MT93L04A Zarlink Semiconductor, MT93L04A Datasheet - Page 21

• Manufacturer Part Number: MT93L04A
• Manufacturer: Zarlink Semiconductor

Preliminary Information
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 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:
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.
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).
ST-BUS
PORT2
(channel N)
(channel N)
Rout
Sin
Programmable
Bypass
m/A-Law/
Linear
Disable Tone
Figure 3 - Echo Canceller Functional Block Diagram
Instability
Detector
Detector
where DTDT is the Double-Talk Detection Threshold.
Lsin and Lrin are signal levels expressed in dBm0.
m/A-Law
Linear/
Offset
Null
Echo Canceller (N), where 0
Lsin > Lrin + 20log10(DTDT)
-
+
Narrow-Band
Attenuator
Non-Linear
Detector
12dB
Processor
Double-Talk
N
Microprocessor
Detector
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Interface
MuteR
Offset
Null
m/A-Law
Linear/
Disable Tone
Path Change
Detector
m/A-Law/
Detector
MuteS
Linear
MT93L04A
(channel N)
(channel N)
ST-BUS
PORT1
Sout
Rin
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