A system whereby speech signals and data can be transmitted simultaneously using a code-breaking technique on the 64kb/s PCM voice channel is proposed. Speech signals, sampled at 8 kHz, are divided into blocks of eight samples; and provided that the square value of the samples exceeds a system threshold, they can be used reliably as a carrier for data transmission. The logical values of the data and the correlation coefficient of each speech segment determine whether frequency inversion scrambling of the samples is employed. The receiver performs the inverse process to recover both the speech and data. Speech distortion is almost unavoidable if data are to be transmitted without error. A simple algorithm has been designed which enables the sample values within each block to jitter according to various explicitly defined rules so as to reduce speech degradation to a minimum. Computer simulations have been performed and data rates of 1000 b/s were achieved without any error on an ideal channel while still m...