Audio Signal Processing and Coding

5.4 SIMULTANEOUS MASKING, MASKING ASYMMETRY, AND THE SPREAD OF MASKING

Masking refers to a process where one sound is rendered inaudible because of the presence of another sound. Simultaneous masking may occur whenever two or more stimuli are simultaneously presented to the auditory system. From a frequency-domain point of view, the relative shapes of the masker and maskee magnitude spectra determine to what extent the presence of certain spectral energy will mask the presence of other spectral energy. From a time-domain perspective, phase relationships between stimuli can also affect masking outcomes. A simplified explanation of the mechanism underlying simultaneous masking phenomena is that the presence of a strong noise or tone masker creates an excitation of sufficient strength on the basilar membrane at the critical band location to block effectively detection of a weaker signal. Although arbitrary audio spectra may contain complex simultaneous masking scenarios, for the purposes of shaping coding distortions it is convenient to distinguish between three types of simultaneous masking, namely noise-masking-tone (NMT) [Scha70], tone-masking-noise (TMN) [Hell72], and noise-masking-noise (NMN) [Hall98].

Table 5.1. Idealized critical band filter bank (after [Scha70]). Band edges and center frequencies for a collection of 25 critical bandwidth auditory filters that span the audio spectrum..

Figure 5.6. Equivalent rectangular bandwidth (ERB). (a) Example ERB for a roex(p) single-parameter ...