Cirrus Logic AN168 - Manual (Page 11)

AN168
AN168REV2 11
The register settings to accomplish the above are as
follows:
reg 0: 47a0 (or c7a0 if internal mic preamp is
used)
reg 1: 26a2
reg 2: 0004 (default)
reg 3: 0006 (default)
reg 4: 0008 (default)
reg 5: 033a
Note: If the mic preamp gain is not easily
adjustable in the test circuit, coarse amounts of gain
can be added by using the TGain control, which can
be set to 0 dB, +6 dB, +9.5 dB, or +12 dB.
1.3.6 Acoustic Sidetone
When the coupling path between the speaker and
the microphone is relatively consistent, linear, and
has a high signal-to-noise ratio (SNR), the CS6422
provides good echo cancellation and makes good
training decisions. In the car environment, the SNR
of the acoustic path can be degraded significantly
by road and engine noise and the separation
between the speaker and the mic. In these systems,
it is often useful to introduce a strong, linear,
predictable coupling path electrically by using an
acoustic sidetone.
The acoustic sidetone provides 3 main benefits:
1) The presence of a strong path decreases conver-
gence time, meaning it decreases the time the
CS6422 spends in half-duplex.
2) The linear path enhances stability in systems in
which the strongest real (air) path is distorted.
Note that even though the echo canceller will
not cancel the nonlinear elements of the acous-
tic echo, it will make better decisions regarding
when to engage the supplementary suppression
algorithms to mask such echo. This results in
improved performance during far-end sin-
gle-talk.
3) The consistent path provides an echo path that
is independent of the acoustic environment,
making the system less sensitive to path chang-
es and noise. This enhances full-duplex perfor-
mance by reducing the tendency of the CS6422
to drop to half-duplex when the driver moves.
The amount of sidetone required depends on
several factors. Typically, a good number is
between -24 dB and -12 dB. To be useful, the
electrical coupling should be about as strong as the
strongest typical air coupling, but not much
stronger. A good starting point for systems whose
peak acoustic coupling is -9 dB is -18 dB of
acoustic sidetone. The acoustic sidetone can be
implemented in CS6422 systems by using the ASdt
control, which is configurable to none, -24 dB,
-18 dB, or -12 dB.
1.4 Echo Canceler Parameter
Optimization
One of the benefits of the CS6422 is its high degree
of configurability. Whereas the number of
parameters may seem daunting at first, there are
only a few that need to be tweaked to optimize
performance. The rest can be set once and left
alone.
1.4.1 Starting Example
The following is an example register configuration
that is useful as a starting point for cellular car
hands-free systems.
Note: Actual performance testing should be
performed in a car, not a lab. This is because the car
and the lab present different acoustic environments
to the echo canceler, and the goal is to optimize the
parameters for the target environment, which
requires testing in that target environment.
The following parameter set assumes that there is
no coupling on the network interface to the phone.
If there is a network coupling path, see the Network
Sidetone and Loop Gain sections below.