zl50212 Zarlink Semiconductor, zl50212 Datasheet - Page 13

• Manufacturer Part Number: zl50212
• Description: 288 Channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

Data Sheet
internally control the switching between Enable Adaptation and Bypass states. The automatic mode is 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 in the Status Registers. Following the detection of a disable tone (TD bit high) on a
given channel, the external controller must switch the echo canceller from Enable Adaptation to Bypass state.
2.4
In systems with very low echo channel return loss (ERL), there may be enough feedback in the loop to cause stability
problems in the Adaptive Filter. This instability can result in variable pitched ringing or oscillation. Should this ringing
occur, the Instability Detector will activate and suppress the oscillations.
The Instability Detector is activated by setting the RingClr bit in Control Register 3 to “1”.
2.5
Single or dual frequency tones (i.e. DTMF tones) present in the receive input (Rin) of the echo canceller for a
prolonged period of time may cause the Adaptive Filter to diverge. The Narrow Band Signal Detector (NBSD) is
designed to prevent this by detecting single or dual tones of arbitrary frequency, phase, and amplitude. When narrow
band signals are detected, adaptation is halted but the echo canceller continues to cancel echo.
The NBSD will be active regardless of the EVP functional state. However the NBSD can be disabled by setting the
NBDis bit to “1” in Control Register 2.
2.6
Adaptive filters in general do not operate properly when a DC offset is present at any input. To remove the DC
component, each EVP incorporates Offset Null filters in both Rin and Sin inputs.
The offset null filters can be disabled by setting the HPFDis bit to “1” in Control Register 2.
2.7
The EVP provides adjustable level pads at Rin, Rout, Sin and Sout. This setup allows signal strength to be adjusted
both inside and outside the echo path. Each signal level may be independently scaled with anywhere from 0 to -12
dB level, in 3 dB steps. Level values are set using the Gains register.
CAUTION: Gain adjustment can help interface the ZL50212 to a particular system in order to provide optimum echo
cancellation, but it can also degrade performance if not done carefully. Excessive loss may cause low signal levels
and slow convergence. Exercise great care when adjusting these values.
The -12 dB PAD bit in Control Register 1 is still supported as a legacy feature. Setting this bit will provide 12 dB of
attenuation at Rin, and override the values in the Gains register.
2.8
The ZL50212 has been certified G.168 (1997), (2000) and (2002) compliant in all 64 ms cancellation modes
(i.e. Normal and Back-to-Back configurations) by in-house testing with the DSPG ECT-1 echo canceller tester.
The ZL50212 has also been tested for G.168 compliance and all voice quality tests at AT&T Labs. The ZL50212 was
classified as “carrier grade” echo canceller.
Instability Detector
Narrow Band Signal Detector (NBSD)
Offset Null Filter
Adjustable Level Pads
ITU-T G.168 Compliance
Zarlink Semiconductor Inc.
ZL50212
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