 waveform is encoded independently signal. In PCM, each sample is quantised to one of 2 amplitude of all the other samples. However, most source signals sampled at levels, where b is the number of binary digits used to represent each or above the Nyquist rate exhibit significant correlation between sample. The rate from the source is therefore bf bits per second, successive samples, i.e. the average change in amplitude between where is the sampling frequency. successive samples is relatively small. An encoding scheme that The quantisation may be represented as in Equation 4.76, where exploits the redundancy in the samples will result in a lower bit rate X represents the quantised value of and 4 represents the quanti- for the source output, and one such system called Differential pulse sation error which is treated as additive noise. code modulation (DPCM) does this by encoding the differences between successive samples rather than the samples themselves. (4.76) A sub-class of DPCM is delta modulation in which the code word is only one binary digit in length. Delta modulators have the advant- If an uniform quantiser is used, the quantisation noise is well age over more conventional PCM systems of having a particularly represented by an uniform probability density function, and the simple architecture, which forms the basis of 'single-chip' coder- mean square value of the quantisation error can be shown to be as decoder (CODEC) integrated circuits. Because delta modulation is in Equation 4.77, where A=2-b = is the step size of the quantiser. a 1-digit code, sampling rate and output digit rate are the same. However, in order for delta modulation to achieve performance (4.77) comparable to that of conventional n-digit PCM systems, the samp- ling rate must be increased substantially over that required for a PCM system. Most real signal sources are nonstationary in nature, which means Measured in decibels, the mean square value of the noise is as in that the variance and the autocorrelation function of the source Equat