MT9300AL datasheet(6/29 Pages) MITEL

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MT9300

Advance Information

Non-Linear Processor (NLP)

After echo cancellation, there is always a small

amount of residual echo which may still be audible.

The MT9300 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

(NLPTHR). TSUP can be calculated by the following

equation:

TSUP = Lrin + 20log10(NLPTHR)

where

NLPTHR

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.

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:

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

where 0 < NLPTHR(dec) < 1

The comfort noise injection 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.

Disable Tone Detector

G.165 recommendation deﬁnes the disable tone as

having

the

following

characteristics:

2100

(

±21Hz) sine wave, a power level between -6 to

-31dBm0, and a phase reversal of 180 degrees (

± 25

degrees) every 450ms (

± 25ms). 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 (

±21Hz) sine wave with a power level

between 0 to -31dBm0. If the disable tone is present

for a minimum of 400 milliseconds, with or without

phase reversal, the Tone Detector will trigger.

The MT9300 has two Tone Detectors per channels

(for a total of 64) in order to monitor the occurrence

of a valid disable tone on both Rin and Sin. Upon

detection of a disable tone, TD bit of the Status

generated (i.e. IRQ pin low). Refer to Figure 4 and to

the Interrupts section.

Figure 4 - Disable Tone Detection

Once a Tone Detector has been triggered, there is

no longer a need for a valid disable tone (G.164 or

G.165) to maintain Tone Detector status (i.e. TD bit

high). The Tone Detector status will only release (i.e.

TD bit low) if the signals Rin and Sin fall below -

30dBm0, in the frequency range of 390Hz to 700Hz,

and below

-34dBm0, in the frequency range of

700Hz to 3400Hz, for at least 400ms. Whenever a

Tone Detector releases, an interrupt is generated

(i.e. IRQ pin low).

The selection between G.165 and G.164 tone

disable is controlled by the PHDis bit in Control

bit is set to 1, G.164 tone disable requirements are

selected.

In response to a valid disable tone, the echo

canceller

must

switched

from

the

Enable

Adaptation state to the Bypass state. This can be

done in two ways, automatically or externally. In

automatic mode, the Tone Detectors internally

control the switching between Enable Adaptation and

Bypass states. The automatic mode can be activated

by setting the AutoTD bit in Control Register 2 to

high. In external mode, an external controller is

needed to service the interrupts and poll the TD bits

TD bit

Rin

Sin

Echo Canceller A

Tone

Detector

Tone

Detector

Status reg

ECA

TD bit

Rin

Sin

Echo Canceller B

Tone

Detector

Tone

Detector

Status reg

ECB

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