CS6420 Datasheet, PDF (27/52 Pages) Cirrus Logic – FULL DUPLEX SPEAKERPHONE CHIP

CS6420
keep it disengaged. We recommend using larger
values of TSBias relative to TSThd settings in or-
der to facilitate ease of near-end speech transmis-
sion. For example, the default setting for TSThd is
15 dB and 18 dB for TSBias.
In some scenarios, especially when the dynamic
range of volume control is significantly large, we
also recommend the use of different combinations
of TSThd and TSBias setting relative to output vol-
ume of the acoustic interface.
Receive Suppression
The “default on” receive suppressor is nominally
attenuating unless far-end speech is present. This
behavior is more consistent with behavior observed
in modern speakerphones, and helps keep noise
levels low.
One side effect of this scheme is that a constant
power signal, such as noise from a noise generator
or a tone, will eventually be attenuated when the
background noise level estimate turns off the re-
ceive suppression speech detector. See the section
on Speech Detection from more details.
RSThd (Register 2, bits 5 and 4) sets the speech de-
tection threshold of the suppressor’s speech detec-
tor. See the Speech Detection section for more
details.
Circuit Design
The design of the CS6420 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 CS6420.
Interface Considerations
Of the CS6420 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
determine how well the echo canceller can per-
form.
Analog Interface
The Analog Interface feeds information about the
echo path to the adaptive filter, so it is critical that
this interface be well designed. Using high-quality
transducers and circuits that guarantee low-distor-
tion and minimal clipping are essential to the suc-
cess of any echo canceller based design.
As mentioned in the Adaptive Filter section, the
adaptive filter assumes that the echo path is linear
and time-invariant. As such, poor quality speakers
are a common cause of poor echo canceller perfor-
mance due to their high distortion. Speakers must
be selected with their linearity in mind. In general,
the speaker should have less than 2% Total Har-
monic Distortion. This will result in distortion
terms 34 dB below the desired signal, enough head-
room for the echo canceller to function adequately.
The other major consideration in the design of the
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.
That which worked for half-duplex speakerphones
will not necessarily work for full-duplex speaker-
phones. Microphone amplifier circuitry is also sus-
pect when looking for sources of clipping and
distortion.
Microcontroller Interface
The Microcontroller Interface is the only asynchro-
nous digital connection to the CS6420, 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.
DS205PP2
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