CT8022 Datasheet, PDF (181/194 Pages) List of Unclassifed Manufacturers – VOIP/VON G.723.1, G279AB TRUESPEECH CO-PROCESSOR

Version: 1.18
PRELIMINARY/CONFIDENTIAL
TrueSpeech® Co-Processor
A.4 CT8022 Digital Canceller
The CT8022 SpeakerPhone system actually contains two digital cancellers:
• an adaptive acoustical canceller which cancels the acoustic echo between microphone and speaker
• an electrical canceller which cancels the echo caused by the 4-to-2 wire telephone line connection
The CT8022 can be operated in two distinct modes:
• half-duplex mode where only the acoustic canceller is used
• full-duplex mode in which both cancellers are active
A.4.1 Half-Duplex Mode
In this half-duplex mode, the loop attenuation selected must be at least 12 dB. In this mode the SpeakerPhone
operates by dynamically distributing the loop attenuation selected between the receive and transmit attenuators (see
figure 1a). The CT8022 makes a decision as to which direction is talking, and then rapidly shifts the attenuation to
the opposite direction. The total attenuation in the loop (receive attenuation plus transmit attenuation) remains
constant. The exception to this is when the CT8022 detects abnormal conditions and loop adjustment mode 01 or 10
is selected. Under these circumstances, the CT8022 can (temporarily) introduce an additional 12 dB of attenuation in
order to prevent howling or feedback. Abnormal conditions exist when the CT8022 determines that it unable to
cancel the acoustic echo by adjusting the echo canceller coefficients. If loop adjust mode 10 is selected, the CT8022
will attempt to restore the original loop attenuation selected by reducing the additional attenuation in steps of 4 dB
once the abnormal circumstances are removed.
Half-duplex mode is the default mode selected when the SpeakerPhone operation is started, before the training pulse
has been sent to the telephone line activating the electrical echo canceller. The SpeakerPhone should always be
started with a loop attenuation of at least 12 dB. The CT8022 operating in half-duplex mode provides for Host
selection of the loop attenuation. This gives the system designer some flexibility in design of the external analog
circuit. It also provides a more natural sounding speakerphone since the non-talking direction is never entirely muted
as with some other speakerphone implementations. Using the lower attenuation levels offered by the CT8022 avoids
the unnatural abrupt switching between active directions, present with alternative implementations, which causes the
user to experience break up of the conversation.
A.4.2 Full-Duplex Mode
In full-duplex mode, the electrical (line-side) echo canceller must be activated by the Host micro-controller. When
instructed, the CT8022 sends a training pulse of short duration to the telephone line to measure the echo (impulse)
response of the line. This pulse is audible to the user, but is of very short duration and is not intrusive. There should
not be any incoming signal from the telephone line when the training pulse is sent. The Host micro-controller must
determine via an algorithm the correct time to send the pulse. On an incoming call, the pulse should be sent 100-200
ms after answering the call (going off-hook). This delay allows time for telephone line conditions to stabilize and for
the ringing signal to cease. On an outgoing call, the pulse should ideally be ideally around 50ms after dialing the
second digit. Again, this allows time for line conditions to stabilize, and avoids interfering with digit detection by the
local telephone exchange. Sending the training pulse at this time also helps ensure that the line is silent, since at this
point no ring tone or busy signal is likely to be present. Sending the training pulse at the end of a speed-dial or re-
dial sequence also works well, provided it is sent before a ringing tone is presented. Sending the pulse after the first
digit is not recommended since sometimes a secondary dial tone is encountered (as in dialing ‘9’ for an outside line).
The Host can determine if training was successful by using the Get Electrical Echo Canceller Quality Factor
command.
CT8022A11AQC FW Revision 0118
DSP GROUP, INC., 3120 SCOTT BOULEVARD
181
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
All specifications are subject to change without prior notice.