GB537966A - Improvements to carrier wave communication systems - Google Patents

Abstract

537,966. Carrier telephony. STANDARD TELEPHONES & CABLES, Ltd., and JACOBSEN, B. B. Jan. 13, 1940, No. 783. [Class 40 (iv)] In a system in which the communication currents are held at or below a given level, signalling currents are sent at a definite higher level for actuating a receiver which does not respond to currents of a lower level. The high-level signal may be merely a prefix, actual signalling currents being transmitted subsequently at a lower level. As shown, telephone currents from lines such as A1 are limited in level as at L and passed to a preferably balanced modulator 7, and modulated carrier currents are de-modulated as at DA and passed to a line such as B1. The transmitter S for auxiliary signals may be arranged to send at a level 3 or 4 db. above the maximum level permitted to the speech currents by apparatus L. Alternatively, the apparatus S may send only a prefix at this level, the effect of this prefix being to remove a loss pad at y to make the receiver more sensitive. At the termination of the prefix a loss-pad is inserted at x to reduce the level of the auxiliary signals and the speech line A1 is cut at q to prevent interference by speech currents. The speech line at the receiving end may be cut at r. Alternatively, the receiver may be connected as part of network N2 to make it inoperable by speech from B1. Signals received at R may be sent back to the sender for checking purposes. The signal impulses may also control the transmission equivalent of the channel concerned.