MT93L04 Zarlink Semiconductor, MT93L04 Datasheet - Page 23

• Manufacturer Part Number: MT93L04
• Description: 128-channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

Adaptive Filter
The adaptive filter adapts to the echo path and generates an estimate of the echo signal. This echo estimate is then
subtracted from Sin. 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 64 ms 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:
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.
ST-BUS
PORT2
(channel N)
(channel N)
Rout
Sin
Programmable
Bypass
m/A-Law/
Linear
Disable Tone
Instability
Detector
Detector
where DTDT is the Double-Talk Detection Threshold.
Figure 4 - Echo Canceller Functional Block Diagram
m/A-Law
Lsin and Lrin are signal levels expressed in dBm0.
Linear/
Offset
Null
Echo Canceller (N), where 0 ≤ N ≤ 31
Lsin > Lrin + 20log10(DTDT)
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Zarlink Semiconductor Inc.
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MT93L04
Narrow-Band
Attenuator
Non-Linear
Detector
12dB
23
Processor
Double-Talk
Microprocessor
Detector
Interface
MuteR
Offset
Null
m/A-Law
Linear/
Disable Tone
Path Change
Detector
m/A-Law/
Detector
MuteS
Linear
Data Sheet
(channel N)
(channel N)
ST-BUS
PORT1
Sout
Rin