Cirrus Logic AN168 - Manual (Page 10)

AN168
10 AN168REV2
maximum value of this curve is then noted, and
gain is added or subtracted at the mic preamp in
order to set this maximum value to -9 dBV (with
TGain set to 0 dB). This procedure is further
described in the Tests section of this note.
The loop gain method uses howling to determine
the optimum mic preamp gain. In short, the phone
network is disconnected from the CS6422, and
TVol, RVol and NSdt are used to create a +9 dB
path between APO and AO inside the CS6422. The
system will howl, go into regenerative feedback, at
the point that the total loop gain reaches a factor of
'1', or 0 dB. This happens whenever the gain
between AO and APO outside the CS6422 reaches
-9 dB. The frequency of the howl is the frequency
of the maximum loop gain, which is dominated by
the speaker, microphone, and the air path between
the two. Figure 8 illustrates.
The loop gain procedure is as follows:
1) Configure the CS6422 with its default configu-
ration, with the exception of the following:
a) Mic = ‘1’ or ‘0’, depending on whether the
internal mic preamp is used or not
b) TSD = RSD = HDD = ‘1’, transmit and re-
ceive suppressors and half-duplex mode are
disabled
c) ACC = NCC = 'cleared', echo cancelers are
forced to a cleared state to prevent updates
d) AECD = NECD = ‘1’, echo cancelers are
disabled
e) TVol = +12 dB
f) NSdt = -12 dB
g) RVol = +9 dB
2) Adjust the mic preamp gain (or the speaker
driver gain) until the system is just on the verge
of howling. At this point the gain between AO
and APO will be the desired -9 dB.
Speaker
Driver
Mic
Preamp
A
O
CS6422
APO
RVol - Receive
Volume
TVol - Transmit
Volume
NSdt - Network
Sidetone
Air Coupling
0dB
-?dB
12dB
+9dB
-12dB
Adjust
?dB
Goal is -
9dB
D
isconn
ect
Ne
tw
or
k
Figure 8. Setting the Acoustic Coupling