Browse Prior Art Database

Method of Measuring and Compensating for Telephone Line Distortion

IP.com Disclosure Number: IPCOM000034468D
Original Publication Date: 1989-Feb-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Locke, ME: AUTHOR

Abstract

As users of voice store and forward equipment require more flexible transmission of messages, it has become necessary to transmit messages between remote equipment. For example, a corporation may have large offices in New York and Los Angeles and desire transmission of voice messages between these two sites. The amount of amplitude distortion (non-flat frequency response) on such lines is variable. Substantial roll-off of the high frequencies (greater than 2 kHz) can cause a serious reduction in intelligibility (in normal 2 way telecommunications, users can interact to make such line acceptable). Substantial rolloff of the low frequencies (less than 1 kHz) can cause user perceived message quality to suffer. A frequency response equalizer (or just equalizer) is generally used to correct for amplitude distortion.

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Method of Measuring and Compensating for Telephone Line Distortion

As users of voice store and forward equipment require more flexible transmission of messages, it has become necessary to transmit messages between remote equipment. For example, a corporation may have large offices in New York and Los Angeles and desire transmission of voice messages between these two sites. The amount of amplitude distortion (non-flat frequency response) on such lines is variable. Substantial roll-off of the high frequencies (greater than 2 kHz) can cause a serious reduction in intelligibility (in normal 2 way telecommunications, users can interact to make such line acceptable). Substantial rolloff of the low frequencies (less than 1 kHz) can cause user perceived message quality to suffer. A frequency response equalizer (or just equalizer) is generally used to correct for amplitude distortion. Traditional equalizers involve making an impulse response measurement of the line (using "white" noise or repetitive impulsive reference signals) and constructing a recursive filter (IIR) from these measurements (using LMS or LPC techniques). This technique results in a equalizer which is recursive (requires high precision calculations), possibly unstable, and of high order. This invention is a method for using the sub-band filter bank used by the speech compression algorithm of the receiving party to perform the function of equalizer with very few operations. A few assumptions are made about the requirements on an equalizer for the application noted in the problem description: 1) phase distortion is unimportant, and 2) sharp transitions in the frequency response of the line are unlikely and unimportant. Phase distortion is unimportant because the purpose of the equalizer is to correct the frequency response of speech, for which phase is unimportant. Sharp transitions are known to occur only rarely on telephone lines. The sequence of events to initialize and execute the equalizer is described below. This is followed by a description of each of the mechanisms used in implementation. 1) System A calls System B via the public telephone network (or a private telephone network, if such is available). 2) System B answers the call and responds with a test transmission usable for characterizing the telephone line. 3) System A analyzes the spectrum of the test transmission of system B. The average error (differen...