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CS6420 Datasheet(PDF) 24 Page - Cirrus Logic |
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CS6420 Datasheet(HTML) 24 Page - Cirrus Logic |
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24 / 52 page  CS6420 24 DS205PP2 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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