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Browse Prior Art Database

Dial Pulse Detector

IP.com Disclosure Number: IPCOM000084020D
Original Publication Date: 1975-Sep-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 37K

Publishing Venue

IBM

Related People

Bringol, CR: AUTHOR [+3]

Abstract

Dial pulse detection is improved with a system having both an analog section made up of a filter and threshold detector, and a digital section made up of a correlator and audio immunity logic.

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Dial Pulse Detector

Dial pulse detection is improved with a system having both an analog section made up of a filter and threshold detector, and a digital section made up of a correlator and audio immunity logic.

The filter serves both to improve the signal-to-noise ratio and the signal-to- audio ratio. Typical dial pulse signals input from telephone line interface circuits are shown in Fig. 1. The leading edge of each pulse is a sharp transition which is comprised of relatively high frequencies. Following these transitions are damped ringing made up of much lower frequencies. On some long distance lines this ringing virtually bridges together the dial pulses and renders proper detection almost impossible.

The input from the telephone line interface circuit is applied to the filter in Fig.
3. This filter has a high-pass characteristic with a break frequency at approximately 3KHz. This characteristic has the effect of attenuating the low- frequency ringing of the dial pulse, while passing the initial sharp transition with little attenuation.

The resultant is shown in Fig. 2, and is a discrete voltage spike with little ringing and having a much improved signal-to-noise ratio over the unfiltered signal shown in Fig. 1. Further, 60-120 Hz hum, common in telephone lines, is greatly reduced.

Since long distance dial pulse detection requires high gain circuitry, audio has the potential of exceeding the detector threshold and appearing as dial pulse detections. The high-pass filter, which attenuates signals below 3KHz, reduces the amount of audio reaching the detector, and thus improves the signal-to-audio ratio. The output of the filter is applied to the threshold detector as shown in Fig.
3.

The detector output is monitored by digital circuitry for input to a serial shift register. A detector output occurring during a given clock period is stored until the next clock period, at which time it is clocked into the shift register. A logical 1 depicts that...