Audio Signal Processing and Coding

5.3 CRITICAL BANDS

Using the absolute threshold of hearing to shape the coding distortion spectrum represents the first step towards perceptual coding. Considered on its own, however, the absolute threshold is of limited value in coding. The detection threshold for spectrally complex quantization noise is a modified version of the absolute threshold, with its shape determined by the stimuli present at any given time. Since stimuli are in general time-varying, the detection threshold is also a time-varying function of the input signal. In order to estimate this threshold, we must first understand how the ear performs spectral analysis. A frequency-to-place transformation takes place in the cochlea (inner ear), along the basilar membrane [Zwic90].

The transformation works as follows. A sound wave generated by an acoustic stimulus moves the eardrum and the attached ossicular bones, which in turn transfer the mechanical vibrations to the cochlea, a spiral-shaped, fluid-filled structure that contains the coiled basilar membrane. Once excited by mechanical vibrations at its oval window (the input), the cochlear structure induces traveling waves along the length of the basilar membrane. Neural receptors are connected along the length of the basilar membrane. The traveling waves generate peak responses at frequency-specific membrane positions, and therefore different neural receptors are effectively “tuned” to different frequency bands according to their locations. For sinusoidal stimuli, ...