ZL50234GDC ZARLINK [Zarlink Semiconductor Inc], ZL50234GDC Datasheet - Page 11

• Manufacturer Part Number: ZL50234GDC
• Description: 8 Channel Voice Echo Canceller
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

Data Sheet
1.6
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”.
1.7
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 Echo Canceller functional state. However the NBSD can be disabled by
setting the NBDis bit to “1” in Control Register 2.
1.8
Adaptive filters in general do not operate properly when a DC offset is present at any input. To remove the DC
component, the ZL50234 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.
1.9
The ZL50234 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 ZL50234 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.
1.10
The ZL50234 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 ZL50234 has also been tested for G.168 compliance and all voice quality tests at AT&T Labs. The ZL50234
was classified as “carrier grade” echo canceller.
2.0
The ZL50234 architecture contains 8 echo cancellers divided into 4 groups. Each group has two echo cancellers
which can be individually controlled (Echo Canceller A (ECA) and Echo Canceller B (ECB)). They can be set in three
distinct configurations: Normal, Back-to-Back, and Extended Delay. See Figures 6, 7, and 8.
Instability Detector
Narrow Band Signal Detector (NBSD)
Offset Null Filter
Adjustable Level Pads
ITU-T G.168 Compliance
Device Configuration
Zarlink Semiconductor Inc.
ZL50234
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