Cirrus Logic AN168 - Manual (Page 13)

AN168
AN168REV2 13
This parameter can be tested using the
Half-Duplex Alternate Counting Test as de-
scribed in the Tests section.
9) RSD controls the enable/disable of the receive
suppression engine. The receive suppressor is a
noise squelch that provides 24 dB of attenua-
tion to the receive path when the far-end talker
is silent. The receive suppressor operates in
half-duplex and full-duplex modes. When the
far-end talker is silent, the signal sent to the
speaker drops to almost nothing, that is, back-
ground noise is not transmitted. If this is unde-
sirable, the receive suppressor can be disabled
by setting RSD to '1', allowing the noise to pass
to the speaker unattenuated.
1.4.3 Network Sidetone
Some cellular phones provide a coupling path
between the network transmit and network receive
paths which is used when a full-duplex earpiece
and mic are used. This sidetone path provides an
echo between the mic and the earpiece to let the
user know that the phone is active, similar to the
sidetone supplied by the Central Office in a
standard analog phone handset.
The presence of a network sidetone, here defined as
a coupling path between the NO pin and the NI pin
on the CS6422, increases the complexity of the
system considerably because it forms a closed loop
with the receive path, the acoustic path, and the
transmit path. When a system has a closed loop,
care must be taken to ensure that the loop gain of
the system remains below a factor of '1', or 0 dB, or
howling, regenerative feedback, will result. Figure
9 illustrates.
It is strongly recommended that the network
sidetone on the cellular phone be disabled if
possible. Most cellular phones that provide only an
analog hands-free interface have a means of
disabling the sidetone. Cellular phones that provide
both analog and digital hands-free interfaces
Speaker
Driver
Mic
Preamp
Air
Coupling
TVol
NO Gain
APO
AO NI
NO
Network
Sidetone
Cell Phone CS6422
Closed Loop
RVol
NI
Gain
Figure 9. Loop Gain with Network Sidetone