MT93L00AB Zarlink Semiconductor, MT93L00AB Datasheet - Page 7

• Manufacturer Part Number: MT93L00AB
• Description: Description = Multi-channel Voice Echo CANceller ;; Package Type = LQFP ;; No. Of Pins = 100
• Manufacturer: Zarlink Semiconductor

Preliminary Information
echo estimation. In Normal configuration, the first
section is dedicated to channel A and the second
section
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
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 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.
energy
(channel N)
(channel N)
ST-BUS
PORT2
to
where DTDT is the Double-Talk
Rout
Sin
Lsin > Lrin + 20log
channel
Detection Threshold.
is
Bypass
Programmable
present
B.
/A-Law/
Linear
Figure 3 - Echo Canceller Functional Block Diagram
Disable Tone
In
Instability
Detector
Detector
10
in
(DTDT)
Linear/
/A-Law
Extended
both
Offset
Null
Echo Canceller (N), where 0
directions
-
+
Delay
Narrow-Band
Attenuator
Detector
Non-Linear
Processor
12dB
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).
guardband, the DTDT is set internally to 0.5625
(-5dB).
In some applications the return loss can be higher or
lower than 6dB. The MT93L00 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:
Example: For DTDT = 0.5625 (-5dB), the
Path Change Detector
Integrated into the MT93L00A is a Path Change
Detector. This permits fast reconvergence when a
major change occurs in the echo channel. Subtle
changes in the echo channel are also tracked
automatically once convergence is achieved, but at a
much slower speed.
The Path Change Detector is activated by setting the
PathDet bit in Control Register A3/B3 to "1". An
Microprocessor
Double-Talk
Detector
Interface
N
MuteR
31
However,
DTDT
Offset
Null
hexadecimal value becomes
hex(
Linear/
/A-Law
where 0 < DTDT
(hex)
0.5625 * 32768
Path Change
Disable Tone
MuteS
Detector
Detector
= hex(DTDT
in
/A-Law/
Linear
order
(dec)
)
(dec)
= 4800h
to
(channel N)
(channel N)
* 32768)
< 1
ST-BUS
PORT1
Sout
Rin
get
MT93L00A
additional
7