CS6420-CS datasheet(29/52 Pages) CIRRUS

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CS6420

DS205PP2

The basic constraint on getting good echo canceller

performance is that the maximum output should

not clip when coupled to the input. For example, if

in a speakerphone, AO provides 1 Vrms to a speak-

er, the reflections reaching the microphone should

present no more than 1 Vrms to the Acoustic ADC.

In fact, it is advisable to allow 6 dB or even 12 dB

of margin, such that in the above example, the sig-

nal present at the Acoustic ADC is 250 mVrms.

After this coupling level is established, the desired

signal gain must be established. To continue from

the previous example, the transmit gain must be ad-

justed to make sure the near-end talker is easy to

hear at the far-end. If the signal from the near-end

talker clips at the ADC, it is not significant to the

echo path because the AEC should not be updating

anyway.

In general, to minimize noise, system gain should

be concentrated before the ADC. However, this is

not practical in all cases, mostly because of the cou-

pling constraint. The CS6420 offers the AGC’d

gains provided by TVol and RVol to help provide

the desired gain.

The CS6420 offers two different programmable

gain sources: TGain/RGain and TVol/RVol. TGain

and RGain switch in different size sampling capac-

itors at the ADC to provide a choice of 0 dB, 6 dB,

9.5 dB, and 12 dB of analog gain. TVol and RVol

introduce digital gain and attenuation in 3 dB steps.

The difference is significant in that the digital gain

will gain up the noise of the ADC as well as the de-

sired signal, whereas the analog gain will not.

Testing Issues

The following tests are suggestions for measuring

echo canceller and half-duplex performance.

ERLE

Echo Return-Loss Enhancement (ERLE) is a mea-

sure of the attenuation that an echo canceller pro-

vides. The number is an expression of the ratio of

the level of signal without the echo canceller com-

pared to the level of signal with the echo canceller.

When measuring ERLE, it is important that any po-

tential acoustic loops be broken; so to measure the

ERLE of the Acoustic Canceller, the NO output

should be disconnected from the rest of the net-

work. This will prevent feedback which could oc-

cur when all of the CS6420’s failsafes are disabled.

The following example outlines the steps necessary

to measure the ERLE of the acoustic echo cancel-

ler.

It is important to choose a good test signal for the

tests to be valid. As mentioned in the Adaptive Fil-

ter section, the CS6420 does not work optimally

with white noise. The best signal to use would be a

repeatable speech signal, like a recording of some-

one counting or saying “ah.”

Use the Microcontroller Interface to disable trans-

mit and receive suppression as well as half-duplex.

Allocate all the taps to the Acoustic Canceller in or-

der to allow full-duplex without the Network Can-

celler being trained. The gains should be set

appropriate for good system performance.

The first measurement is a baseline figure of per-

formance with no echo canceller. Use the Micro-

controller Interface to clear the acoustic canceller

coefficients. Inject the test signal at NI and measure

the rms voltage at NO. This measurement gives the

baseline coupling level (denominator).

Use the Microcontroller Interface to set the acous-

tic canceller coefficients to normal which will al-

low the adaptive filter to adapt. Inject the test signal

at NI and allow a few seconds for the filter to adapt.

Measure the rms voltage at NO. This measurement

gives the cancelled echo level (numerator).

Convert both voltages to decibels and subtract the

echo cancelled level from the baseline level to cal-

culate the ERLE. At the factory, with known good

components, we typically see 30 dB of ERLE with

speech.

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