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MT9300 Datasheet(PDF) 5 Page - Mitel Networks Corporation |
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MT9300 Datasheet(HTML) 5 Page - Mitel Networks Corporation |
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5 / 29 page  Advance Information MT9300 5 Figure 3 - Echo Canceller Functional Block Diagram Linear/ µ/A-Law + Non-Linear Processor Offset Null µ/A-Law/ Linear Linear/ µ/A-Law Microprocessor Interface Double-Talk Detector Disable Tone Detector Narrow-Band Detector µ/A-Law/ Linear Offset Null Echo Canceller (N), where 0 ≤ N ≤ 31 Sout Rin Sin Rout - Programmable Bypass Disable Tone Detector (channel N) (channel N) (channel N) (channel N) ST-BUS ST-BUS PORT2 PORT1 12dB Attenuator MuteR MuteS Each echo canceller in the MT9300 has four functional states: Mute, Bypass, Disable Adaptation and Enable Adaptation. These are explained in the section entitled Echo Canceller Functional States. Adaptive Filter For each group of echo cancellers, the Adaptive Filter is a 1024 tap FIR adaptive 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. In Back-to Back configuration, the first section is used in the receive direction and the second section is used in the transmit direction for the same channel. 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 using the previous converged echo profile. A double-talk condition exists whenever the relative signal levels of Rin (Lrin) and Sin (Lsin) meet the following condition: Lsin > Lrin + 20log10(DTDT) where DTDT is the Double-Talk Detection Threshold. Lsin and Lrin are 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. The convergence speed is shown by the CONV bit in the Status Register. In G.168 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 some applications the return loss can be higher or lower than 6dB. The MT9300 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 |
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