Cirrus Logic AN168 - Manual

Copyright
© Cirrus Logic, Inc. 2006
(All Rights Reserved) http://www.cirrus.com
AN168
Application Note
ACOUSTIC PATH DESIGN FOR FULL-DUPLEX CELLULAR
HANDS-FREE CAR KITS
This application note describes a design procedure
coupled with some testing procedures to enable a
system designer to implement a low cost
full-duplex cellular hands-free system for cars
using the CS6422 Enhanced Echo Cancelling IC.
This application note focuses on the design of the
acoustic path, that is, the path between the acoustic
output (AO) and the acoustic input (APO) of the
CS6422. The acoustic path contains the speaker
driver, the speaker, the air path between the
speaker and the microphone, the microphone, and
the microphone preamp.
Additionally, a suggested set of CS6422
configuration parameters is presented as well as
some system-level tests that can be used to
optimize the parameters for a particular
environment.
1. DESIGN PROCESS AND
CONSIDERATIONS
There are four parts to the hands-free design
process: mechanical design, electrical design, echo
canceler coefficient optimization, and testing. This
note will investigate all four.
1.1 Design Flow
The design flow for full-duplex systems is as
follows:
1) Design the mechanical and electrical systems
for low distortion, specifically less than
2% THD across frequency.
2) Install the equipment in the target test system,
usually a car.
3) Tweak the mic preamp gain to achieve -9 dB
acoustic coupling.
4) Load the starting point example CS6422 regis-
ter configuration.
5) Perform parameter optimization tweaking
tests.
6) Test under actual driving conditions. If neces-
sary, modify speaker/mic placement and test
again.
MAR ‘06
AN168REV2