Method and system for detecting at a selected station an alerting signal

Abstract

A method and apparatus for detecting a dual tone alerting signal in the presence of speech for the purpose of initiating transfer of special service information on a telephone line is described. The apparatus includes two channels for separately detecting each tone. Each channel includes means for comparing the signaling energy around one of the tones with the energy in a selected weighted guard band portion of the voiceband. Based on this comparison, a determination is made whether a tone is or is not present. A pulse signal formed from the coincidences of tone detection in each channel is used by timing circuitry to determine whether the dual tone alerting signal is actually present or whether speech energy caused both tones to be erroneously detected (talkoff). Characteristics of this pulse signal are analyzed to determine whether an alerting signal is present. An alerting signal is detected if parameters of these characteristics fall within determined ranges, which ranges are continuously updated based on the parameters of these signal characteristics determined from previously detected alerting signals. If, however, the signal characteristic parameters of a present potential alerting signal match those of a previously determined talkoff signal, a talkoff is presumed even when these parameters fall within ranges in which an alerting signal would otherwise be detected.