Forward Error Correction (Fec) Theory Of Operation - VBrick 9000 Admin Manual

In order to better use this feature, it is useful to understand some details of its design. The
algorithm reverts to the calculated latency whenever a relevant configuration parameter is
changed. At this point it observes the timing of the encoding and transmission video frames
and determines an optimal value for the buffering required on the decoder to avoid dropped
frames. This buffering value is measured in time and is the
adjusts gap only—the overhead is set by the user either by utilizing the
setting via the
Custom
value more rapidly than reducing the gap value. If the gap is too large, latency is increased,
but video quality is retained. If the gap is too low, it is likely that frames will be dropped
leading to reduced perceived quality.
The algorithm has the capability to adjust its behavior based on a number of factors. The
algorithm may be "aggressive" in optimizing latency by reducing the gap to a minimum and
accepting the possibility of a limited number of dropped frames. The algorithm may also be
"conservative" by keeping the gap higher and thus reducing the likelihood of dropped frames
at the cost of increased latency. To determine how to tune the algorithm the encoder software
uses the transport stream
resolution, the video frame rate, the video connection (Standard Definition or High
Definition), and the rate control setting. If the transport stream
and the
Rate Control Setting
latency, the administrator is primarily interested in low latency and so the auto gap algorithm
is aggressive in controlling latency. On the other hand, if the transport stream
Variable High Delay
interested in high quality and latency if of less interest so the latency control algorithm is
conservative in controlling latency. The remaining factors are combined to tune the algorithm
based on video input and output parameters. In general, if the variation in encoding time is
more variable, in order to not drop excessive frames, the algorithm must be more
conservative. Lower frame rates and lower resolutions lead to more variation in encoding and
thus to higher latency and a less aggressive auto gap algorithm.

Forward Error Correction (FEC) Theory of Operation

Forward Error Correction (FEC) is in general a system of error control for data transmission
whereby the sender adds systematically generated redundant data to its messages. The basic
trade-offs that must be determined in configuring FEC is the bandwidth overhead and
increased latency caused by FEC traded off against the level of error recovery provided by
the algorithm. A deeper understanding of the statistics of errors in the network and the
details of the FEC algorithm configuration will provide the ability to optimize these trade-
offs.
A key concept with FEC systems on IP networks is that, because of the UDP checksums,
channel bit errors get translated into packet losses. In addition to this, buffer and re-route
issues cause burst packet losses. The combination of packet losses from the three sources—
gross reordering, bit-error induced losses, and burst losses needs to be low enough so that
the FEC scheme is not broken more than the negotiated error rate. Because any bit errors
cause the packet to be discarded there is no requirement for an error correction scheme that
can handle errored packets—every packet will either arrive correct or not at all.
An RTP payload format for Generic Forward Error Correction Packets has been defined in
the RFC 2733 to enable error correction of realtime media. This standard allows the use of
traditional error correcting codes. A major advantage of this scheme is that it can be used
with any video format standards (MPEG, SDI, SDTI, etc.) as long as it is encapsulated in
VBrick 9000 Encoder Admin Guide
option. The algorithm seeks to reduce frame loss by increasing the gap
configuration, the template configuration, the video
Mode
(on Encoder Configuration > Video Input page) is lowest
and the Rate Control Setting is best quality, the administrator is primarily
Advanced Transport Stream Settings
. The algorithm
PTS-PCR Gap
selection or by
Mode
is
Mode Variable Low Delay
Mode
is
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