ZL38001
Data Sheet
1.0 Functional Description
The ZL38001 device contains an acoustic echo cancellers, as well as the many control functions necessary to
operate the echo canceller. The ZL38001 provides clear signal transmission in both audio path directions to ensure
reliable voice communication, even with low level signals. The ZL38001 does not use variable attenuators during
double-talk or single-talk periods of speech, as do many other acoustic echo cancellers for speakerphones.
Instead, the ZL38001 provides high performance full-duplex operation similar to network echo cancellers, so that
users experience clear speech and uninterrupted background signals during the conversation. This prevents
subjective sound quality problems associated with “noise gating” or “noise contrasting”.
The ZL38001 uses an advanced adaptive filter algorithm that is double-talk stable, which means that convergence
takes place even while both parties are talking1. This algorithm allows continual tracking of changes in the echo
path, regardless of double-talk, as long as a reference signal is available for the echo canceller.
The echo tail cancellation capability of the acoustic echo canceller has been sized appropriately (112 ms) to cancel
echo in an average sized office with a reverberation time of less than 112 ms.
In addition to the echo cancellers, the following functions are supported:
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Control of adaptive filter convergence speed during periods of double-talk, far end single-talk and near-end
echo path changes
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Control of Non-Linear Processor thresholds for suppression of residual non-linear echo
Howling detector to identify when instability is starting to occur and to take action to prevent oscillation
Narrow-Band Detector for preventing adaptive filter divergence caused by narrow-band signals
Offset Nulling filters for removal of DC components in PCM channels
Limiters that introduce controlled saturation levels
Serial controller interface compatible with Motorola, National and Intel microcontrollers
PCM encoder/decoder compatible with µ/A-Law ITU-T G.711, µ/A-Law Sign-Mag or linear 2’s complement
coding
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Automatic gain control on the receive speaker path
1.1 Adaptation Speed Control
The adaptation speed of the acoustic echo canceller is designed to optimize the convergence speed versus
divergence caused by interfering near-end signals. Adaptation speed algorithm takes into account many different
factors such as relative double-talk condition, far end signal power, echo path change and noise levels to achieve
fast convergence.
1.2 Advanced Non-Linear Processor (ADV-NLP)1
After echo cancellation, there is likely to be residual echo which needs to be removed so that it will not be audible.
The ZL38001 uses an NLP to remove low level residual echo signals which are not comprised of background noise.
The operation of the NLP depends upon a dynamic activation threshold, as well as a double-talk detector which
disables the NLP during double-talk periods.
The ZL38001 keeps the perceived noise level constant, without the need for any variable attenuators or gain
switching that causes audible “noise gating”. The noise level is constant and identical to the original background
noise even when the NLP is activated.
The NLP can be disabled by setting the NLP- bit to 1 in the AEC control registers.
1. Patent pending.
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Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
