure anneal process for integrated circuits