Cirrus Logic AN168 - Manual (Page 8)

AN168
8 AN168REV2
1.3.4 Acoustic Coupling
Figure 5 shows the three most common places for
distortion to be introduced into the acoustic path.
These are the speaker driver, the speaker, and
clipping at the A/D converter after the mic preamp.
With careful choice of the speaker and speaker
driver gain, we can eliminate the first two by using
the techniques previously discussed. The third
distortion source, clipping at the A/D converter, is
controlled by limiting the amount of acoustic
coupling.
The acoustic coupling is defined as the gain (or
loss) between the AO pin and the APO pin on the
CS6422, with TGain set to 0 dB. If TGain is set to
a non-zero value, then the TGain value is added to
the AO/APO gain number to compute the amount
of acoustic coupling.
The acoustic coupling is determined by 5 factors:
the speaker driver gain, the speaker efficiency, the
air coupling between the speaker and the
microphone, the microphone sensitivity, and the
mic preamp gain. Assuming the speaker and mic
have been chosen, the remaining design variables
are the speaker driver gain, the mic preamp gain,
and the speaker and mic position.
Usually, the speaker driver gain is chosen based on
the linearity requirements previously described.
The speaker and mic placement are determined by
ergonomic factors and the desired acoustic path
stability described above. The remaining variable
is the mic preamp gain, which is typically set such
that the worst-case acoustic coupling is between
-9 dB and -15 dB, the first number being the
preferred design target, as shown in Figure 6.
The acoustic path response is highly frequency
dependent. The contributions of the speaker driver
and the mic preamp to the frequency response are
essentially negligible since both of these amplifiers
typically have a stable and well-behaved frequency
response. The dominant factors in the frequency
response of the acoustic path are the speaker's
inherent frequency response, the microphone's
inherent frequency response, and the frequency
response of the path between the speaker and the
mic which is strongly affected by the speaker's
housing. The flatter the frequency response, the
better the echo cancellation.
Figure 7 shows an example acoustic path frequency
response for a speaker and microphone separated
by approximately one meter.
The signal at APO will visibly clip for signals
greater than +5 dBV (5 Vpp). Keep in mind that the
acoustic A/D converter clips at 0 dBV (2.8 Vpp)
when TGain is set to 0 dB.
1.3.5 Setting the Mic Preamp Gain
As stated above, the design goal is to have the
worst-case value for the acoustic coupling, the
highest value across the frequency band of interest,
less than or equal to -9 dB. Strictly speaking, it
need only be less than 0 dB to avoid clipping at the
acoustic A/D converter. The additional 9 dB
provides margin for component tolerance variation
(dominated by speaker variation), component
installation (dominated by speaker/mic placement),
and acoustic path variation (dominated by the
position of the driver, passengers, and objects in the
car). The mic preamp gain is adjusted to achieve
the desired level of acoustic coupling.
There are two methods that can be used to set the
acoustic coupling: the frequency response method
and the loop gain method. The frequency response
method is good because it provides frequency
response information that can be used to increase
the quality of the system (flat frequency response is
desired). The loop gain method is quick, easy, and
requires no additional test hardware beyond the
ability to configure the CS6422's registers.
In the frequency response method, the acoustic
path frequency response, the gain between the AO
and the APO pins on the CS6422, is measured by
automated test equipment and plotted. The