Abstract :
Modern communications systems are increasingly using digital transmission, at various digit rates, for the many advantages thereby obtained. there are many methods that can be used for converting analogue speech signals to digital form. the aim is to produce very good speech quality using simple equipment at a low digit rate, but any application inevitably involves choosing the appropriate compromise between these three conflicting attributes. the paper reviews the most important of currently known methods, including systems already in widespread use and those still in the research stage. thel problem of assessing speech transmission performance is also discussed.
