CS6422
CS6422
Fs
0dB
Fs
0dB
Fs
0dB
-30dB
-30dB
-30dB
t
t
t
(a) Input Signal
(b) AGC Attenuation
(c) AGC Gain
Fs
Fs
0dB
0dB
-30dB
-30dB
t
t
(d) Input Signal
(e) AGC Gain
Figure 11. How the AGC works (TVol = +30 dB)
Any signal greater than 30 dB below full-scale (a),
is scaled down to 30 dB (b). This signal is then
scaled up +30 dB (the reference level) to provide
the final output (c). Note that the combination of at-
tenuation and gain results in less than +30 dB total
gain being applied. If the input signal is below
30 dB below full-scale (d), no attenuation is added
and the full +30 dB of gain is applied to the signal
(e).
4.1.4 Suppression
Echo cancellation is somewhat of a misnomer in
that echo is merely attenuated, not entirely can-
celled. Some residual echo still exists after the
summing node. This residual echo, though low in
amplitude, may be audible when the near-end talk-
er is not speaking. Suppression further attenuates
the echoed signal, preventing the far-end listener
from hearing echo.
When the reference level is set to +0 dB, the AGC
is effectively disabled. Volume control is imple-
mented by digital attenuation in 3 dB steps from
this point on down. The maximum gain is +30 dB,
and the minimum is -12 dB in 3 dB steps. The low-
est gain setting (1111) mutes the signal path. The
signal scaling takes place in between the two can-
cellers, and so does not disturb the echo canceller
as changing gain in the echo path, for example at
the speaker driver, would (see Section 4.1.1.2,
“Adaptive Filter” for more details).
The CS6422 employs supplementary echo suppres-
sion which adds attenuation on top of the cancella-
tion to remove the residual echo. For example, the
CS6422 will engage extra attenuation in the trans-
mit path whenever only the far-end talker is speak-
ing. However, if the near-end talker starts speaking,
this attenuation is removed and the system relies on
the near-end talker’s speech to mask the residual
echo.
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DS295F1
CIRRUS [ CIRRUS LOGIC ]
