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Vocoder Filter Design: Practical Considerations

1968; Acoustical Society of America; Volume: 43; Issue: 4 Linguagem: Inglês

10.1121/1.1910899

1520-9024

Roger M. Golden,

Acoustic Wave Phenomena Research

Design optimization procedures for vocoder bandpass filters have evolved as a result of successful digital computer simulations of voice-excited vocoders and spectrum channel vocoders. These procedures yield designs that result in improved performance of the channel vocoder. The procedures, implemented by computer programs, permit choosing the proper response characteristics for the various bandpass and low-pass filters required in a channel vocoder. Examination of response curves shows why the nearly critical-damping time characteristics of Bessel designs are preferred compared to the underdamped, although relatively constant, magnitude response of Butterworth and elliptic designs. In addition to impulse response, consideration is also given to: (a) Filter spectral-resolving power (i.e., passband and attenuation-band characteristics); and (b) filter magnitude-phase characteristics at the frequency crossover point of contiguous filters. For vocoder designs employing unequal bandwidth filters, channel characteristics may be preserved either by using higher-order filters for the wider frequency bands or by combining an appropriate number of narrow-band equal-bandwidth filters in parallel. Attention to magnitude-phase characteristics requires evaluation of resultant gain at different crossover frequencies when two filters are combined in parallel. This evaluation is performed for filters combined with and without phase inversion and leads to a choice of a proper bandwidth scale factor. Such evaluation also shows why vocoder performance can sometimes be improved by shifting the phase of alternate channels by 180°.