User Manual

AT-OMNI-121 / AT-OMNI-122
131
FEC Details
Matrix Size, Overhead, and Latency
FEC and Video Bitrate
• FEC can only work if a single packet from each row/column is missing. Multiple packets missing from each row/
column will cause FEC to fail.
• Due to the above, a smaller matrix is more robust, as there is a better chance of errors not occurring in the same
row/column.
• FEC has a bitrate overhead that is inversely proportional to the matrix size: the bigger the matrix, the less bitrate
overhead is generated.
• FEC has a latency overhead that is directly proportional to the matrix size: the bigger the matrix, the more
latency is introduced.
» As of v1.0.0, OmniStream does not explicitly synchronize audio and video. Therefore, FEC conguration can
have a noticeable impact on lip sync. The tables below should be used to keep the audio/video lip sync as
tight as possible.
• FEC latency overhead is also inversely proportional to bitrate: the higher the bitrate, the less FEC latency is
introduced.
» For applications where lip sync is very critical, using a higher audio sampling rate, and thus a higher audio
bitrate, can result in more accurate lip sync.
• The bitrate congured on the video encoder includes FEC overhead and will automatically adjust itself depending
on the bitrate needed for FEC.
• FEC overhead can be calculated using the following formulas:
• The following table provides a few examples of how this works.
Video rate =
Configured bit rate
1 +
Rows + Columns
Rows x Columns
FEC rate = Configured bit rate - Video rate
FEC / matrix usage Congured bit rate Used for video Used for FEC
FEC disabled 900 Mbps 900 Mbps 0 Mbps
FEC enabled, 4x4 900 Mbps 600 Mbps 300 Mbps
FEC enabled, 10x10 900 Mbps 750 Mbps 150 Mbps
FEC enabled, 20x20 900 Mbps 818 Mbps 82 Mbps
FEC enabled, 4x4 450 Mbps 300 Mbps 150 Mbps
FEC enabled, 10x10 450 Mbps 375 Mbps 75 Mbps
FEC enabled, 20x20 450 Mbps 409 Mbps 41 Mbps
Appendix