Abstract A signal recognition circuit (10) is disclosed which is capable of distinguishing between the digit tones used for dialing in multifrequency tone telephone sets, and spurious signals at the same frequencies, such as noise and voice. The discriminator comprises, in cascade, an AGC amplifier (11), an adjustable threshold comparator (12), and a signal validation circuit (13). The operation of the discriminator is based upon the fact that the true dialing tones have envelopes of constant magntiude whereas the spurious signals tend to have pronounced envelope variations. By the appropriate adjustment of the comparator threshold, these variations show up as gaps in the comparator output signal, which are readily detected by the validation circuit.
A multifrequency tone receiver comprising: first and second signal paths; said first path including, in cascade, a first bandpass filter, an AGC amplifier, and a first signal comparator; said second path including, in cascade, a second bandpass filter, and a second signal comparator; means connecting the input ports of said first and second filters to a common signal path; and means connecting the output ports of said first and second signal comparators to a frequency detector and validation circuit.
Claim 2
Dependent Claim
79 words
A multifrequency tone receiver comprising: first and second signal paths; said first path including, in cascade, a first bandpass filter, an AGC amplifier, and a first signal comparator; said second path including, in cascade, a second bandpass filter, and a second signal comparator; means connecting the input ports of said first and second filters to a common signal path; and means connecting the output ports of said first and second signal comparators to a frequency detector and validation circuit.
Claim 3
Dependent Claim
24 words
The receiver according to claim 1 wherein said second path includes a second AGC amplifier connected between said second filter and said second comparator.
Claim 4
Dependent Claim
28 words
The receiver according to claim 2 wherein each AGC amplifier produces an output signal whose envelope exceeds a given level over a specified dynamic range of input terminals.
Claim 5
Dependent Claim
34 words
The receiver according to claim 1 wherein each comparator compares the signal applied thereto with a reference signal, and produces an output signal when the envelope of said applied signal exceeds said reference signal.
Claim 6
Dependent Claim
28 words
The receiver according to claim 1 wherein said AGC amplifier produces an output signal whose envelope exceeds a given level over a specified dynamic range of input signals.