CDB6420 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

Fs

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-30dB

Fs

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-30dB

Fs

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-30dB

CS6420

Fs

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-30dB

(a) Input Signal

t

(b) AGC Attenuation

t

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-30dB

(c) AGC Gain

t

(d) Input Signal

t

(e) AGC Gain

t

Figure 9. How AGC works (TVol = 00000)

transmit channel will engage extra attenuation

whenever only the far-end talker is speaking. How-

ever, if the near-end talker starts speaking, the at-

tenuation is removed and the system relies on the

near-end talker’s speech to mask residual echo.

Suppression causes some modulation of the per-

ceived background noise which may be distracting

to some users. As a result, it may be desirable to

limit the suppression attenuation to the minimum

necessary. The CS6420 provides TSAtt (Register

3, bits 9 and 8) to control the amount of attenuation

introduced by suppression in the transmit channel.

Receive suppression attenuates by 24 dB.

The suppression in the transmit suppression and

that in the receive direction work very differently.

The transmit suppression works in a “default off”

mode while the receive suppression is “default on.”

Transmit Suppression

The transmit suppressor attenuates the transmit

path when only far-end speech is present, hence the

name “default off.” This ensures that the suppres-

sion engages only when necessary.

Recall that the purpose of Transmit Suppression is

to mask residual echo by inserting additional

loss/attenuation in the transmit path in the scenario

when only far-end speech is present; the residual

echo, if any, in double-talk being masked by near-

end speech assuming reasonable levels of ERLE.

There are two controls/tweekable parameters for

governing the behavior of Transmit Suppression.

The two controls are adjustable through the Micro-

controller Interface, and they are TSThd (Register

3, bits 5 and 4) and TSBias (Register 3, bits 7 and

6). TSThd is the primary control and should be ad-

justed before changing the value of TSBias from its

default setting. TSThd sets the ERLE expectation

to be used in discriminating between near-end

speech and far-end echo. This control setting will

by far predominate in affecting the manner in

which Transmit Suppression behaves.

TSBias is a secondary control. This is to be adjust-

ed after the system designer is more or less satisfied

with the behavior of Transmit Suppression with the

TSThd set. It affects the ease with which a near-end

talker may disengage Transmit Suppression and

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DS205PP2

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