MT93L16 Mitel Networks Corporation, MT93L16 Datasheet - Page 5

• Manufacturer Part Number: MT93L16
• Description: CMOS Low-voltage Acoustic Echo CANceller
• Manufacturer: Mitel Networks Corporation

Preliminary Information
Howling Detector (HWLD)
(4. Patent Pending)
The Howling detector is part of an Anti-Howling
control, designed to prevent oscillation as a result of
positive feedback in the audio paths.
The HWLD can be disabled by setting the AH- bit to
1 in the (MC) control register.
Offset Null Filter
To ensure robust performance of the adaptive filters
at all times, any DC offset that may be present on
either the Rin signal or the Sin signal, is removed by
highpass filters. These filters have a corner
frequency placed at 40Hz.
The offset null filters can be disabled by setting the
HPF- bit to 1 in the LEC or AEC control registers.
Limiters
To prevent clipping in the echo paths, two limiters
with variable thresholds are provided at the outputs.
The Rout limiter threshold is in Rout Limiter Register
1 and 2. The Sout limiter threshold is in Sout Limiter
Register. Both output limiters are always enabled.
User Gain
The user gain function provides the ability for users
to adjust the audio gain in the receive path (speaker
path). This gain is adjustable from -24dB to +21dB in
3dB steps. It is important to use ONLY this user gain
function to adjust the speaker volume. The user gain
function in the MT93L16 is optimally placed between
the two echo cancellers such that no reconvergence
is necessary after gain changes.
The gain can be accessed through Receive Gain
Control Register.
AGC
The AGC function is provided to limit the volume in
the speaker path. The gain of the speaker path is
automatically
conditions:
•
•
•
When clipping of the receive signal occurs.
When initial convergence of the acoustic echo
canceller detects unusually large echo return.
When howling is detected.
reduced
during
4
the
following
The AGC can be disabled by setting the AGC- bit to
1 in MC control register.
Mute Function
A pcm mute function is provided for independent
control of the Receive and Send audio paths. Setting
the MUTE_R or MUTE_S bit in the MC register,
causes quiet code to be transmitted on the Rout or
Sout paths respectively.
Quiet code is defined according to the following
table.
Bypass Control
A PCM bypass function is provided to allow
transparent transmission of pcm data through the
MT93L16. When the bypass function is active, pcm
data passes transparently from Rin to Rout and from
Sin to Sout, with bit-wise integrity preserved.
When the Bypass function is selected, most internal
functions are powered down to provide low power
consumption.
The BYPASS control bit is located in the main control
MC register.
Adaptation Enable/Disable
Adaptation control bits are located in the AEC and
LEC control registers. When the ADAPT- bit is set to
1, the adaptive filter is frozen at the current state. In
this state, the device continues to cancel echo with
the current echo model.
When the ADAPT- bit is set to 0, the adaptive filter is
continually updated. This allows the echo canceller
to adapt and track changes in the echo path. This is
the normal operating state.
MT93L16 Throughput Delay
In all modes, voice channels always have 2 frames of
delay. In ST-BUS/GCI operation, the D and C
channels have a delay of one frame.
(quiet code)
+Zero
Table 1 - Quiet PCM Code Assignment
complement
LINEAR
16 bits
0000h
2’s
MAGNITUDE
A-Law
SIGN/
80h
-Law
FFh
-Law
MT93L16
CCITT (G.711)
A-Law
D5h
5