CDB6422 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

CS6422

loop (near-end speaker/mic to far-end speaker/mic)

during a double-talk scenario (where both near-end

and far-end parties are talking at the same time).

The CS6422 implements an optional attenuation

feature that introduces a programmable amount of

loss in the transmit and/or receive directions dur-

ing double-talk to alleviate stability concerns with-

out sacrificing speaker volume. This allows for

higher speaker volume levels at both ends of the

call without compromising stability.

4.1.4.4 Noise Guard

Noise Guard is an optional noise squelch feature

that operates in the transmit path (near-end micro-

phone to far-end speaker). In traditional systems, if

the near-end talker is located in a noisy environ-

ment, the near-end system will remain in transmit

mode and transmit that noise to the far-end listener.

While this may be bothersome to the far-end listen-

er using a standard handset, this creates a real prob-

lem if the listener is using a traditional half-duplex

speakerphone because the far-end phone may stay

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.

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 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

determine how well the echo canceller can per-

form.

4.2.1.1 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 Section 4.1.1.2, “Adaptive Filter”,

the adaptive filter assumes that the echo path is lin-

ear and time-invariant. As such, poor quality

speakers are a common cause of poor echo cancel-

ler performance due to their high distortion. Speak-

ers must be selected with their linearity in mind. In

general, the speaker should have less than 2% Total

Harmonic Distortion (THD). This will result in dis-

tortion terms 34 dB below the desired signal,

enough headroom 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.

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.

4.2.1.2 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.

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DS295F1

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