| 摘要 |
A signalling tone detector for use in a communications network carrying voice traffic incorporating single frequency and dual frequency signalling tones comprises a first output path, a second output path, and a switch arranged to selectively couple an input signal to one or other of the output paths. The first output path comprises an automatic frequency control circuit for determining a mean frequency of an input signal, comparison means for comparing the mean frequency with stored frequency values corresponding to single tone signalling frequencies and mean values of pairs of dual tone signalling frequencies, a first discriminator for determining the presence of either a single frequency or a pair of frequencies, and a second discriminator for providing a signal output indicative of the presence of a single frequency or pair of frequencies. The second output path comprises a phase locked loop arranged to respond in frequency and phase to modem signalling tones and to detect phase reversals in a those tones. The detector is used for the selective disablement of an echo canceller in the presence of signalling tones.
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