TrueSpeech® Co-Processor
PRELIMINARY/CONFIDENTIAL
Version: 1.18
9.15.7.2
Programming the Loop Adjustment Attenuation
When the AEC detects error-state amplification in excess of the Loop Adjustment Threshold, by default it adds 12dB
of echo suppression in addition to the echo suppression (loop attenuation) selected by the Set Echo Suppressor
Attenuation command. The maximum amount of total echo suppression in operation is limited to 28dB.
The Host can program the amount of additional echo suppression introduced using the following command:
Command Status Response
C1
C2
E015
000X
E015
000X
where X is between 0 and 7 and represents units of 4dB steps.
For example, the default attenuation added is 12 dB, so X=3.
When the AEC detects that the error conditions that caused the need for the additional echo suppression (loop
attenuation) no longer exist, it is removed automatically.
9.15.8
Setting AEC Microphone Noise Cut-Off Level When Canceling Telephone Line Echoes
When operating the AEC, the CT8022 checks the microphone input level, and if it is below the microphone noise
cut-off level, adaptation of the AEC is disabled. This helps the AEC prevent mis-training in the presence of low
level acoustic noise. This also helps deal with prevention of mis-training if the microphone is fitted with a mute
switch.
When the AEC is used to cancel electrical telephone line echoes from a telephone line 2-to-4 wire hybrid, it may be
desirable to reduce the cut-off level since telephone line environments typically have less noise. This is particularly
true if operating with a hybrid that provides a high degree of echo cancellation. This may result in the echo signal
created by the hybrid falling below the Microphone Noise Cut-Off Level. In this case, the AEC may not train since
the signal level of the echo is considered too low.
To set the AEC Microphone Noise Cut-Off Level use the command:
C1:
S1:
C2:
S2:
E007H
E007H
cut-off level
cut-off level
The default cut-off level is 0. In the CT8020 and CT8021 devices, the default value is 50 (decimal) which
represents a signal level of around -35 dBm0.
For telephone line echoes, this threshold can be safely set to zero.
116
DSP GROUP, INC., 3120 SCOTT BOULEVARD CT8022A11AQC FW Revision 0118
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
All specifications are subject to change without prior notice.
ETC [ ETC ]
