MT8924AS datasheet(5/14 Pages) MITEL

5 / 14 page

Preliminary Information

MT8924

8-7

Noise Suppression

When noise suppression is enabled for a specific

input channel then the PCM bytes for this channel,

when

below

the

selected

threshold

level,

are

converted to PCM bytes corresponding to the

minimum PCM code level before being added to the

conference sum.

The four threshold levels available correspond to the

first, fifth, ninth and sixteenth step of the first

segment. These are 1/4096, 9/4096, 16/4096, and

32/4096 with respect to full scale A-Law, and 1/8159,

9/8159, 16/8159, and 32/8159 with respect to full

scale

µ-Law (see Table 1).

PCM Format Selection

PCM

digital

code

assignment

programmable and achieved through the use of

Instruction 5 (see Table 2). The available formats are

CCITT

G.711

A-Law

µ-Law, with true-sign

Alternate

Digit

Inversion

true-sign/Inverted

Magnitude coding.

Output clock Cko provides a reference time base for

a digital time/space crosspoint switch. Normally this

signal is identical to the master clock input Cki.

When operating with the extra bit selection, through

Instruction 5, Cko is low for two clock periods, which

allows operation of the MT8924 with the 1.544 MHz

PCM frame format (see Figure 10).

Transparent Mode

The MT8924 can operate in transparent mode. In

this case the PCM input (DSTi) is passed unmodified

through the MT8924 to the output (DSTo) with a

delay of one frame and one channel. This feature

allows attenuation of specific channels that are not

connected to a conference.

Tone Insertion

The MT8924 provides for tone insertion into PCM

output channels by using the two input pins TD and

TF. An externally generated square wave tone

applied to the TF input will generate a level

corresponding to 1/10 of the full scale accumulator

value when TD is High. Only channels connected in

a conference with the insertion tone bit (IT) active

will have the PCM coded tone at their output (see

Instruction 1).

Testing and Diagnostic Feature

For testing and diagnostic purposes, a

status

instruction

has

been

provided

that

indicates

conference location and attenuation level for each

channel requested. This data

appears

the

databus upon status request.

Programmable Control

Instruction 1 : Conference Mode Connection

This function connects a PCM channel to a

conference.

The

control

information

from

the

microprocessor consists of two data bytes and one

control byte. The first byte contains the conference

number (bits D0-D3) and the Start bit S (D4). When

S is High, the accumulator registers connected to a

conference are initialized. S set to High is only

required in Instruction 1 of the first channel

connected to a new conference, otherwise S is set

LOW to bring other channels into the conference.

The second byte contains the number of the

channel to be connected (D0-D4), and the Insert

Tone Enable bit IT (D5). When IT and TD are both

High all the channels belonging to that conference

are enabled using the insert tone function. The

third byte contains a four bit opcode (D0-D3) plus

information about the attenuation level and noise

suppression to be applied to the specific channel.

Instruction 2 : Transparent Mode Connection

This

function

sets

PCM

channel

for

transparent

mode

operation.

The

control

information from the microprocessor consists of

one data byte and one control byte.

The first byte contains the channel number, and

the second byte contains a four bit opcode (D0-D3)

and

information

about

attenuation

and

noise

suppression levels to be applied to the specific

channel. PCM data on this channel is not added to

any conference, but is transferred to the PCM

output after a full frame pulse plus one channel

delay. It is not affected by the tone control pins (TF,

TD).

Instruction 3 : Disconnection

This function disconnects a PCM channel from a

conference.

The

control

information

from

the

microprocessor consists of one data byte and one

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