Cirrus Logic AN168 - Manual (Page 4)

AN168
4 AN168REV2
1.2 Mechanical Design
The performance of full-duplex hands-free designs
is strongly influenced by the mechanical hardware,
far more so than comparable half-duplex systems.
Upgrading a half-duplex design by adding a
full-duplex echo controller without changing the
half-duplex mechanical hardware typically results
in a system whose performance is unacceptable.
This section describes the critical parameters of the
mechanical design that ensure quality full-duplex
operation.
The mechanical design consists of speaker and
microphone component selection, speaker housing
and mounting, and speaker and microphone
placement in the car..
1.2.1 Selecting the Acoustic Components
1.2.1.1 Speaker Requirements
The quality of the speaker in a full-duplex system
is critical to system performance because echo
cancelers are sensitive to signal distortion. Because
the echo canceler uses a linear filter to model the
acoustic path, the acoustic path to be modeled must
be linear in order for the echo canceler to work
well. The total worst-case distortion in the acoustic
path, which includes the speaker driver, the
speaker, the microphone, and the microphone
preamp, should be less than 2% THD across
frequency.
The speakers in automotive hands-free systems are
typically driven with a maximum RMS power
between 0.5 and 5 Watts. In order to maintain
2% THD or less, it is necessary to install a speaker
whose RATED power is at least twice as large as
the maximum power to be driven. For example, if
we wish to drive 2 Watts into the speaker, then the
speaker's RATED power should be 4 Watts or
greater. The RATED power of a speaker is the
power at which the distortion performance is
specified. The typical distortion specification for a
speaker operating at its RATED power is either 5%
or 10% THD, depending on how the manufacturer
specifies distortion.
Speakers are also specified with a MAX power
rating. The MAX power is the power level above
which the speaker can be damaged. The RATED
power, if it is given, is typically about half as large
as the MAX power. Thus if the RATED power is
not given, a good rule is to assume that the RATED
power is about half of the MAX power.
NOTE: The above RATED power/MAX power
generalization does not hold for new generation
ultra-thin Mylar speakers. The poor distortion
performance of these thin speakers makes them
unsuitable for full-duplex car designs. Thick
speakers exhibit a more linear behavior than thin
speakers of equal diameter and are preferred in
full-duplex designs.
1.2.1.2 Microphone Requirements
Less care is needed in microphone selection than in
speaker selection. Almost any standard
inexpensive electret microphone will work because
microphones are inherently fairly linear devices.
Microphones that cancel background noise due to
their mechanical construction are preferred over
those that do not. Microphones that are
omnidirectional are preferred over those that are
directional.
Speaker
Figure 1. Speaker Distortion