ZL38065
1.1
Adaptive Filter
Data Sheet
The adaptive filter adapts to the echo path and generates an estimate of the echo signal. This echo estimate is then
subtracted from Sin. For each group of echo cancellers, the adaptive filter is a 1024 tap FIR adaptive filter which is
divided into two sections. Each section contains 512 taps providing 64 ms of echo estimation. In
Normal
configuration,
the first section is dedicated to channel A and the second section to channel B. In
Extended Delay
configuration,
both sections are cascaded to provide 128 ms of echo estimation in channel A. In
Back-to Back
configuration,
the first section is used in the receive direction and the second section is used in the transmit
direction for the same channel.
The ZL38065 offers industry leading convergence speeds, both in initial convergence and reconvergence. A
sample test result from G.168-2002 Test 2A can be seen in Figure 5. This test result demonstrates one of the many
conditions where the Zarlink device offer sub 50 ms initial convergence times (G.168 Test 2A, Hybrid 5, 40 ms
delay, ERL=24dB, Lrin=0dBm0). Full G.168 test results across all hybrids and test conditions are available upon
request.
Figure 5 - Sample G.168 Test 2A Convergence Result
1.2
Double-Talk Detector
Double-Talk is defined as those periods of time when signal energy is present in both directions simultaneously.
When this happens, it is necessary to disable the filter adaptation to prevent divergence of the Adaptive Filter
coefficients. Note that when double-talk is detected, the adaptation process is halted but the echo canceller
continues to cancel echo using the previous converged echo profile. A double-talk condition exists whenever the
relative signal levels of Rin (Lrin) and Sin (Lsin) meet the following condition:
Lsin > Lrin + 20log
10
(DTDT)
where DTDT is the Double-Talk Detection Threshold. Lsin and Lrin are signal levels expressed in dBm0.
A different method is used when it is uncertain whether Sin consists of a low level double-talk signal or an echo
return. During these periods, the adaptation process is slowed down but it is not halted. The slow convergence
speed is set using the Slow sub-register in Control Register 4. During slow convergence, the adaptation speed is
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