Version: 1.18
PRELIMINARY/CONFIDENTIAL
TrueSpeech® Co-Processor
9.15.6.5
Freezing the AEC
In the locked state, the AEC coefficients are updated if the measured attenuation exceeds the Lock Threshold. To
cause a complete freeze of the AEC, where no updating of the AEC coefficients takes place, simply program the
Lock and Un-Lock Thresholds to large values and force the AEC into the locked state.
For example:
Command Status Response
C1
C2
C1
C2
C1
C2
E01B
7FFF
E014
FF00
E012
C004
E01B
7FFF
E014
FF00
E012
C004
program the un-lock threshold to a large
positive value
program the lock threshold to a large
negative value
Force the AEC into the locked state
9.15.7
Controlling the Automatic Loop Adjustment Attenuation
When the AEC is operating under normal conditions, it constantly monitors the effect of the echo cancellation
process. The intent of the AEC is to reduce the amplitude of any echo signals passing through it. When a signal
originating at the near end (a signal caused by the local user talking, and not an echo of the speaker output) passes
through the AEC, the signal should pass through without any change in amplitude. Under either of these conditions,
the output signal level from the AEC block should never be larger than the input signal level.
If the AEC detects that its output level is greater than its input level, this indicates a fault condition. This state may
arise if the acoustic echo path is rapidly changed, for example by suddenly moving the microphone to a position
immediately in front of the speaker. Under these circumstances, the AEC by default will increase the amount of echo
suppression in use to try to prevent positive feedback in the audio loop (howling).
9.15.7.1
Programming the Loop Adjustment Threshold
The Host can program the Loop Adjustment Threshold at which the additional loop attenuation is activated:
Command Status Response
C1
C2
E01A
E01A
XXXX
XXXX
where XXXX is the Loop Adjustment Threshold in dB.
For example, the default threshold is +1 dB, so XXXX=1dB.
The automatic loop adjustment feature can be disabled by programming the Loop Adjustment Threshold to a large
positive value, e.g. XXXX=7FFFH, or by programming the amount of attenuation added to zero (see below).
It is also possible to disable the automatic loop adjustment feature with the Speakerphone Configuration Command
described in Section 9.21.
Note: When using the AEC concurrently with speech compression modes, this particular command can
only be sent after enabling the AEC and entering the playback and record states.
CT8022A11AQC FW Revision 0118 DSP GROUP, INC., 3120 SCOTT BOULEVARD
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
115
All specifications are subject to change without prior notice.
ETC [ ETC ]
