Browse Prior Art Database

Call Progress Tone Processor

IP.com Disclosure Number: IPCOM000039359D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Toma, SK: AUTHOR [+2]

Abstract

A simple, inexpensive, reliable circuit means is described for decoding U.S. call progress telephony tones, specifically dial, busy and ringback. Today there is an emphasis in the telecommunications industry on merging the communication capability of the public switched network with the digital computer. A key requirement is the ability to process call progress tones signalled to the telephone. In the U. S., these tones are specified in the EIA RS-464 standard.

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Call Progress Tone Processor

A simple, inexpensive, reliable circuit means is described for decoding U.S. call progress telephony tones, specifically dial, busy and ringback. Today there is an emphasis in the telecommunications industry on merging the communication capability of the public switched network with the digital computer. A key requirement is the ability to process call progress tones signalled to the telephone. In the U. S., these tones are specified in the EIA RS-464 standard. Each tone consists of the sum of two discrete frequencies, as shown below: Dial Tone: 350Hz + 440Hz

Busy Tone: 480Hz + 620Hz

Ringback: 440Hz + 480Hz The technique described for detection of these tones uses a simple two-bit decode with the following definition: Bit 0 Bit 1 Tone 0 0 No Tone

1 0 Dial Tone

0 1 Busy Tone

1 1 Ringback

(Image Omitted)

To accomplish this decode definition requires separation of the ringback frequencies (440Hz, 480Hz). The ability to separate and detect these frequencies accurately is the key feature of this technique. The input 1 to the call progress tone processor 2 contains energy in the voice band (300Hz - 3400Hz) consisting of call progress tone energy and noise. The amplitude of the tone varies depending on the length of the telephone line and the type of call in progress, i.e., local or long distance. Generally speaking, the tone will have a minimum amplitude around -33dbm (17mvrms). An Automatic Gain Control (AGC) with an approximate dynamic range from -33dbm through 0dbm is needed to raise the energy level to a constant, known amplitude. The energy is then processed by a standard band-split filter 4 typically used for dual tone multi-frequency detection. Such a filter is characterized by the use of switched capacitor filtering techniques which require a high frequency clock to determine the filter bands. A 3.579MHz colorburst crystal is used to separate the high and low frequencies used in DTMF tone generation. These filters consist of two separate bandpass filters yielding two outputs each, one digital and one analog. Our technique uses the analog outputs...