Secure digital speech communication

A novel form of waveform coding is used, designated 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 Critical Point Coder uses a new approach to waveform coding. Rather than transmitting information about each sample as other waveform coders, the Critical Point Coder transmits information for only samples deemed critical to high quality reconstruction. The four major functions performed by the Critical Point Coder are (1) detecting which samples are critical, called critical points e.g. those points in the waveform where it is changing direction most rapidly, generally the peaks and valleys; (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. For stabilizing the bit rate, stabilization intervals of fixed duration are divided into equal analysis intervals, with logic to select from three modes for quantizing and encoding the vectors during each stabilization interval, depending upon the stepsize.