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MT9300B

Multi-Channel Voice Echo Canceller

Zarlink Semiconductor Inc 

MT9300B

Data Sheet

Non-Linear Processor (NLP)

After echo cancellation, there is always a small amount of residual echo which may still be audible. The MT9300B

uses an NLP to remove residual echo signals which have a level lower than the Adaptive Suppression Threshold

(TSUP in G.168). 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

by an additional 36 dB. 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.

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:

where 0 < NLPTHR(dec) < 1

NLPTHR(hex) = hex(NLPTHR(dec) * 32768)

The comfort noise injector can be disabled by setting the INJDis bit to “1” in Control Register A1/B1. It should be

noted that the NLPTHR is valid and the comfort noise injection is active only when the NLP is enabled.

If the comfort noise injector is unable to correctly match the level of the background noise (because of peculiar

spectral characteristics, for example), the injected level can be fine-tuned using the Noise Scaling register. A

neutral value of 80(hex) will prevent any scaling. Values less than 80(hex) will reduce the noise level, values greater

than 80(hex) will increase the noise level. The scaling is done linearly.

Example:

To decrease the comfort noise level

by 3 dB, the register value would be

10 ^ (-3 / 20) * 128

= 0.71 * 128

= 91(dec)

= 5B(hex)

The default factory setting for the Noise Scaling register should be adequate for most operating environments. It is

unlikely that it will need to be changed. It has also been set to a value which will ensure G.168 compliance.

Disable Tone Detector

G.165 recommendation defines the disable tone as having the following characteristics: 2100 Hz (±21 Hz) sine

wave, a power level between -6 to -31 dBm0, and a phase reversal of 180 degrees (± 25 degrees) every 450 ms (±

25 ms). If the disable tone is present for a minimum of one second with at least one phase reversal, the Tone

Detector will trigger.

G.164 recommendation defines the disable tone as a 2100 Hz (±21 Hz) sine wave with a power level between 0 to

-31 dBm0. If the disable tone is present for a minimum of 400 milliseconds, with or without phase reversal, the Tone

Detector will trigger.

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Zarlink Semiconductor Inc.

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