| 摘要 |
1532103 Frequency detection NORTHERN TELECOM Ltd 23 Feb 1977 [3 March 1976] 07674/77 Heading G4H A multiple frequency signalling receiver comprises input means 1-2 for receiving voice frequency signals; means 2a, 2b for filtering the received signals into separate high and low bands; means, e.g. 18-19, for generating high band and low band threshold levels 16a, 16b in response to the amplitudes of signals in the high and low bands respectively; a high band detector 10a including a high band digital circuit 20 for detecting ones of predetermined frequencies of the signals in the high band and for indicating the frequency detected; first means 15 responsive to the low band threshold level 16b to transmit to the high band digital circuit only those signals in the high band which exceed the level 16b; a low band detector 10b including a low band digital circuit for detecting ones of predetermined frequencies of the signals in the low band and for indicating the frequency detected; second means responsive to the high band threshold level 16a to transmit to the low band digital circuit only those signals in the low band which exceed level 16a; and output means 6, 25, 28, &c. for indicating the frequencies detected in response to a simultaneous and constant indication of frequency detection from both detectors extended over a predetermined period of time. |
