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Tone Detector

IP.com Disclosure Number: IPCOM000085592D
Original Publication Date: 1976-May-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 67K

Publishing Venue


Related People

Davis, MD: AUTHOR [+2]


Tones generated by a push-button telephone are detected and tested to insure that they are valid tones and not simulated signals produced by audio.

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Tone Detector

Tones generated by a push-button telephone are detected and tested to insure that they are valid tones and not simulated signals produced by audio.

The generated tones are comprised of two frequencies. One is low band and the other is high band. Elimination filters separate the tone signals into the two frequency components and each frequency is detected separately. Each frequency component is then passed through a zero-crossing detector and the resulting square wave has an amplitude that is referenced to a reference voltage. This square wave drives a set of detector circuits.

Each detector circuit is comprised of a level detector and a bandpass filter having a resonant frequency equal to one of the tone frequencies. If the frequency of the square wave is within the detection bandwidth of one of the detectors, a train of pulses will appear at the output of that detector. The output of the detectors is input to decision logic. A valid tone signal must be present for a predetermined time before the decision logic will recognize it as valid.

From the drawing, a high-band elimination filter 1 and a low-band elimination filter 3 are made up of conventional series and parallel LC resonant circuits that provide attenuation at undesirable frequencies. A zero-crossing detector and driver 2 has an operational amplifier OA-1 used as a zero-crossing detector, limiter, and inverter. Operational amplifier 0A-2 is used as a buffer to provide a low-impedance source to drive detector circuits 5 and 6. When the input of OA-1 is positive, the output is saturated at the negative supply voltage, and Q1 is off. The output of 0A-2 is equal to the reference voltage V ref. When the input of 0A- 1 is negative, Q1 is on and saturated, and the output is equal to the saturation voltage of Q1.

Detector circuits 5-9 are made up of an RCL bandpass filter and a level detector. The RCL bandpass filter is tuned to the frequency to be detected. The reference voltage V ref applied to the driver circuit 2 is also applied to the detector circuits 5-9 to bias the threshold of the level detector. Therefore, the overall stability and accuracy of the detection is improved. Potentiometer P1 is used to adjust the bandwidth of detection by changing the threshold of the level detector. Resistors R4 and R5 provide...