GB782650A - Improvements in or relating to receivers for electric carrier communication systems - Google Patents

Abstract

782,650. Code telegraphy. STANDARD TELEPHONES & CABLES, Ltd. March 29, 1956, No. 9941/56. Class 40 (3). [Also in Group XXXIX] A receiver for telegraph type signals, e.g. telephone supervisory signals, which are transmitted by amplitude modulation of a carrier current and have a pilot wave transmitted therewith, comprises a two-condition device which is controlled by the mark-to-space and space-to-mark transitions of the rectified signal wave and an arrangement including rectifiers whereby a bias potential is derived from the pilot wave for holding the two-condition device in a specified one of the two conditions when no signals are being received, the bias being arranged not to affect the times at which the device changes its condition when signals are received. The primary winding of transformer 14, Fig. 2, is tuned to the signalling frequency by a condenser 15 and the accepted bandwidth is determined by a resistor 16. Pre-amplification is effected in a tube 17, and a differential signalling relay 57 in the anode of a tube 18 is actuated by the rectified signalling current after reflex amplification in valve 18. The A.C. signal pulses amplified in valve 18 are rectified in 37 after a time delay determined by a signal filter comprising elements 27 and 32-35, and superimposed on a bias wave which comprises the standing bias developed by the H.T. supply across resistor 49 and a varying bias across a resistor 45 derived from the signal through rectifiers 40, 41. The latter varying bias the form of which is shown in Fig. 3D ensures actuation of the signalling relay at times corresponding to the mark-to-space and spaceto-mark transitions of the original signal in the manner described in Specification 693,769. Potentiometer 38 allows the obtention of the correct amplitude relationship between the rectified signal and the bias. The rectified pilot current is applied to resistor 45 through a rectifier 56 which is blocked when the potential on line 46 due to the signal rectified by 40, 41 exceeds the pilot voltage. The composite D.C. signal returned to the grid of valve 18 is modified by a variable cathode resistor 31 which provides negative feedback and limited as regards negative excursions by a rectifier 30. In the absence of a signal tone the pilot voltage determines the current flow through the valve and holds the relay in the specified condition. A multi channel telephone system in which a single pilot serves a number of signal receivers is described with reference to Fig. 1 (not shown) and the application of the invention to a voice frequency telephone system is also suggested.