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MT9123AP Datasheet(PDF) 6 Page - Mitel Networks Corporation |
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MT9123AP Datasheet(HTML) 6 Page - Mitel Networks Corporation |
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6 / 32 page  8-50 MT9123 Preliminary Information Adaptive Filter The adaptive filter is a 1024 tap FIR filter which is divided into two sections. Each section contains 512 taps providing 64ms of echo estimation. In Normal configuration, the first section is dedicated to channel A and the second section to channel B. In Extended Delay configuration, both sections are cascaded to provide 128ms of echo estimation in channel A. Double-Talk Detector Double-Talk is defined as those periods of time when signal energy is present in both directions simultaneously. When this happens, it is necessary to disable the filter adaptation to prevent divergence of the adaptive filter coefficients. Note that when double-talk is detected, the adaptation process is halted but the echo canceller continues to cancel echo. A double-talk condition exists whenever the Sin signal level is greater than the expected return echo level. The relative signal levels of Rin (Lrin) and Sin (Lsin) are compared according to the following expression to identify a double-talk condition: Lsin > Lrin + 20log10(DTDT) where DTDT is the Double-Talk Detection Threshold. Lsin and Lrin are the relative signal levels expressed in dBm0. A different method is used when it is uncertain whether Sin consists of a low level double-talk signal or an echo return. During these periods, the adaptation process is slowed down but it is not halted. Controllerless Mode In G.165 standard, the echo return loss is expected to be at least 6dB. This implies that the Double-Talk Detector Threshold (DTDT) should be set to 0.5 (-6dB). However, in order to get additional guardband, the DTDT is set internally to 0.5625 (-5dB). In controllerless mode, the Double-Talk Detector is always active. Controller Mode In some applications the return loss can be higher or lower than 6dB. The MT9123 allows the user to change the detection threshold to suit each application’s need. This threshold can be set by writing the desired threshold value into the DTDT register. The DTDT register is 16 bits wide. The register value in hexadecimal can be calculated with the following equation: DTDT(hex) = hex(DTDT(dec) * 32768) where 0 < DTDT(dec) < 1 Example: For DTDT = 0.5625 (-5dB), the hexadecimal value becomes hex( 0.5625 * 32768) = 4800h Non-Linear Processor (NLP) After echo cancellation, there is always a small amount of residual echo which may still be audible. The MT9123 uses an NLP to remove residual echo signals which have a level lower than the Adaptive Suppression Threshold (TSUP in G.165). This threshold depends upon the level of the Rin (Lrin) reference signal as well as the programmed value of the Non-Linear Processor Threshold register (NLPTHR). TSUP can be calculated by the following equation: TSUP = Lrin + 20log10(NLPTHR) where NLPTHR is the Non-Linear Processor Threshold register value and Lrin is the relative power level expressed in dBm0. When the level of residual error signal falls below TSUP, the NLP is activated further attenuating the residual signal to less than -65dBm0. To prevent a perceived decrease in background noise due to the activation of the NLP, a spectrally-shaped comfort noise, equivalent in power level to the background noise, is injected. This keeps the perceived noise level constant. Consequently, the user does not hear the activation and de-activation of the NLP. Controllerless Mode The NLP processor can be disabled by connecting the NLP pin to Vss. Controller Mode The NLP processor can be disabled by setting the NLPDis bit to 1 in Control Register 2. The NLPTHR register is 16 bits wide. The register value in hexadecimal can be calculated with the following equation: |
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