Tone detection circuitry

Claim 1

Tone detection circuitry for determining the receipt of a tone signal having a frequency within a selected one of a plurality of bandwidths including: timing resistance; timing capacitance; a tone decoder operatively connected to said timing resistance and said timing capacitance, said tone decoder providing a signal output at a free running frequency that is inversely proportional to the product of said timing capacitance and timing resistance and providing a signal output at the frequency of an input tone signal to said decoder when such input tone signal is within the response bandwidth of the decoder; and control means operatively connected to said tone decoder, said timing resistance and said timing capacitance, said control means providing a calibration routine which is operable subsequent to each energization of the control means for determining the product of said timing resistance and said timing capacitance for each of the plurality of bandwidths for establishing the free running frequency of said decoder as close as possible to the center frequency of each of the plurality of bandwidths, providing a retrievable record based on the product of the timing resistance and timing capacitance determined for establishing the free running frequency of said tone decoder for each of the plurality of bandwidths and providing for the selective retrieval of said retrievable record for any one of the plurality of bandwidths and establishing the product of said timing resistance and said timing capacitance in accordance with the retrieved record.