Version: 1.18
PRELIMINARY/CONFIDENTIAL
TrueSpeech® Co-Processor
9.15.3
Controlling the AEC Adaptation (Training) Rate
The Host may limit the AEC adaptation (training) rate to limit the CT8022 processing resources consumed by the
AEC. This also reduces the adaptation rate of the AEC (increases the time taken to train or converge the AEC).
Reducing the adaptation rate increases the time taken for the AEC to train, but it also increases the time taken for the
AEC to diverge (un-train). Reducing the adaptation rate increases the stability of the AEC.
The Host may control the AEC adaptation rate in real-time. Therefore, the Host may enable rapid adaptation in the
initial stages of a call to quickly train the AEC, and then select slower adaptation in the later stages of a call. This
increases the stability of the AEC and makes the AEC more immune to double-talk conditions, which can destabilize
the AEC.
The processing load (MIPS) consumed by the AEC feature is the sum of three main components:
•
•
•
control overhead
actual cancellation
adaptation (training)
The control overhead is approximately constant.
The cancellation load is proportional to the AEC Tail Length and does not vary frame-by-frame.
The peak adaptation load is proportional to the AEC Tail Length, but switches On or Off on a sample-by-sample
basis, as determined by the internal control function (speech direction).
For a given frame size and Tail Length, the processing load due to the first two factors is fixed.
The adaptation load varies on a frame-by-frame basis and depends on the speech direction which is evaluated on a
sample-by-sample basis within the speech frame.
To prevent the Host from over-committing the CT8022 processing resources (by configuring an inappropriate
combination of Tail Length, Speech Coder and Frame Size) , the AEC function monitors the CT8022 processing
load in real-time. If the AEC senses that the CT8022 DSP processor is over-committed, it can automatically throttle
(limit) the AEC adaptation process to reduce the amount of processing resources consumed by the AEC adaptation.
The AEC adaptation throttle limits the number of adaptation cycles that may run during the processing of each
speech frame. By default, the maximum number of adaptation cycles is set to the number of samples in the speech
frame. For example: a 30ms speech frame contains 240 samples, so the AEC may run the AEC adaptation operation
up to a maximum of 240 times during the speech frame. The number of adaptation cycles actually run depends on
the measured speech direction for each speech sample within the frame. Adaptation of the AEC is enabled only
when the speech direction is in the playback (decompression) direction. If the speech direction is sensed to be in the
record (compression) direction, no adaptation (training) takes place. The speech direction is sensed on a sample-
by-sample basis, so the number of adaptation cycles that run in a particular speech frame can vary from a minimum
of zero, to a maximum equal to the speech frame length. For a 30ms speech frame, the number of adaptation cycles
can vary from 0 to 240.
The AEC adaptation cycle limit is set equal to the speech frame size at the beginning of each full-duplex speech
session. For each speech frame that the AEC senses the CT8022 running out of processing resources, the AEC
adaptation cycle limit is reduced (in decrements of 16).
The Host may override the maximum and minimum AEC adaptation cycle limits by programming a maximum and
minimum value for the AEC adaptation cycle limit. If the maximum and minimum values are programmed such that
they are equal, then the AEC adaptation cycles operate with a fixed limit.
Whenever the AEC cycle adaptation limit is set to a value lower than the current frame size, the peak processing load
due to the AEC adaptation (training) process is reduced. The side effect of this is that the AEC adaptation (training)
time is correspondingly increased.
CT8022A11AQC FW Revision 0118 DSP GROUP, INC., 3120 SCOTT BOULEVARD
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
103
All specifications are subject to change without prior notice.
ETC [ ETC ]
