Secure digital speech communication

This invention relates to a secure digital speech communication system, and more particularly to waveform coding with digital samples. A novel form of waveform coding is used, designing a Critical Point Coder (CPC). It operates by transmitting only those points in a waveform which are significant for defining its overall structure. The receiver reconstructs the missing points in the waveform using some sort of approximating interpolation. The CPC uses a new approach to waveform coding. Rather than transmitting information about each sample as other waveform coders, the CPC transmits information for only samples deemed critical to high quality reconstruction. The four major functions performed by the CPC are (1) detecting which samples are critical, called critical points: (2) quantizing the vectors which are formed between successive critical points: (3) dealing with the non-uniform bit rate since critical points do not occur regularly: and (4) reconstructing the waveform between the endpoints of the quantized vectors.