Preliminary Information
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 is register
programmable and achieved through the use of
Instruction 5 (see Table 2). The available formats are
CCITT G.711 A-Law or
µ-Law,
with true-sign
Alternate Digit Inversion or 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
MT8924
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 on 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 up a 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
8-7
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