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Figure 2-3 illustrates how the packet multiplexer uses a destination queue to create an output
multiplex:
Figure 2-3
Creating an output multiplex
Input stream A
A
Input stream B
B
Input stream C
In this example, the multiplexer routes three different input packet streams labeled A, B, and C
to the RF modulator for output. As a packet from stream A enters the queue, the multiplexer
transfers it to its associated buffer in the RF modulator destination queue. The multiplexer
handles packets from streams B and C in the same way.
The multiplexer then builds the output multiplex for the RF modulator by extracting packets
from each buffer in the RF modulator queue in sequence (that is, one packet from the A buffer,
one from the B buffer, and so on).
The input devices that supply the data control the PID numbers assigned to incoming packet
streams. As a result, it is possible that packet streams received from two different devices use
the same PID number. To avoid collision of these packets when multiplexed for output, the
OM 1000 re-assigns the PID numbers of streams routed to the same destination. PID numbers
can be re-assigned to any value or left unchanged, as necessary, to guarantee PID uniqueness in
the output. PID number re-assignment for a packet stream is defined in the PID map for that
stream.
Inserting PID 0 and PID 1 Control Streams
Depending on your system configuration, the Digital Consumer Terminal (DCT) control-stream
input to the OM 1000 can include the PID 0 and PID 1 control streams required to define the
OOB transport multiplex. If not, these streams are added to the multiplex before it is input to
the RF modulator.
The internal MPEG message stores the PID 0 and PID 1 control streams received from the
DAC 6000 and inserts packets containing messages from these streams into the multiplexer at
defined intervals. The messages are input to the internal MPEG message generator through the
Ethernet and they include data to PID 0 and PID 1 packets, as well as commands specifying
how often the generator should insert the packets into the multiplexer.
RF modulator
queue
A
A
B
B
C
C
Overview
Output multiplex
B
A
C
B
A
OM 1000 Installation and Operation Manual
2-5
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