| 摘要 |
881,779. Voice-frequency signalling. TESLA, NARODNI PODNIK. June 26, 1959 [July 9, 1958], No. 22005/59. Class 40 (4). In a V.F. receiver having a circuit Z1, Fig. 1, tuned to receive the signal frequency and a blocking circuit Z2 tuned to reject that frequency, the latter is brought into circuit by the energization of a slow-release relay F which is dependent on the receiving relay P. A twofrequency receiver is also described, Fig. 2 (not shown). As applied to working between two exchanges A, B, Fig. 3, relays F at both ends of the junction are de-energized normally and also while a call is being set up, so that the blocking circuits Z2 are ineffective. When the called party at exchange B replies, a relay H (not shown) responds and sends a signal to connect up a similar relay at exchange A. Relays F energize at both exchanges so that the V.F. receivers are now protected against operation by speech which is unlikely to contain a pure signalling frequency of sufficient duration to release relay F. |
