| 摘要 |
1402119 Voice frequency receivers; automatic exchange systems SIEMENS AG 13 Sept 1972 [14 Sept 1971 11 Nov 1971] 42459/72 Headings H4R and H4K A tone receiver includes a speech protection and tone frequency determining circuit which counts clock pulses during two consecutive half-cycles of a tone and only verifies the tone if the counts correspond within limits, the count for the first half-cycle or the complete cycle being used to indicate the tone frequency. The application is to two tone receivers, e.g. for push button dialling signals and one such circuit may be provided for each frequency band (Fig. 1, not shown) or alternatively one circuit is shared between the two frequency bands and is used consecutively therefor via a changeover switch (Fig. 4, not shown). In one circuit a forward counting counter RZ1-RZ4, Fig. 2, is used which is driven by clock pulses. Gates M1 and M2 define a range of counts in which the negative transistion should occur for a particular frequency and gates E1 and E2 define the complete cycle count longer for the same frequency, a signal being produced at output Q of bi-stable device FF4 if the half and fiull wave transitions occur in the correct count ranges. In practice a plurality of circuits DS are provided in conjunction with the counter, one for each possible frequency to be detected by the circuit. A second circuit uses a backwards and forwards counter, Fig. 3, and pairs of gates U1 O1 ... U4, 04 are used to define the half-cycle count ranges for a respective frequency to be detected. The speech protection is afforded by gates G1, G2 which define counts 125 above and below zero respectively. The counter is triggered by a positive going transition and for a valid frequency the count G1 should be passed through twice without the count G2 being passed. Fig. 5 (not shown), illustrates the modifications by way of additions to the arrangement of Fig. 3, where the same circuit is to be used consecutively for both frequency bands and assuming a 2 out of 8 code. Essentially two sets of bi-stable devices HSB are provided, and one set is enabled for each position of a changeover switch which couples outputs of one of the two clipped band passed waveforms to the circuit. |
