Cirrus Logic CS6422 - Manual (Page 41)

CS6422
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4.3.2.2 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 CS6422 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.
4.3.2.3 Half-Duplex Switching
Although the CS6422 transitions from half-duplex
to full-duplex operation from reset after only a few
utterances are passed through the system, the per-
formance of half-duplex is critical to the end-user
in cases where the echo canceller is not adequate.
The half-duplex switching characteristics can be
subjectively tested with the following procedure:
Set the CS6422 Microcontroller Interface to the
nominal register values for the system, but clear the
acoustic and network echo canceller coefficients.
This will force the CS6422 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 Section 4.1.2, “Half-
Duplex Mode” and Section 3.2.2, “Register Defini-
tions”for more details.
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