MT9300B データシートの表示（PDF） - Zarlink Semiconductor Inc

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MT9300B

Multi-Channel Voice Echo Canceller

Zarlink Semiconductor Inc 

MT9300B

Data Sheet

Adaptive Filter

The adaptive filter adapts to the echo path and generates an estimate of the echo signal. This echo estimate is then

subtracted from Sin. For each group of echo cancellers, the adaptive filter is a 1024 tap FIR adaptive filter which is

divided into two sections. Each section contains 512 taps providing 64ms of echo estimation. In Normal

configuration, the first section is dedicated to channel A and the second section to channel B. In Extended Delay

configuration, both sections are cascaded to provide 128 ms of echo estimation in channel A. In Back-to Back

configuration, the first section is used in the receive direction and the second section is used in the transmit

direction for the same channel.

Double-Talk Detector

Double-Talk is defined as those periods of time when signal energy is present in both directions simultaneously.

When this happens, it is necessary to disable the filter adaptation to prevent divergence of the Adaptive Filter

coefficients. Note that when double-talk is detected, the adaptation process is halted but the echo canceller

continues to cancel echo using the previous converged echo profile.

A double-talk condition exists whenever the relative signal levels of Rin (Lrin) and Sin (Lsin) meet the following

condition:

Lsin > Lrin + 20log10(DTDT)

where DTDT is the Double-Talk Detection Threshold. Lsin and Lrin are signal levels expressed in dBm0.

A different method is used when it is uncertain whether Sin consists of a low level double-talk signal or an echo

return. During these periods, the adaptation process is slowed down but it is not halted.

In the G.168 standard, the echo return loss is expected to be at least 6 dB. This implies that the Double-Talk

Detector Threshold (DTDT) should be set to 0.5 (-6 dB). However, in order to get additional guardband, the DTDT

is set internally to 0.5625 (-5 dB).

In some applications the return loss can be higher or lower than 6 dB. The MT9300B allows the user to change the

detection threshold to suit each application’s need. This threshold can be set by writing the desired threshold value

into the DTDT register.

The DTDT register is 16 bits wide. The register value in hexadecimal can be calculated with the following equation:

DTDT(hex) = hex(DTDT(dec) * 32768)

where 0 < DTDT(dec) < 1

Example: For DTDT = 0.5625 (-5dB), the

hexadecimal value becomes

hex(0.5625 * 32768) = 4800H

Path Change Detector

Integrated into the MT9300B is a Path Change Detector. This permits fast reconvergence when a major change

occurs in the echo channel. Subtle changes in the echo channel are also tracked automatically once convergence

is achieved, but at a much slower speed.

The Path Change Detector is activated by setting the PathDet bit in Control Register A3/B3 to "1". An optional path

clearing feature can be enabled by setting the PathClr bit in Control Register A3/B3 to "1". With path clearing turned

on, the existing echo channel estimate will also be cleared (i.e. the adaptive filter will be filled with zeroes) upon

detection of a major path change.

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Zarlink Semiconductor Inc.

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