TrueSpeech® Co-Processor
PRELIMINARY/CONFIDENTIAL
Version: 1.18
The value of this parameter indicates the degree of echo cancellation from the telephone line. The precise threshold
value, which indicates success or failure, depends to an extent on the design and amount of gain of the external
hardware. Once the electrical echo canceller has been successfully trained, the loop attenuation can be safely reduced
to zero or one of the other full-duplex attenuation factors. Since it is difficult to anticipate all circumstances under
which the speakerphone may be used, a facility for the user to manually send the pulse should be provided in
addition to the automatic training pulse control logic described. The speakerphone system should provide a manual
speakerphone reset push button that the user can press to activate full duplex mode if the automatic control logic fails
for any reason.
A.5
Volume Control
The CT8022 includes volume control for both the line output level (8dB range) and speaker output level (28dB
range). The volume should be adjusted using the CT8022 rather than by changing external physical gain. In this way,
the CT8022 is aware of the change in loop gain directly and can internally compensate for it, instead of having to
adaptively adjust for the change. The CT8022 also supports speaker and microphone mute.
A.6
Dial Tone Detection
The CT8022 includes special handling for constant-level tone signals, such as a dial tone. The voice detectors used
for controlling the loop attenuation ignore signals of this nature. When a steady tone is detected for a period of
approximately 1-second, priority (or minimum attenuation) is automatically switched to the line side so that the user
will hear the dial tone at full volume. Without this special handling, the CT8022 would see that both the receive and
transmit directions had constant signal energy, and would distribute the loop attenuation equally between the two
directions. Under these circumstances, the dial tone level heard by the user would be attenuated.
A.7
Performance
The acoustical echo canceller can handle echo delays up to 10ms (approximately 3.4 meters or 11 feet in sound
propagation). The canceller supplies 34dB cancellation for white noise in less than 0.5 seconds with the speaker
CODEC output short circuit connected to the microphone CODEC input.
The electrical echo canceller generates a training pulse of 16ms duration with an instantaneous peak output voltage
of 350mV. The maximum line echo duration that will be cancelled is 23ms. With the output CODEC signal short
circuit connected to the input CODEC signal, the electrical echo canceller achieves 34 dB of cancellation (measured
with white noise). In this test, the echo canceller performance is limited primarily by the quantization noise
associated with the D-to-A and A-to-D translations performed by the µ-law CODEC.
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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 ]
