MT9196AS datasheet(8/38 Pages) MITEL

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MT9196

Preliminary Information

7-134

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

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

Pad1

Pad0

Attenuation (dB)

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

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

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

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.

MT9196AS Datasheet(PDF) 8 Page - Mitel Networks Corporation

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