Cirrus Logic AN168 - Manual (Page 7)

AN168
AN168REV2 7
1.3.3 Volume Control
In most half-duplex systems, volume control is
implemented by changing the gain of the speaker
driver. In a full-duplex system, this is undesirable
because gain changes in the acoustic path require
the echo canceler to readapt, resulting in elevated
levels of residual echo during the training process
or a temporary drop to half-duplex operation.
In CS6422 systems, it is best to implement volume
changes by using the RVol control. The RVol
control provides up to +30 dB of AGC'ed gain to
the receive path, and because the output of the
RVol control is fed both to the echo canceler and to
the DAC driving the speaker, changes in RVol do
not cause changes in the acoustic path, which keeps
the echo canceler from having to readapt. This
portion of the signal flow diagram is shown in
Figure 4.
In general, the RVol control should be set to a value
between +6 dB and +30 dB. In systems which have
a network sidetone (a coupling path between NO
and NI supplied by the phone), the maximum RVol
value may need to be limited due to loop gain
concerns. See the sections entitled Network
Sidetone and Loop Gain for more information.
R
1
R
2
+20 dB
+
CS6422
A0
Attenuation Gain
+
Figure 3. Generic Speaker Driver Configuration
D/A
Acoustic
Echo
Canceler
Σ
A/D
Microcontroller
RVol
TVol
3
+
-
Speaker
Driver
Mic
Preamp
CS6422
Figure 4. Using RVol to Implement Volume Control