zl50235 Zarlink Semiconductor, zl50235 Datasheet - Page 12

• Manufacturer Part Number: zl50235
• Description: 16 Channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

The Path Change Detector is activated by setting the PathDet bit in Control Register 3 to “1”. An optional path
clearing feature can be enabled by setting the PathClr bit in Control Register 3 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.
2.4
After echo cancellation, there is always a small amount of residual echo which may still be audible. The ZL50235
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:
where 0 < NLPTHR
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.
NLP Selection
Reject uncanceled echo as noise
Reject double-talk as noise
Noise level estimator ramping scheme
Noise level ramping rate
Noise level scaling
Non-Linear Processor (NLP)
Feature
(dec)
< 1
NLPTHR
Table 1 - Comparison of NLP Types
TSUP = Lrin + 20log
NLPSel (Control Register 3)
NLRun1 (Control Register 3)
NLRun2 (Control Register 3)
InjCtrl (Control Register 3)
NLInc (Noise Control)
Noise Scaling
(hex)
Zarlink Semiconductor Inc.
Register or Bit(s)
= hex(NLPTHR
ZL50235
12
10
(NLPTHR)
(dec)
* 32768)
NLP Default
Advanced
16(hex)
5(hex)
Value
1
1
1
1
not supported)
not supported)
not supported)
not supported)
Default Value
Original NLP
0 (feature
0 (feature
0 (feature
0 (feature
74(hex)
C(hex)
Data Sheet