CDB6422 データシートの表示（PDF） - Cirrus Logic

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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

Fs

0dB

-30dB

Fs

0dB

-30dB

Fs

0dB

-30dB

CS6422

Fs

0dB

-30dB

(a) Input Signal

t

(b) AGC Attenuation

t

Fs

0dB

-30dB

(c) AGC Gain

t

(d) Input Signal

t

(e) AGC Gain

t

Figure 11. How the AGC works (TVol = +30 dB)

Any signal greater than 30 dB below full-scale (a),

is scaled down to 30 dB (b). This signal is then

scaled up +30 dB (the reference level) to provide

the final output (c). Note that the combination of at-

tenuation and gain results in less than +30 dB total

gain being applied. If the input signal is below

30 dB below full-scale (d), no attenuation is added

and the full +30 dB of gain is applied to the signal

(e).

When the reference level is set to +0 dB, the AGC

is effectively disabled. Volume control is imple-

mented by digital attenuation in 3 dB steps from

this point on down. The maximum gain is +30 dB,

and the minimum is -12 dB in 3 dB steps. The low-

est gain setting (1111) mutes the signal path. The

signal scaling takes place in between the two can-

cellers, and so does not disturb the echo canceller

as changing gain in the echo path, for example at

the speaker driver, would (see Section 4.1.1.2,

“Adaptive Filter” for more details).

4.1.4 Suppression

Echo cancellation is somewhat of a misnomer in

that echo is merely attenuated, not entirely can-

celled. Some residual echo still exists after the

summing node. This residual echo, though low in

amplitude, may be audible when the near-end talk-

er is not speaking. Suppression further attenuates

the echoed signal, preventing the far-end listener

from hearing echo.

The CS6422 employs supplementary echo suppres-

sion which adds attenuation on top of the cancella-

tion to remove the residual echo. For example, the

CS6422 will engage extra attenuation in the trans-

mit path whenever only the far-end talker is speak-

ing. However, if the near-end talker starts speaking,

this attenuation is removed and the system relies on

the near-end talker’s speech to mask the residual

echo.

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DS295F1

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