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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

CS6420, is controlled by GB. The network echo

canceller does not support graded beta.

Graded beta is an architectural enhancement to the

CS6420 which takes advantage of the fact that

acoustic echoes tend to decay exponentially with

time. The CS6420 can increase the beta, or update

gain, for the coefficients of the adaptive filter

which occur earlier in time and decrease it for those

that occur later in time, which increases conver-

gence speed while maintaining stability. In order to

make this improvement, there is an implicit assump-

tion that the decay rate of the echo is known. The

graded beta control allows the system designer to ad-

just this. For very acoustically live rooms, use either

no decay (00) or slight decay (11). Cars and acousti-

cally dead rooms can benefit from the most rapid de-

cay (01).

ACC - Acoustic Coefficient Control

The coefficients of the AEC adaptive filters in the

CS6420 are controlled by ACC. The default posi-

tion (00) yields normal operation, which means the

coefficients are free to adjust themselves to the

echo path in order to cancel echo. When set to the

clear position (01), the adaptive filter coefficients

are all held at zero, so the echo canceller is effec-

tively disabled. Note that unless the half-duplex

mode is disabled, this will force the CS6420 into

half-duplex mode. The freeze position (10) causes

the coefficients to hold their current values.

RVol - Receive Volume Control

Volume in the receive path is set by RVol. The vol-

ume control in the receive direction is implemented

by a peak-limiting automatic gain control (AGC)

and digital attenuation at the near-end output DAC.

The AGC is discussed in detail in the Design Con-

siderations section. See the sub-section on AGC

for a full explanation of how it functions.

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 -60 dB in 3 dB steps. The low-

est gain setting (11111) mutes the receive path.

The default setting for the receive reference level is

+18 dB.

TGain - Transmit Analog Gain

TGain selects the amount of additional on-chip an-

alog gain to be supplied to the acoustic input of the

CS6420. A programmable gain amplifier (PGA)

exists before each ADC which allows 0 dB, 6 dB,

9.5 dB, or 12 dB of gain to be added to the signal

path. The acoustic side defaults to 0 dB of gain.

Note: Changing the analog gain will change the full-

scale voltage as applied to the input pin. Make

sure that the ADC input does not clip with the gain

stage on.

DS205PP2

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