The channel vocoder

The channel vocoder is described. This device achieves bandwidth compression greater than that of the bandpass compressor but less than that of the formant vocoder. To date it has been much more widely used than any other kind of vocoder. The channel vocoder exploits the insensitivity of the aural mechanism to phase, and only attempts to reproduce the short time power spectrum of the speech waveform. The spectral envelope of the speech is measured with a bank of filters and ascribed wholly to the vocal tract filter, while the excitation is estimated to be either a quasi-periodic pulse train, or noise. There are several methods of combining these extracted parameters to reconstruct the speech. Several configurations of the channel vocoder are described and the factors which affect the specification of design parameters for channel vocoders are considered.