Gradient Encoding for Low-Bit-Rate Stored Speech Applications
S. V. Ahamed · Bell System Technical Journal · 1978
In stored speech applications, the waveform of the message is completely specified and can be effectively used to reduce the bit rate at which the message may be synthesized. In gradient encoding, we propose to match the gradient of the output wave of the differential decoder with the required gradient between discrete clock cycles. When the required gradient is very steep the bit pattern selected maximizes the rate of change of the decoder voltage, otherwise appropriate bits of opposite polarity are inserted to match the amplitude of the decoder voltage with the required voltage at the discrete clock cycles. The performances of gradient encoding and conventional encoding are presented as corresponding signal-to-noise ratios under different inputs and circuit conditions. Further, our preliminary results indicate that gradient encoding can lead to comparable quality of speech at about half the bit rate of the conventional encoding between 32 to 24 kbaud.