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MT9123

Dual Voice Echo Canceller

Mitel Networks 

MT9123

Preliminary Information

Adaptive Filter

The adaptive filter is a 1024 tap FIR 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 128ms of echo estimation in

channel A.

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.

A double-talk condition exists whenever the Sin

signal level is greater than the expected return echo

level. The relative signal levels of Rin (Lrin) and Sin

(Lsin) are compared according to the following

expression to identify a double-talk condition:

Lsin > Lrin + 20log10(DTDT)

where DTDT is the Double-Talk Detection Threshold.

Lsin and Lrin are the relative 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.

Controllerless Mode

In G.165 standard, the echo return loss is expected

to be at least 6dB. This implies that the Double-Talk

Detector Threshold (DTDT) should be set to 0.5

(-6dB). However, in order to get additional

guardband, the DTDT is set internally to 0.5625

(-5dB). In controllerless mode, the Double-Talk

Detector is always active.

Controller Mode

In some applications the return loss can be higher or

lower than 6dB. The MT9123 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

Non-Linear Processor (NLP)

After echo cancellation, there is always a small

amount of residual echo which may still be audible.

The MT9123 uses an NLP to remove residual echo

signals which have a level lower than the Adaptive

Suppression Threshold (TSUP in G.165). This

threshold depends upon the level of the Rin (Lrin)

reference signal as well as the programmed value of

the Non-Linear Processor Threshold register

(NLPTHR). TSUP can be calculated by the following

equation:

TSUP = Lrin + 20log10(NLPTHR)

where NLPTHR is the Non-Linear Processor

Threshold register value and Lrin is the relative

power level expressed in dBm0.

When the level of residual error signal falls below

TSUP, the NLP is activated further attenuating the

residual signal to less than -65dBm0. To prevent a

perceived decrease in background noise due to the

activation of the NLP, a spectrally-shaped comfort

noise, equivalent in power level to the background

noise, is injected. This keeps the perceived noise

level constant. Consequently, the user does not hear

the activation and de-activation of the NLP.

Controllerless Mode

The NLP processor can be disabled by connecting

the NLP pin to Vss.

Controller Mode

The NLP processor can be disabled by setting the

NLPDis bit to 1 in Control Register 2.

The NLPTHR register is 16 bits wide. The register

value in hexadecimal can be calculated with the

following equation:

8-50

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