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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

efficients can change. If beta is too low, the adap-

tive filter will be slow to adapt. Conversely, if it is

too high, the filter will be unstable and will create

unwanted noise in the system.

In most echo canceller implementations, the beta is

a fixed value for all the filter coefficients. In some

situations, though, through knowledge of the char-

acteristics of echo path response, the beta can be

varied for groups of coefficients. This preserves

stability by allowing the beta to be higher for some

coefficients and compensating by reducing beta be-

low nominal for others.

For example, acoustic echo tends to decay expo-

nentially, so the first taps need to be large and the

later taps will be small. Having a large beta for the

first taps will allow those taps to be adapted faster,

while having a small beta for the later taps will

keep the filter stable. This has an added benefit of

suppressing the spurious taps mentioned in the Pre-

Emphasis Filter section above.

The Microcontroller Interface allows four settings

for graded beta: none, 0.19 dB/ms, 0.38 dB/ms, and

0.75 dB/ms. Use 0.75 dB/ms for acoustically dead

rooms or cars, and 0.19 dB/ms or no grading of beta

for large, or acoustically live rooms.

Update Control

As mentioned in the Theory of Operation section,

the update control algorithms are the heart of any

useful echo canceller implementation. Aside from

telling the adaptive filter when to adapt, they are re-

sponsible for correcting performance when the path

changes too quickly for the filter. For example, if

the adaptive filter is actually adding signal power

instead of cancelling, the update control algorithms

will reset the adaptive filter to cleared coefficients,

forcing it to restart.

Speech Detection

The CS6420 detects speech by using power estima-

tors to track deviations from a background noise

power level. The power estimators filter and aver-

age the raw incoming samples from the ADC.

A background noise level is established by a regis-

ter that increases 3 dB at intervals determined by

NseRmp (Register 2, bits 7 and 6). When the power

estimator level rises, the background noise level

will slowly increase to try to match it. When the

power estimator level is below the background

noise level, the background noise level is quickly

reset to match the power estimator level. This

method allows significant flexibility in tracking the

background noise level.

Speech is detected when the power estimator level

rises above the background noise level by a given

threshold. The half-duplex receive speech detector

threshold is set by RHDet (Register 2, bits 11 and

10), the half-duplex transmit speech detector

threshold is set by THDet (Register 3, bits 11 and

10), and the receive suppression speech detector

threshold is set by RSThd (Register 2, bits 5 and 4).

The transmit speech detectors for both half-duplex

and suppression default to 5 dB.

Note that constant power signals which persist for

long durations, such as tones from a signal genera-

tor, will be detected as speech only as long as the

background noise level has not risen to within the

speech detection threshold of the signal power.

When a tone has persisted for long enough, the

background noise level will be equal to the power

estimator level, and so the tone will no longer be

considered speech. This duration is dependent

upon the power difference between the signal and

the ambient noise power, as well as NseRmp. It

should be noted that the CS6420 has a tone detector

to prevent updates when tones are present and allow

tones to persist regardless of the speech detectors.

Half-Duplex Mode

In cases where the system relies on the echo cancel-

ler for stability, a fail-safe mechanism must be in

place for instances when the echo canceller is not

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DS205PP2

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