GB921663A - Improvements in or relating to voice-frequency line signalling systems - Google Patents

Abstract

921,663. Voice frequency; frequency-shift signalling. STANDARD TELEPHONES & CABLES Ltd. June 23, 1961 [June 24, 1960], No. 22222/60. Class 40 (3). In a voice frequency line signalling system information is passed between terminal equipments by frequency shift signals. The line attenuates the signals to differing degrees dependent on their frequency, and means is provided at at least one of the terminal equipments for attenuating, relative to signals for which the line attenuation is relatively high, signals for which the line attenuation is relatively low. The system described includes error detection by means of redundancy check and error correction by repetition. During normal operation, information passes to a unit 11 which produces frequency shift signals. These signals are adjusted in power level by unit 12, such that signals suffering higher loss on the line are sent at a higher level, and are fed to line via a high pass filter. At the receiving station the signals pass through filter 15 to unit 25, where their levels are further adjusted, and to unit 16 where they are converted to binary code prior to processing in unit 17. Unit 17 derives the redundancy of the received signals which is sent to the transmitter by frequency shift, but using frequencies far removed from those of the forward direction signals. The received redundancy signals pass via low-pass filter 21 to redundancy check unit 24. This unit compares the received signal with the corresponding transmitted signal, stored in unit 23, and in the event of non-equality changes switch 10 so that the stored signals are repeated. Signals in the forward direction are sent at a much faster rate than those simultaneously sent in the backward direction, and hence the level of the forward signals can be higher than that of the backward signals without exceeding the maximum permitted power in the channel.