Results of investigation of adaptive speech codes

A search for ways of increasing the effectiveness of speech signals in digital form lead to the appearance of various methods of encoding, to reduce the excessiveness of specific properties of the speech signal. It is customary to divide speech codes into two large classes: codes of signal parameters (or vocoders), and codes of the signal form (CSF. In telephony, preference is given to a second class of systems, which maintains naturalness of sound. The class of CSF expanded considerably because of the development of codes based on the frequency representation of a signal. The greatest interest is given to such methods of encoding as pulse modulation (PCM), differential PCM (DPCM), and delta modulation (DM). However, developers of digital transmission systems find it difficult to compile a complete pattern of the applicability of specific types of codes. The best known versions of the codes are evaluated by means of subjective-statistical measurements of their characteristics. The results obtained help developers to draw conclusions regarding the applicability of the codes considered in various communication systems.