Speech-level control in telephone instruments

The range of conditions to be catered for in the design of a general-purpose telephone instrument is discussed to show that the human, environmental and telephone-system variables are such that no one instrument can give complete satisfaction over the whole range. A system of speech-level control is described whereby instruments may be developed which have greater usefulness in some of the more extreme conditions. Experimental circuits which offer improved sidetone performance, high gain and noise rejection are shown.