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CS6422-CSZ Datasheet(PDF) 41 Page - Cirrus Logic |
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CS6422-CSZ Datasheet(HTML) 41 Page - Cirrus Logic |
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41 / 48 page  CS6422 41 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. CS6422 DS295F1 41 |
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