MT9196
Register, address 19h). The minimum digital gain is
limited to -24 dB regardless of the mathematical
result of this operation. The path without loss reverts
to the gain value programmed into the Digital Gain
Register.
The magnitude of the switched loss defaults to 12 dB
on power up but can be programmed to between 0
and 21 dB using the Pad0 - Pad2 control bits
(address 1Ch).
Pad2
0
0
0
0
1
1
1
1
Pad1
0
0
1
1
0
0
1
1
Pad0
0
1
0
1
0
1
0
1
Attenuation (dB)
0
3
6
9
12
15
18
21
Preliminary Information
comfortable group listening level after the handset
user has adjusted their listening level as required.
Since the anti-howling circuit has dynamic control
over the transmit and receive gain control registers,
it is recommended that this function be turned off
momentarily when DTMF tone generation is
required. This will ensure that the proper transmit
levels are attained.
Transmit Noise Reduction (squelch)
The transmit signal may be muted to eliminate
transmission of excessive background noise.
In this mode the signal level in the transmit path is
compared with the high level threshold stored at
address 1Dh. When the transmit signal level
exceeds this threshold no loss is inserted into the
transmit path. After exceeding the high level
threshold the transmit signal level is then compared
to a low level threshold stored at address 1Eh. When
the transmit signal level falls below this threshold the
transmit digital gain is reduced by the programmed
amount (Pad0-2) and comparison reverts back to the
high threshold level. The receive path gain is not
altered by transmit noise reduction.
Receive Noise Reduction (squelch)
The receive signal may be muted to eliminate
background noise resulting from a poor trunk
connection.
In this mode the signal level in the receive path is
compared with the high level threshold stored at
address 1Dh. When the receive signal level exceeds
this threshold no loss is inserted into the receive
path. After exceeding the high level threshold the
receive signal level is then compared to a low level
threshold stored at address 1Eh. When the receive
signal level falls below this threshold the receive
digital gain is reduced by the programmed amount
(Pad2-0) and comparison reverts back to the high
threshold level. The transmit path gain is not altered
by receive noise reduction.
Tx/Rx Switched Loss
In this mode the programmed switched loss is
inserted into either the transmit or receive path
dependent only upon activity in the receive path. If
receive path activity is above the programmed high
level threshold then the switched loss is inserted into
the transmit path. If receive path activity is below the
programmed low level threshold then the switched
loss is inserted into the receive path.
Switched Loss for Group Listening (anti-howling)
Group listening is defined as a normal handset
conversation with received speech also directed to
the loudspeaker for third party observation. In this
mode, if the handset microphone is moved into close
proximity of the loudspeaker a feedback path will
occur resulting in a singing connection. To prevent
this the anti-howling circuit introduces a switched
loss into either the transmit or receive paths
dependent upon the transmit path speech activity.
Loss switching is determined by comparing the
signal level in the transmit path with the high level
threshold stored at address 1Dh. When the transmit
signal level exceeds this threshold the programmed
loss is switched from the transmit path to the receive
path. Once switching has occurred the transmit
signal level is then compared to a low level threshold
stored at address 1Eh. When the transmit signal
level falls below this threshold the programmed loss
is switched from the received path back to the
transmit path and comparison reverts back to the
high threshold level.
Since the received digital gain control is used to set
the listening level of the received speech, for both
handset receiver and loudspeaker, it is necessary to
provide additional gain in the loudspeaker path so
that its receive level can be controlled independently
from the receiver output. The Gain0 to Gain3 control
bits (address 0Bh) are used to boost the loudspeaker
output to a comfortable listening level for the third
parties in group listening. Generally the Gain3 bit
should be set to logic 1 in this mode. This increases
the gain programmed via the Gain0 - Gain2 bits by a
factor of 8 dB. In group listening a speaker gain
setting of 4 to 16 dB will be required to set a
7-134
MITEL [ MITEL NETWORKS CORPORATION ]
