MT9196
Anti-Howl
Data Sheet
IDPC includes an Anti-Howling circuit plus speaker gain control circuit to allow for group listening operation.
Although this is the main function of the circuit there are additional modes in which it may be used as defined by the
MS1 and MS0 control bits (address 1Ch).
MS1
0
0
1
1
MS0
0
1
0
1
Operational Mode
Tx noise reduction (squelch)
Rx noise reduction (squelch)
switched loss group listening
(anti-howling)
Tx/Rx switched loss
The circuit is enabled by setting the Anti-howl Enable bit (address 1Ch) and selecting the required operational
mode (MS0 & MS1) as described.
For all modes of operation the switching levels and inserted loss are programmed as follows.
Switching decisions are made by comparing either the transmit or the receive signal level to threshold levels stored
in the High Threshold Register (address 1Dh) and the Low Threshold Register (address 1Eh). Threshold data is
encoded in PCM sign-magnitude format excluding the sign bit. For example; THh0 - THh3 encode the PCM step
number while THh4 - THh6 encode the PCM chord number for the high threshold. Similarly for the THl0 - THl6 bits
of the low threshold. The difference between the high and low threshold levels provides the circuit with hysteresis to
prevent uncontrolled operation. The low level threshold must never be programmed to a value higher than the one
stored in the high level threshold. If this occurs the circuit will become unstable.
Loss is implemented, in the chosen path, by subtracting the value set by the Pad0 - Pad3 control bits from the
appropriate gain value set by the RxG0 - RxG3 or TxG0 - TxG3 control bits (see Digital Gain 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
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.
8
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
