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A TELEPHONE EQUALIZATION SENSING CIRCUIT

IP.com Disclosure Number: IPCOM000005564D
Original Publication Date: 1985-Oct-01
Included in the Prior Art Database: 2001-Oct-16
Document File: 2 page(s) / 98K

Publishing Venue

Motorola

Related People

W. David Pace: AUTHOR

Abstract

Automatic gain control of telephone speech circuits as a function of the distance from the telephone to the central switching network is known as loop length equalization. Varying speech circuit gains with loop length is intended to offset signal losses within the telephone cable and "equalize" transmission levels at each and. A pro- perly designed telephone will operate on a wide range of loop lengths with only minor variations in loudness.

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MOlVROLA Technical Developments Volume 5 October 1985

A TELEPHONE EQUALIZATION SENSING CIRCUIT

by W. David Pace

   Automatic gain control of telephone speech circuits as a function of the distance from the telephone to the central switching network is known as loop length equalization. Varying speech circuit gains with loop length is intended to offset signal losses within the telephone cable and "equalize" transmission levels at each and. A pro- perly designed telephone will operate on a wide range of loop lengths with only minor variations in loudness.

   A key element in any equalization circuit in the sensing by which the telephone circuit determines the loop length. Typical telephone loops are supplied from a private or public exchange which sources 48V dc through 400 ohm source resistance. The cable from the exchange to the telephone adds 82 ohm per kilofoot (#26 AWG) to the source resistance of the loop. The loop length is determined from voltage or currant measurements within the telephone. Traditionally, loop length has bean estimated from loop current by sensing the voltage across a known, low value resistor.

   Two sources of error plague the traditional sensing means in more complex feature telephones. These telephones have peripheral circuits operating in parallel with the speech circuit which also draw dc currant from the loop. Current diverted from the speech circuit to power these peripherals does not flow through the aqualiza- tion sensing resistor. The result is an error in the speech circuit equalization.

   A second error in the traditional sensing means results from variations in the terminal voltage of the telephone. The terminal voltage often must be increased to accommodate dialing or speakerphone functions. When the tar- minal voltage increases, the currant fed from the 48V battery through the loop resistance decreases. Again the traditional equalization circuit will error in estimating the loop length. A new equalization sensing means will be described which monitors both loop current and loop voltage to reduce these errors.

Simple calculation of the loop dc resistance R, will yield the following equation:

R, =

v, - VT...