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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

Convergence Time

Convergence time is a measure of how quickly the

adaptive filter can model the echo path. From

cleared coefficients, the training signal is injected

into the echo canceller and the time for the ERLE

to reach a given threshold value is the convergence

time. Different customers will have different

threshold levels, so Crystal does not specify con-

vergence time.

The following example will measure convergence

time for the acoustic echo canceller:

Set up the system as for the ERLE test. Clear the

acoustic canceller coefficients through the Micro-

controller Interface. Apply the training signal to

NI, set the coefficients to normal, and simulta-

neously start a timer. Once the measured ERLE

reaches the threshold the system designer desires,

stop the timer. The elapsed time is the convergence

time. A good value for the threshold would be the

AErle value from Register 3, since this would be

the time for the CS6420 to go from half-duplex

mode to full-duplex mode.

A good tool for this measurement is a digital stor-

age oscilloscope set to a slow sweep so that about

five seconds of signal is shown on the screen. One

channel of the oscilloscope should monitor the

ADC input (for an uncancelled reference), and an-

other channel should monitor the echo cancelled

output. This technique is especially effective when

speech is the training signal.

We see about 2-5 seconds of training time using

known good equipment. This time assumes contin-

uous speech as the training signal. Pauses will ex-

tend the convergence time.

Half-Duplex Switching

Although the CS6420 transitions from half-duplex

operation from reset after only a few utterances are

passed through the system, the performance of the

half-duplex is critical to the end-user in cases

where the echo canceller is not adequate. The half-

duplex switching characteristics can be subjective-

ly tested with the following procedure:

Set the CS6420 Microcontroller Interface to the

nominal register values for the system, but clear the

acoustic and network echo canceller coefficients.

This will force the CS6420 to remain in half-duplex

mode.

The most useful test of practical performance

found at Crystal has been the “alternating counting

test.” In this test the person at the near-end counts

all the odd numbers and the person at the far-end

counts all the even numbers. This tests the inter-

ruptibility of the half-duplexer. During testing, sys-

tem parameters for the half-duplex may need to be

changed to accommodate the level of performance

expected for the product. See the Half-Duplex and

Register Definitions sections for more details.

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DS205PP2

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