Half duplex integral vocoder modem system

Abstract

Disclosed is a system whereby in a transmit mode analog speech is sampled d converted digitally to 12 bits of accuracy and then fed into a fast Fourier transform (FFT) processor which analyzes the speech into spectral and pitch parameters. These parameters are then quantized into a data stream which acts as an input to a differential phase shift keying modulator. The modulator constructs a multi-tone modem output signal from a 25 tone stack which is converted to an analog signal which is fed into a communications channel. In the receive mode a modem input signal from the communications channel is fed through the same analog to digital converter which was used for input speech. A data stream is now provided which is coupled back into the same digital FFT processor which now operates to provide pitch spectral coefficients which are then separated and used by a synthesizer to reconstruct a speech waveform. The speech waveform is applied to a digital analog converter which is the same converter used to produce the line signal when the processor operates in the transmit mode. The FFT processor implements a single FFT algorithm which is used for both vocoder and modem processing in both the transmit and receive modes.