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ZL38003 Datasheet(PDF) 10 Page - Zarlink Semiconductor Inc |
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ZL38003 Datasheet(HTML) 10 Page - Zarlink Semiconductor Inc |
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10 / 51 page  ZL38002 Data Sheet 10 Zarlink Semiconductor Inc. 2.2 Noise Suppression The ZL38002 also utilizes noise suppression which can be used to reduce idle channel noise from emanating from the acoustic end. By setting a threshold value in the lower nibble (bits 0-3) of the MCR2 register on page 0 address 01H, idle channel noise below the threshold is zero-forced. The threshold limits ranges from 0 to 16 (based on a 16 bit 2's complement) with a value of 0 disabling suppression. 2.3 Adaptation Speed Control The adaptation speed of the acoustic echo canceller is designed to optimize the convergence speed versus divergence caused by interfering near-end signals. Adaptation speed algorithm takes into account many different factors such as relative double-talk condition, far end signal power, echo path change and noise levels to achieve fast convergence. 2.4 Advanced Non-Linear Processor (ADV-NLP) After echo cancellation, there is likely to be residual echo which needs to be removed so that it will not be audible. The ZL38002 uses an NLP to remove low level residual echo signals which are not comprised of background noise. The operation of the NLP depends upon a dynamic activation threshold, as well as a double-talk detector which disables the NLP during double-talk periods. The ZL38002 keeps the perceived noise level constant, without the need for any variable attenuators or gain switching that causes audible “noise gating”. The noise level is constant and identical to the original background noise even when the NLP is activated. The NLP can be disabled by setting the NLP- bit to 1 in the AEC control registers. 2.5 Narrow Band Signal Detector (NBSD)1 Single or multi-frequency tones (e.g,. DTMF, or signalling tones) present in the reference input of an echo canceller for a prolonged period of time may cause the adaptive filter to diverge. The Narrow Band Signal Detector (NBSD) is designed to prevent this divergence by detecting single or multi-tones of arbitrary frequency, phase, and amplitude. When narrow band signals are detected, the filter adaptation process is stopped but the echo canceller continues to cancel echo. The NBSD can be disabled by setting the NB- bit to 1 in the MC control registers. 2.6 Howling Detector (HWLD)1 The Howling detector is part of an Anti-Howling control, designed to prevent oscillation as a result of positive feedback in the audio paths. The HWLD can be disabled by setting the AH- bit to 1 in the (MC) control register. 2.7 Programmable High Pass Filter Programmable high pass filters are place at the Sin and Rin ports. These filters have two functions, one to remove any DC offset that may be present on either the Rin or the Sin port and two, to filter low frequency noise such as road noise (below 300 Hz). The offset null filters can be disabled by setting the HPF- bit to 1 in the AEC control registers. 1. Patented |
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