Amplitude modulation spectrogram (
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
’log’. To illustrate the difference between linear
linearly-scaled and logarithmically-scaled modulation filters, the corresponding
filter bank responses are shown in Fig. 33. 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. 33.
Fig. 33 Transfer functions of 15 linearly-scaled (left panel) and 9
logarithmically-scaled (right panel) modulation filters.
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'
requested number of filters
ams_nFilter will be linearly-spaced
omitted, the number of filters will be set to 15 by default.
ams_fbType = 'log'
- Directly define a vector of centre frequencies, e.g.
ams_cfHz = [4 8 16
...]. In this case, the parameters
ams_nFilter are ignored.
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.
requested number of filters
ams_nFilter will be spaced logarithmically
as power of two between
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
ams_dsRatio larger than 1. A full list of AMS feature
parameters is shown in Table 31.
Table 31 List of parameters related to
|Filter bank type (
|Number of modulation filters (integer)
|Lowest modulation filter centre frequency in Hz
|Highest modulation filter centre frequency in Hz
|Vector of modulation filter centre frequencies in Hz
|Downsampling ratio of the IHC representation
|Window duration in s
|Window step size in s
The functionality of the AMS feature processor is demonstrated by the script
DEMO_AMS and the corresponding four plots are presented in
Fig. 34. 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.
Fig. 34 Speech signal (top left panel) and the corresponding IHC representation
(top right panel) using 23 frequency channels spaced between 80 and 8000 Hz.
Linear AMS features (bottom left panel) and logarithmic AMS features
(bottom right panel). The response of the modulation filters are stacked on
top of each other for each IHC frequency channel, and each frequency
channel is visually separated by a horizontal black line. The individual
frequency channels, ranging from 1 to 23, are labels at the left hand side.
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