Browse Prior Art Database

Improvement to Echo Cancellers

IP.com Disclosure Number: IPCOM000035329D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Roulier, G: AUTHOR

Abstract

In a digital voice transmission system, as represented in the figure, some of the remote speech to be provided to the user at the local speech terminal is, in fact, fed back to the originator as an echo. Echoes are due to mismatching in the hybrid transformer (H) load impedance. They are generally very troublesome and as much as possible should be removed.

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Improvement to Echo Cancellers

In a digital voice transmission system, as represented in the figure, some of the remote speech to be provided to the user at the local speech terminal is, in fact, fed back to the originator as an echo. Echoes are due to mismatching in the hybrid transformer (H) load impedance. They are generally very troublesome and as much as possible should be removed.

Conventionally, echoes are cancelled by using an adaptive echo canceller including a digital filter, the coefficients of which are dynamically adjusted using an adapter device. The filter synthesizes an echo replica and subtracts the replica from the echo-spoiled incoming speech. The adapter device sets and updates the filter coefficients in order to minimize the residual echo, i.e., the error.

Both echo canceller parts are conventional. They implement algorithms using a microprogram-controlled microprocessor. Due to finite arithmetic of the processor, the echo cancellation is not optimal and generates noise. This could be improved by implementing the following algorithm: - Whenever local speech is detected by the speech compression algorithm, updating is bypassed, preventing the taps from diverging. - Taps may be clipped to improve convergence.

When using a finite arithmetic machine, with 16-bit words and 12 x 12 multiplier, experience shows that values which should be zero are not and may even be very high.

The implemented solution consists in setting to zero the taps which are ...