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USING SPECTRAL DISTANCE MEASURES FOR VOCODER ERROR DETECTION

IP.com Disclosure Number: IPCOM000007288D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2002-Mar-12
Document File: 2 page(s) / 117K

Publishing Venue

Motorola

Related People

Donald Newberg: AUTHOR [+3]

Abstract

Digital radio systems employ vocoders to reduce the total number of bits needed to transport the voice information. For example, the ASTRO radio sys- tem uses VSELP a vocoder which encodes speech as a set of parameters representing the energy, pitch, excitation, and vocal tract response of the original speech. Under low signal conditions, this voice infor- mation will become corrupted by bit errors, resulting in degraded or unintelligible speech quality. Forward error correction is therefore used to lessen the effect of errors, with the transmitter adding extra parity bits to the VSELP information and the receiver using these bits to correct bit errors that may have occurred. Due to bandwidth limitations, the error correction codes are generally not strong enough to correct all of the bit errors that occur in normal operation. Because the error correction decoding will some- times fail to correct bit errors, it is desired to have an algorithm that will detect this situation and engage a process to mitigate its effects.

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MOTOROLA Technical Developments Volume 23 October 1994

USING SPECTRAL DISTANCE MEASURES FOR VOCODER ERROR DETECTION

by Donald Newberg, Brad Hiben and Bob Logalbo

mitigation should be performed. This situation causes invalid VSELP parameters to be used, degrading the speech quality.

BACKGROUND OF THE PROBLEM

  Digital radio systems employ vocoders to reduce the total number of bits needed to transport the voice information. For example, the ASTRO radio sys- tem uses VSELP a vocoder which encodes speech as a set of parameters representing the energy, pitch, excitation, and vocal tract response of the original speech. Under low signal conditions, this voice infor- mation will become corrupted by bit errors, resulting in degraded or unintelligible speech quality. Forward error correction is therefore used to lessen the effect of errors, with the transmitter adding extra parity bits to the VSELP information and the receiver using these bits to correct bit errors that may have occurred. Due to bandwidth limitations, the error correction codes are generally not strong enough to correct all of the bit errors that occur in normal operation. Because the error correction decoding will some- times fail to correct bit errors, it is desired to have an algorithm that will detect this situation and engage a process to mitigate its effects.

  The current error mitigation scheme for the ASTRO radio depends on the error detection prop- erties of the ECC codewords. The Golay codewords used to protect the VSELP parameters can correct three or less bit errors. If exactly four errors occur, they cannot be corrected but their presence is detected, a condition termed an erasure. If more than four errors occur, either an erasure will be detected or the data will be decoded to a valid but incorrect codeword, depending on the placement of the errors. Whenever a codeword erasure is detected, the data is known to be bad. The error mitigation process uses previously stored VSELP parameters in place of the known bad ones, mitigating the effect ofthe bit errors, If no erasure is detected, the parameter is assumed to be good and will be used. At low bit error rates, this method will correct or detect bit errors reliably. As the BER rises, however, the error correction becomes sporadically overwhelmed with bit rerors, corrupting the data but giving no indication that

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DESCRIPTION OF THE SOLUTION

  The LPC parameters in the VSELP vocoder rep- resent the vocal tract, or spectral shape, of the speech. Because it...