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Digital Voice Station Scrambling

IP.com Disclosure Number: IPCOM000047865D
Original Publication Date: 1983-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Croisier, A: AUTHOR

Abstract

In modern telephone systems, the computer-controlled Private Automatic Branch Exchange (PABX) and the voice stations exchange voice and other signals in digitized form. In many circumstances, those digitized signals are comprised of repetitive bit patterns which produce a signal which, if radiated from the telephone line, may create Radio Frequency Interference (RFI). This article describes a simple technique to reduce RFI, in which the bit bursts exchanged by the stations and the PABX are randomly inverted. Most of the time, a voice station, even when active, transmits at a near-zero level which causes the six to seven most significant bits of the digitized voice sample to be always the same. Furthermore, the signalling bit values are nearly always zero.

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Digital Voice Station Scrambling

In modern telephone systems, the computer-controlled Private Automatic Branch Exchange (PABX) and the voice stations exchange voice and other signals in digitized form. In many circumstances, those digitized signals are comprised of repetitive bit patterns which produce a signal which, if radiated from the telephone line, may create Radio Frequency Interference (RFI). This article describes a simple technique to reduce RFI, in which the bit bursts exchanged by the stations and the PABX are randomly inverted. Most of the time, a voice station, even when active, transmits at a near-zero level which causes the six to seven most significant bits of the digitized voice sample to be always the same. Furthermore, the signalling bit values are nearly always zero. This creates a repetitive pattern which produces a signal having mainly a single frequency component which, if radiated from the telephone line, may create RFI. Typically, the voice stations are divided into groups, and each group is attached to a Line Interface Module (LIM) which is part of the PABX. In those systems, in which communications between the LIMs and the voice stations operate in the so- called ping-pong mode, the LIMs and the voice stations exchange information signals in the form of bit bursts, each comprising a start bit, a digitized voice sample, and a signalling bit. To reduce RFI, it has been proposed in the past to scramble the bit sequence in each burst, ther...