Digitized Voice-Excited Vocoder for Telephone-Quality Inputs, Using Bandpass Sampling of the Baseband Signal
Bernard Gold, J. Tierney · The Journal of the Acoustical Society of America · 1965
A voice-excited vocoder transmits a portion of the baseband signal to serve as the synthesizer excitation signal. When the input speech comes from a carbon-button transmitter via a telephone line, the transmitted baseband must begin above 300 cps. Thus, a low pass sampling scheme for baseband digitization is inefficient. A particular bandpass sampling procedure that is applicable to a digitized VEV is described in this note. The baseband signal is sampled by two pulse trains each of frequency equal to the center frequency of the signal spectrum but displaced from each other by one-quarter of the sampling period. To recover the signal, both sample trains are applied to a desampling filter identical to the original baseband filter, This technique makes possible the specification of a 9600 bit ser VEV for ordinary telephone speech. Testing was done on a laboratory vocoder together with a general purpose computer simulating a real-time sampler-quantizer, Six bit quantization of the bandpassed samples was shown necessary to produce synthetic speech that was samples was shown necessary to produce synthetic speech that was compatable in quality to that produced by the VEV in the analog mode.