Forward error correction (FEC) panel
3b
3c
3a
Figure 28.
Audio Encoder FEC Panel
NOTES:
•
FEC is
only
applied to the audio data channel; it is
not
applied to the video
channel.
•
FEC is effective only for RTP transports. It should not be used for TCP
transport.
•
FEC is not required, and is not applied when the encoder is configured in
stereo mode.
Forward error correction can be applied to the audio data in data streams that use the
RTP transport to protect audio data against data loss. Forward error correction enables the
decoder to recreate data that has been lost or corrupted without retransmission. FEC adds
additional data to the stream that the decoder can use to detect and reconstruct any lost
packets. The FEC data is sent separately from the image and audio data.
The level of FEC can be set to protect against different levels of data loss. This produces a
corresponding change in the amount of data that is transported and thus, the bit rate is
increased or decreased in proportion to the amount of FEC applied.
The VN-Matrix 325 products implement the recommendations defined by the PRo-MPEG
Code of Practice #3 (CoP3)/SMPTE 2022 standard.
Ñ
FEC Mode
— The FEC can be applied as either column only or row and
column.
•
Row FEC
protects against the loss of a single packet. It is the
simplest method of protection and results in the lowest overhead in
terms of bit rate and latency. However, it cannot handle consecutive packet losses
(burst packet loss).
•
Column FEC
protects against burst packet loss, providing that the burst of packet
losses are not in the same row. Since correction data is only calculated on one
dimension, column FEC is known as 1-dimensional (1-D) FEC.
By using both FEC methods together, 2-dimensional (2-D) FEC, the combined effect
protects against more loss combinations than either scheme by itself.
P1
P2
P3
P4
F
R
1
The chart at left shows the FEC scheme applied
to a 4 column x 5 row structure. This scheme
adds 5 row FEC packets (F
R
1 through F
R
5) and 4
column FEC (F
C
1 through F
C
4) packets to every
20 data packets. This scheme can protect for
burst packet losses up 5 packets long.
P5
P6
P7
P8
F
R
2
P9
P10
P11
P12
F
R
3
P13
P14
P15
P16
F
R
4
P17
P18
P19
P20
F
R
5
F
C
1
F
C
2
F
C
3
F
C
4
VN-Matrix 325 Serial Digital Over IP CODEC • Operation
40
