# Amplitude modulation spectrogram (modulationProc.m)¶

The detection of envelope fluctuations is a very fundamental ability of the human auditory system which plays a major role in speech perception. Consequently, computational models have tried to exploit speech- and noise specific characteristics of amplitude modulations by extracting so-called amplitude modulation spectrogram (AMS)features with linearly-scaled modulation filters [Kollmeier1994], [Tchorz2003], [Kim2009], [May2013a], [May2014a], [May2014b]. The use of linearly-scaled modulation filters is, however, not consistent with psychoacoustic data on modulation detection and masking in humans [Bacon1989], [Houtgast1989], [Dau1997a], [Dau1997b], [Ewert2000]. As demonstrated by [Ewert2000], the processing of envelope fluctuations can be described effectively by a second-order band-pass filter bank with logarithmically-spaced centre frequencies. Moreover, it has been shown that an AMS feature representation based on an auditory-inspired modulation filter bank with logarithmically-scaled modulation filters substantially improved the performance of computational speech segregation in the presence of stationary and fluctuating interferers [May2014c]. In addition, such a processing based on auditory-inspired modulation filters has recently also been successful in speech intelligibility prediction studies [Joergensen2011], [Joergensen2013]. To investigate the contribution of both AMS feature representations, the amplitude modulation processor can be used to extract linearly- and logarithmically-scaled AMS features. Therefore, each frequency channel of the IHC representation is analysed by a bank of modulation filters. The type of modulation filters can be controlled by setting the parameter ams_fbType to either ’lin’ or ’log’. To illustrate the difference between linear linearly-scaled and logarithmically-scaled modulation filters, the corresponding filter bank responses are shown in Fig. 34. The linear modulation filter bank is implemented in the frequency domain, whereas the logarithmically-scaled filter bank is realised by a band of second-order IIR Butterworth filters with a constant-Q factor of 1. The modulation filter with the lowest centre frequency is always implemented as a low-pass filter, as illustrated in the right panel of Fig. 34.

Similarly to the gammatone processor described in Gammatone (gammatoneProc.m), there are different ways to control the centre frequencies of the individual modulation filters, which depend on the type of modulation filters

• ams_fbType = 'lin'
1. Specify ams_lowFreqHz, ams_highFreqHz and ams_nFilter. The requested number of filters ams_nFilter will be linearly-spaced between ams_lowFreqHz and ams_highFreqHz. If ams_nFilter is omitted, the number of filters will be set to 15 by default.
• ams_fbType = 'log'
1. Directly define a vector of centre frequencies, e.g. ams_cfHz = [4 8 16 ...]. In this case, the parameters ams_lowFreqHz, ams_highFreqHz, and ams_nFilter are ignored.
2. Specify ams_lowFreqHz and ams_highFreqHz. Starting at ams_lowFreqHz, the centre frequencies will be logarithmically-spaced at integer powers of two, e.g. 2^2, 2^3, 2^4 … until the higher frequency limit ams_highFreqHz is reached.
3. Specify ams_lowFreqHz, ams_highFreqHz and ams_nFilter. The requested number of filters ams_nFilter will be spaced logarithmically as power of two between ams_lowFreqHz and ams_highFreqHz.

The temporal resolution at which the AMS features are computed is specified by the window size ams_wSizeSec and the step size ams_hSizeSec. The window size is an important parameter, because it determines how many periods of the lowest modulation frequencies can be resolved within one individual time frame. Moreover, the window shape can be adjusted by ams_wname. Finally, the IHC representation can be downsampled prior to modulation analysis by selecting a downsampling ratio ams_dsRatio larger than 1. A full list of AMS feature parameters is shown in Table 31.

Table 31 List of parameters related to 'ams_features'.
Parameter Default Description
ams_fbType 'log' Filter bank type ('lin' or 'log')
ams_nFilter [] Number of modulation filters (integer)
ams_lowFreqHz 4 Lowest modulation filter centre frequency in Hz
ams_highFreqHz 1024 Highest modulation filter centre frequency in Hz
ams_cfHz [] Vector of modulation filter centre frequencies in Hz
ams_dsRatio 4 Downsampling ratio of the IHC representation
ams_wSizeSec 32E-3 Window duration in s
ams_hSizeSec 16E-3 Window step size in s
ams_wname 'rectwin' Window name

The functionality of the AMS feature processor is demonstrated by the script DEMO_AMS and the corresponding four plots are presented in Fig. 35. The time domain speech signal (top left panel) is transformed into a IHC representation (top right panel) using 23 frequency channels spaced between 80 and 8000 Hz. The linear and the logarithmic AMS feature representations are shown in the bottom panels. The response of the modulation filters are stacked on top of each other for each IHC frequency channel, such that the AMS feature representations can be read like spectrograms. It can be seen that the linear AMS feature representation is more noisy in comparison to the logarithmically-scaled AMS features. Moreover, the logarithmically-scaled modulation pattern shows a much higher correlation with the activity reflected in the IHC representation.

 [Bacon1989] Bacon, S. P. and Grantham, D. W. (1989), “Modulation masking: Effects of modulation frequency, depths, and phase,” Journal of the Acoustical Society of America 85(6), pp. 2575–2580.
 [Dau1997b] Dau, T., Püschel, D., and Kohlrausch, A. (1997b), “Modeling auditory processing of amplitude modulation. II. Spectral and temporal integration,” Journal of the Acoustical Society of America 102(5), pp. 2906–2919.
 [Ewert2000] (1, 2) Ewert, S. D. and Dau, T. (2000), “Characterizing frequency selectivity for envelope fluctuations,” Journal of the Acoustical Society of America 108(3), pp. 1181–1196.
 [Houtgast1989] Houtgast, T. (1989), “Frequency selectivity in amplitude-modulation detection,” Journal of the Acoustical Society of America 85(4), pp. 1676–1680.
 [Joergensen2013] Jørgensen, S., Ewert, S. D., and Dau, T. (2013), “A multi-resolution envelope-power based model for speech intelligibility,” Journal of the Acoustical Society of America 134(1), pp. 1–11.
 [Kim2009] Kim, G., Lu, Y., Hu, Y., and Loizou, P. C. (2009), “An algorithm that improves speech intelligibility in noise for normal-hearing listeners,” Journal of the Acoustical Society of America 126(3), pp. 1486–1494.
 [Kollmeier1994] Kollmeier, B. and Koch, R. (1994), “Speech enhancement based on physiological and psychoacoustical models of modulation perception and binaural interaction,” Journal of the Acoustical Society of America 95(3), pp. 1593–1602.
 [May2013a] May, T. and Dau, T. (2013), “Environment-aware ideal binary mask estimation using monaural cues,” in IEEE Workshop on Applications of Signal Processing to Audio and Acoustics (WASPAA), pp. 1–4.
 [May2014a] May, T. and Dau, T. (2014), “Requirements for the evaluation of computational speech segregation systems,” Journal of the Acoustical Society of America 136(6), pp. EL398– EL404.
 [May2014b] May, T. and Gerkmann, T. (2014), “Generalization of supervised learning for binary mask estimation,” in International Workshop on Acoustic Signal Enhancement, Antibes, France.
 [May2014c] May, T. and Dau, T. (2014), “Computational speech segregation based on an auditory-inspired modulation analysis,” Journal of the Acoustical Society of America 136(6), pp. 3350-3359.
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