CS6420-CS datasheet(27/52 Pages) CIRRUS

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CS6420

DS205PP2

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.

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