7 FEC Forward Error Correction according to SMPTE 2022-1
In telecommunication, information theory, and coding theory, forward error
correction (FEC) or channel coding is a technique used for controlling errors
in data transmission over unreliable or noisy communication channels. The
central idea is the sender encodes their message in a redundant way by
using an error-correcting code (ECC).
The redundancy allows the receiver to detect a limited number of errors that
may occur anywhere in the message, and often to correct these errors
without retransmission. FEC gives the receiver the ability to correct errors
without needing a reverse channel to request retransmission of data, but at
the cost of a fixed, higher forward channel bandwidth.
The FEC doesn’t support the audio from the external audio interface.
FEC Enable: for video and audio separately selectable
Size of matrix: settings between 4 and 10
To calculate the redundant packets, all packets to be transmitted are placed
in a matrix. The size of the array determines how many packets can be
recovered. The smaller the matrix, the more bandwidth is provided to the
redundant packets (= more overhead) and more packets can be recovered.
But the larger the matrix, more packets must be buffered before sending, so
the latency is greater. The lower the bandwidth, the greater is this effect.
e.g.: low bandwidth (1 frame = 17 Ethernet packets) and a size of 10 x 10, 6
images need to be cached before the redundant packets may be calculated.
Thus, at 60 Hz refresh rate, the network delay at the receiver to 1/60 Hz * 6 =
100 ms must be increased to ensure the restoration works. With a high
bandwidth (1 picture = 250 Ethernet packets), the additional delay is only
1/60 Hz * 100/250 = 7 ms. In the example with set low bandwidth (1 frame =
17 Ethernet packets), the die size is set to 4 x 4, so only one image needs to
be cached, thus the additional delay is only 17 ms.
