Study of Vocoder Filters by Computer Simulation
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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.