Study of Vocoder Filters by Computer Simulation
Charles M. Rader · The Journal of the Acoustical Society of America · 1964
A number of vocoders, identical except for the bandpass filters employed, were simulated on a digital computer. Each vocoder used a two-bank synthesizer and was excited by undistorted 3.3-kc/sec speech. Thus, the system provides no compression, but the effect of the filter banks is not masked by the effects of improper excitation. By listening to A-B comparisons of vocoder outputs with various filter banks, it was possible to compare the effects of different types of bandpass filters on speech quality. In general, we have found that a vocoder filter should have an impulse response whose envelope resembles a short, single-lobed pulse. A well-known filter that satisfies this requirement is the Bessel filter. We have also verified that the amplitude-frequency responses of adjacent filters in a bank should intersect at 3 dB of attenuation. Attention was concentrated on 3-pole filters of the Tchebyshev and Butterworth-Thompson types, including the Bessel filter. We briefly studied 1-pole filters, 4-pole Bessel filters, and Lerner filters. The filter bandwidths were uniformly 120 cps, with 26 filters covering the 3.3-kc/sec band.