CS6422
CS6422
loop (near-end speaker/mic to far-end speaker/mic) determine how well the echo canceller can per-
during a double-talk scenario (where both near-end form.
and far-end parties are talking at the same time).
4.2.1.1 Analog Interface
The CS6422 implements an optional attenuation
The Analog Interface feeds information about the
feature that introduces a programmable amount of
echo path to the adaptive filter, so it is critical that
loss in the transmit and/or receive directions dur-
this interface be well designed. Using high-quality
ing double-talk to alleviate stability concerns with-
transducers and circuits that guarantee low-distor-
out sacrificing speaker volume. This allows for
tion and minimal clipping are essential to the suc-
higher speaker volume levels at both ends of the
cess of any echo canceller based design.
call without compromising stability.
As mentioned in Section 4.1.1.2, “Adaptive Filter”,
the adaptive filter assumes that the echo path is lin-
4.1.4.4 Noise Guard
Noise Guard is an optional noise squelch feature ear and time-invariant. As such, poor quality
that operates in the transmit path (near-end micro-
speakers are a common cause of poor echo cancel-
phone to far-end speaker). In traditional systems, if ler performance due to their high distortion. Speak-
the near-end talker is located in a noisy environ- ers must be selected with their linearity in mind. In
ment, the near-end system will remain in transmit general, the speaker should have less than 2% Total
mode and transmit that noise to the far-end listener.
While this may be bothersome to the far-end listen- tortion terms 34 dB below the desired signal,
er using a standard handset, this creates a real prob- enough headroom for the echo canceller to function
Harmonic Distortion (THD). This will result in dis-
lem if the listener is using a traditional half-duplex adequately.
speakerphone because the far-end phone may stay
The other major consideration in the design of the
in receive mode and not allow the far-end talker to
be heard. Noise guard eliminates this problem by
squelching the transmit channel at the near-end un-
less near-end speech is detected, permitting the far-
end speakerphone to switch normally during the
conversation.
analog interface is that the circuitry that processes
the transducer signals not clip or distort it. For ex-
ample, a common problem is the use of a speaker
amplifier with a fixed gain, which clips when driv-
ing the speaker. Although the distortion may not be
objectionable to the human ear, it will prevent the
adaptive filter from modeling the path correctly.
Speakers and microphones which worked for half-
duplex speakerphones will not necessarily work for
full-duplex speakerphones. Microphone amplifier
circuitry is also suspect when looking for sources
of clipping and distortion.
4.2
Circuit Design
The design of the CS6422 interface circuitry plays
an important role in achieving optimum perfor-
mance. The actual circuit design is important, espe-
cially the analog interface. Proper grounding and
layout will help minimize the noise that might get
coupled into the CS6422.
4.2.1.2 Microcontroller Interface
The Microcontroller Interface is the only asynchro-
nous digital connection to the CS6422, so it is the
most likely place for digital noise coupling to be a
problem. The interface itself is fairly straightfor-
ward and requires only three pins from a microcon-
troller.
4.2.1 Interface Considerations
Of the CS6422 interfaces, the analog interface and
the microcontroller interface are the most impor-
tant to pay special attention to during circuit de-
sign. The analog interface especially will
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DS295F1
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