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24
SEL-MDX1
Installation and Operation Manual
FEC
Copyright © 2011, Harris Corporation
PID
This parameter sets the packet identifier (PID) value associated with this VC. When PID is
selected in the Function parameter, the packet identifier specifies which packets to extract
from the interface's transport stream. Other VC functions ignore this setting.
The default setting of -1 -cannot occur in a transport stream. If the channel is configured for
single-PID processing, this gives a "safe" default value that cannot match any packets (and
thus avoids duplicating a PID in an active program). Additionally, channels configured for
other functions use -1 to indicate the absence of specific PID extraction.
Values from 1 to 8190 are remapped to PID 59 before they are transfered through the VC.
When processing a full program from a multiplex, the system remaps the program number
to 1 and all the PIDs starting at 32. For a full program, PID 59 corresponds to the first
assigned data PID within the program. Because a single-PID channel normally would carry
data, the designated input PID is re-mapped to 59. For symmetry, a multiplexer VC inserts a
single PID also uses packets from PID 59.
CAUTION:
Do not use "0" as a value. This value extracts the stream's program allocation table
(PAT), disabling program processing.
Use this parameter to select the mode of forward error correction (FEC) for the selected IP
virtual channel. This value applies to IP channels transmitting MPEG transport stream data,
generating RTP parity packets according to RFC 2733. This value should be set to
None-UDP or None-RTP for MPEG receivers and VCs configured for other channel
functions.
To enhance error recovery, parity packets can be computed on a repeating, conceptual grid
overlaid on the data packets. This grid has period columns (L) and order rows (D), as
described by Pro-MPEG Code of Practice #3, January 2003.
With forward error correction enabled, the correction packets are sent to and received from
a different port from the transport stream packets. The channel's transmit rate includes only
the transport stream data; error correction overhead (and IP overhead) must be separately
computed to determine the entire bandwidth for the video-on-IP stream.
In the MPEG data stream, each IP packet carries up to 7 transport stream packets (as set in
the TS Packets Per IP parameter), possibly wrapped in additional protocols. Each FEC parity
packet carries error correction data for those transport stream packets. That FEC payload
rides in an ethernet frame with a 14-byte header, carrying an IP packet with a 20-byte IP
header, an 8-byte UDP header, a 12-byte RTP header, and a 16-byte FEC header.
None-UDP and None-RTP disable forward error correction for this virtual channel. The
Low setting is equivalent to generating 1 parity packet for every 16 data packets. Medium
is equivalent to generating 5 parity packets for every 40 data packets, and High is
equivalent to generating 20 parity packets for every 80 data packets.
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