CDB6403 datasheet(10/54 Pages) CIRRUS

Part #	CDB6403

Description	Echo-Cancelling Codec

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Manufacturer	CIRRUS [Cirrus Logic]

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The Central Processing Unit (CPU) does all the

other miscellaneous processing, like update con-

trol and double talk detection. This processing

has a critical influence on overall echo-cancella-

tion performance.

Double-talk detection is a

particularly important part of this processing.

Double-talk detection and other algorithms were

carefully developed and validated at Crystal un-

der real-world conditions.

To increase the CS6403’s echo return-loss en-

h an cement (ERLE), supplemental echo

suppression is used. A sophisticated voice-detec-

tion algorithm is used to reduce echo with

minimal impact on conversation quality, assuring

the highest quality conversation.

Figure 3 describes the functional behavior of the

CS6403 in a typical application.

Digital data

from the far-end interface comes into SDI of the

CS6403 where it is acted upon by the various

algorithms running in the CPU.

First, a High-

Pass Filter eliminates DC offset and low

frequency noise before sending the far-end input

data onto the summing node for the network

echo canceller.

Assuming there is no speech

from the near-end, the signal after the summing

node is unaffected.

The signal then passes on to the AGC/Volume

Control block where the signal level is boosted,

if necessary. The volume control is implemented

in part by the AGC (for more details, see the

section entitled "Embedded Signal Processing

Functions").

The signal then passes on to the Non-linear Echo

Control block which controls the half-duplex

failsafe and the supplementary echo suppressor

which act to enhance and supplement the per-

formance of the echo canceller.

The signal after this block is then fed to both the

speaker output and the input to the acoustic echo

canceller (implemented by the AFP).

The

speaker output couples to the microphone input

by various echo paths.

The signal received at

the microphone is then filtered and sent on to the

summing node of the Acoustic Echo Canceller.

The Acoustic Echo Canceller constructs a model

of the echo paths between the speaker and mi-

crophone and processes its input signal with its

digital representation of the echo paths. As such,

its output should very closely match the input

from the microphone and so the output from the

summing node should be a very small signal,

which is referred to as the "error signal." This

error signal is fed back to the echo canceller to

let it adapt its performance should there be a

change in the echo path.

The Non-linear Echo Control block following

the summing node further attenuates any vestiges

of signal received at the microphone that origi-

nated from the speaker. This signal is then sent

to the far-end output by SDO as well as to the

input of the Network Echo Canceller, where a

function similar to that performed by the Acous-

tic Echo Canceller is performed.

-

+

SDI

SDO

SPKROUT

CS6403

MICIN

Near-End Output

Near-End Input

Far-End Input

Far-End Output

DSP or

PCM Codec

*

* Optional

*

Non-linear

Echo

Control

High Pass

Filter

High Pass

Filter

Network

Echo

Canceller

Acoustic

Echo

Canceller

AGC/

Volume

Control

Non-linear

Echo

Control

Figure 3. Functional Diagram

CS6403

10

DS192PP6

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