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Preliminary Information
MT9122
8-21
Functional Description
The MT9122 architecture contains two individually
controlled echo cancellers (Echo Canceller A and B).
They can be set in three distinct configurations:
Normal, Back-to-Back and Extended Delay (see
Figure 3). Under Normal configuration, the two echo
cancellers are positioned in parallel providing 64
millisecond echo cancellation in two channels
simultaneously. In Back-to-Back configuration, the
two echo cancellers are positioned to cancel echo
coming from both directions in a single channel. In
Extended-Delay
configuration,
cancellers are internally cascaded into one 128
millisecond echo canceller.
the
two
echo
Each echo canceller contains the following main
elements (see Figure 1).
Adaptive Filter for estimating the echo channel
Subtracter for cancelling the echo
Double-Talk detector for disabling the filter
adaptation during periods of double-talk
Non-Linear Processor for suppression of
residual echo
Disable Tone Detectors for detecting valid
disable tones at the input of receive and send
paths
Narrow-Band Detector for preventing Adaptive
Filter divergence caused by narrow-band
signals
Offset Null filters for removing the DC
component in PCM channels
12dB attenuator for signal attenuation
Serial controller interface compatible with
Motorola, National and Intel microcontrollers
PCM encoder/decoder compatible with
μ
/A-
Law ITU-T G.711,
μ
/A-Law Sign-Mag or linear
2’s complement coding
The MT9122 has two modes of operation:
Controllerless and Controller Controllerless mode is
intended for applications where customization is not
required. Controller mode allows access to all
registers for customizing the MT9122 operation.
Refer to Table 7 for a complete list. Controller mode
is selected when CONFIG1 and CONFIG2 pins are
both connected to Vss.
Each echo canceller in the MT9122 has four
functional states: Mute Bypass Disable Adaptation
and Enable Adaptation. These are explained in the
section entitled Echo Canceller Functional States.
Figure 3 - Device Configuration
Rin
Rout
Sout
Sin
echo
path A
Optional -12dB pad
PORT 2
PORT 1
echo
path B
+
-
channel A
channel A
+
-
channel B
channel B
E.C.A
E.C.B
a) Normal Configuration (64ms)
+
-
channel A
channel A
E.C.A
Sin
Sout
Rout
Rin
b) Extended Delay Configuration (128ms)
PORT 2
PORT 1
+
E.C.A
Sin
Sout
Rout
Rin
c) Back-to-Back Configuration (64ms)
-
E.C.B
+
-
echo
path
echo
path
PORT 2
PORT 1
echo
path A
Adaptive
Filter (64ms)
Adaptive
Filter (64ms)
Optional -12dB pad
Adaptive
Filter (64ms)
Optional -12dB pad
Adaptive
Filter (64ms)
Optional -12dB pad
Adaptive Filter
(128 ms)
Optional -12dB pad