CDB6422 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

CS6422

4. DESIGN CONSIDERATIONS

When designing the CS6422 into a system, it is im-

portant to keep several considerations in mind.

These concerns can be loosely grouped into three

categories: algorithmic considerations, circuit de-

sign considerations, and system design consider-

ations.

4.1 Algorithmic Considerations

The CS6422 facilitates full-duplex hands-free

communication via many algorithms running on

the Digital Signal Processor that is the core of the

CS6422. Among these are the algorithms that per-

form the adaptive filtering, the half-duplex switch-

ing, digital volume control, and supplementary

echo suppression.

4.1.1 Full-Duplex Mode

Full-duplex hands-free communication is achieved

through a technique called adaptive filtering. The

basic principle behind adaptive filtering is that the

acoustic path between speaker and microphone can

be modeled by a transfer function which can be dy-

namically determined by an adaptive digital filter.

This principle assumes good update control and

speech/tone detection algorithms to prevent the fil-

ter from mistraining.

4.1.1.1 Theory of Operation

Figure 10 illustrates how the adaptive filter can

cancel echo and reduce loop gain. The echo path of

the system is between points B and C: the speaker

to microphone coupling. A signal injected at A

(sometimes called a “training signal”) is sent both

to B, the input of the echo path, and to F, the input

of the adaptive filter. The signal at B is modified by

the acoustic transducers (speaker and microphone)

and the environment, and received at point C (as an

“Echo”). Meanwhile, assume that the adaptive fil-

ter has exactly the right transfer function to match

the echo path BC, and so the signal at point D is ap-

proximately equal to the signal at point C. After

these are subtracted by the summing element, all

that is left is the error signal at point E, which

should be very small.

If a person were to speak into the microphone at

point C, that signal would pass through the sum-

ming element unchanged because the adaptive fil-

ter had no comparable input to subtract out. In this

manner, the person at A and the person at C may si-

multaneously speak and A will not hear his own

echo.

In the real world, the echo path is not static. It will

change, for example, when people move in the

A

B

F

Adaptive Filter

D-

E

Σ+

C

Figure 10. Simplified Acoustic Echo Canceller Block Diagram

32

DS295F1

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