The Effect of FEC
DVB-S and DVB-S2 transmitters allow the selection of different values of Forward Error Correction. Put simply, forward error correction steals data capacity that would otherwise be used for video and audio, and uses it to correct errors to allow weaker signals to be decoded.
The fraction (for example 2/3) defines how much of the data capacity is available for headers, audio and video. The remainder is used for error correction.
The typical signal to noise required for decoding in each mode and FEC are listed below, together with indicative header+audio+video (payload) data rates for 1000 kS. Note that DVB-S2 has a smaller header so is more efficient in that it allows for more audio and video in the payload for the same symbol rate. It also needs slightly less SNR to decode, because it is easier for the receiver to lock to the more advanced FEC method.
| Standard | Modulation | FEC | SNR Required | Payload Bitrate Available |
|---|---|---|---|---|
| DVB-S | QPSK | 1/2 | 1.7 dB | 1000 kbits/s |
| DVB-S | QPSK | 2/3 | 3.3 dB | 1333 kbits/s |
| DVB-S | QPSK | 3/4 | 4.2 dB | 1500 kbits/s |
| DVB-S | QPSK | 5/6 | 5.1 dB | 1666 kbits/s |
| DVB-S | QPSK | 7/8 | 5.8 dB | 1750 kbits/s |
| DVB-S2 | QPSK | 1/4 | -2.3 dB | 500 kbits/s |
| DVB-S2 | QPSK | 1/3 | -1.2 dB | 666 kbits/s |
| DVB-S2 | QPSK | 2/5 | -0.3 dB | 800 kbits/s |
| DVB-S2 | QPSK | 1/2 | 1.0 dB | 1000 kbits/s |
| DVB-S2 | QPSK | 3/5 | 2.3 dB | 1200 kbits/s |
| DVB-S2 | QPSK | 2/3 | 3.1 dB | 1333 kbits/s |
| DVB-S2 | QPSK | 3/4 | 4.1 dB | 1500 kbits/s |
| DVB-S2 | QPSK | 4/5 | 4.7 dB | 1600 kbits/s |
| DVB-S2 | QPSK | 5/6 | 5.2 dB | 1666 kbits/s |
| DVB-S2 | QPSK | 8/9 | 6.2 dB | 1778 kbits/s |
| DVB-S2 | QPSK | 9/10 | 6.5 dB | 1800 kbits/s |
| DVB-S2 | 8PSK | 3/5 | 5.5 dB | 1800 kbits/s |
| DVB-S2 | 8PSK | 2/3 | 6.6 dB | 2000 kbits/s |
| DVB-S2 | 8PSK | 3/4 | 7.9 dB | 2250 kbits/s |
| DVB-S2 | 8PSK | 5/6 | 9.4 dB | 2500 kbits/s |
| DVB-S2 | 8PSK | 8/9 | 10.7 dB | 2667 kbits/s |
| DVB-S2 | 8PSK | 9/10 | 11.0 dB | 2700 kbits/s |
| DVB-S2 | 16APSK | 2/3 | 9.0 dB | 2667 kbits/s |
| DVB-S2 | 16APSK | 3/4 | 10.2 dB | 3000 kbits/s |
| DVB-S2 | 16APSK | 4/5 | 11.0 dB | 3200 kbits/s |
| DVB-S2 | 16APSK | 5/6 | 11.6 dB | 3333 kbits/s |
| DVB-S2 | 16APSK | 8/9 | 12.9 dB | 3556 kbits/s |
| DVB-S2 | 16APSK | 9/10 | 13.2 dB | 3600 kbits/s |
| DVB-S2 | 32APSK | 3/4 | 12.8 dB | 3750 kbits/s |
| DVB-S2 | 32APSK | 4/5 | 13.7 dB | 4000 kbits/s |
| DVB-S2 | 32APSK | 5/6 | 14.3 dB | 4167 kbits/s |
| DVB-S2 | 32APSK | 8/9 | 15.7 dB | 4444 kbits/s |
| DVB-S2 | 32APSK | 9/10 | 16.1 dB | 4500 kbits/s |