Parametric coding of speech spectra

We suggest that a class of speech coders can be designed from criteria that are perception-specific. Such a class represents a middle ground between ’’waveform’’ coders and speech-specific ’’source’’ coders. We argue that, within perceptually acceptable limits, a bandpass portion of a speech signal can be represented by its short-time spectral amplitude ‖ Sn ‖ and its phase derivative ?n. Both parameters are evaluated at the center frequency ωn of the passband whose spectral width is Wn rad/s. We show that the short-time spectral amplitude is identical to the Hilbert envelope An of the passband waveform. As a by-product, we show that the bandwidth occupied by both An2 and An2 ?n is the same as that for their parent bandpass signal, but their spectra are translated to the range 0 to Wn. No comparable statements can be adduced for An and ?n, but on practical grounds, we conjecture that useful low-pass limits of less than Wn/2 can be applied to both. Finally, we outline techniques to utilize the passband parameters An, ?n, An2 and An2 ?n in digital voice coding.