ZL50233

Data Sheet

9

Zarlink Semiconductor Inc.

1.4

Non-Linear Processor (NLP)

After echo cancellation, there is always a small amount of residual echo which may still be audible. The ZL50233

uses Zarlink’s patented Advanced 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:

where NLPTHR is the Non-Linear Processor Threshold register value and Lrin is the relative power level expressed

in dBm0. The NLPTHR register is 16 bits wide. The register value in hexadecimal can be calculated with the

following equation:

When the level of residual error signal falls below TSUP, the NLP is activated further attenuating the residual signal

by an additional 30 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 comfort noise injector can be disabled by setting the INJDis bit to “1” in Control Register 1. It should be noted

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

The patented Advanced NLP provides a number of new and improved features over the original NLP found in

previous generation devices. Differences between the Advanced NLP and the original NLP are summarized in

Table 1.

The NLPSel bit in Control Register 3 selects which NLP is used. A “1” will select the Advanced NLP, “0” selects the

original NLP. (See page 24 for Control Register 3 bit description)

Feature

Register or Bit(s)

Advanced

NLP Default

Value

Original NLP

Default Value

NLP Selection

NLPSel (Control Register 3)

1

0 (feature

not supported)

Reject uncanceled echo as noise

NLRun1 (Control Register 3)

1

0 (feature

not supported)

Reject double-talk as noise

NLRun2 (Control Register 3)

1

0 (feature

not supported)

Noise level estimator ramping scheme

InjCtrl (Control Register 3)

1

0 (feature

not supported)

Noise level ramping rate

NLInc (Noise Control)

5(hex)

C(hex)

Noise level scaling

Noise Scaling

16(hex)

74(hex)

Table 1 - Comparison of NLP Types