Voice-operated level control system for telephone networks

The operation of a telephone system is impaired by circuit noise. In the case of a radio telephone link the question arises of maintaining the signal/noise ratio for minimum transmitter carrier power, since an increase of the latter is usually unattainable. The signal for a given carrier power increases with the depth of modulation. It is always desirable to operate close to maximum modulation. The speech level of conversations on ordinary telephone networks may vary as much as 30 db. If the radio transmitter has been adjusted to avoid serious overloading on the high levels its percentage modulation may be 30 db below optimum on low speech values with a corresponding reduction in signal/noise ratio at the listening point. This loss of 30 db is avoided by a technical operator who increases the l.f. gain of the transmitter on low speech levels. If transmitter gain control is to be used it should be completely automatic. The fundamental requirement is that the voltage giving the automatic control shall be a function of the average peak voltages of the speech integrated over an interval of time. Circuit diagrams and the theory of the apparatus which has been evolved are given. Tests have been made on a 2-wire to 2-wire circuit between Belfast and Glasgow repeater stations with satisfactory results.