Vocoder Filter Design: Practical Considerations

Abstract

Successful digital computer simulation of a spectrum channel vocoder, [R. M. Golden, J. Acoust. Soc. Am. 35, 805(A) (1963) has also led to new filter design procedures for vocoder bandpass filters. These procedures yield designs that result in improved performance of the spectrum channel vocoder. The procedures are accomlished by computer aided design programs. The programs permit the examination and modification of the time and frequency response characteristics of the various bandpass and low-pass filters required by a spectrum channel vocoder. Such examination readily shows why the Bessel (or Linear Phase) designs are to be preferred over filter designs such as Butter-worth and Elliptic. In addition to choice of prototype filter design, consideration must also be given to filter resolving power (i.e., passband and stop-band characteristics) and to the frequency cross-over point of contiguous filters. This latter consideration accounts for the fact that performance of some vocoders has been improved by the simple device of shifting the phase of alternate channels by 180°. For many vocoder configurations, equalization of filter group delay characteristics is necessary between various channels. This factor is important for those vocoder designs using increasing bandwidth filters for the higher frequency channels. Equal bandwidth filters combined in pairs or triplets achieve both equalization and the desired increase in bandwidth.