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 waveform.
| Standard | Modulation | FEC | SNR Required | Bitrate Available |
|---|---|---|---|---|
| DVB-S | QPSK | 1/2 | 1.7 dB | 1000 kS |
| DVB-S | QPSK | 2/3 | 3.3 dB | 1333 kS |
| DVB-S | QPSK | 3/4 | 4.2 dB | 1500 kS |
| DVB-S | QPSK | 5/6 | 5.1 dB | 1666 kS |
| DVB-S | QPSK | 7/8 | 5.8 dB | 1750 kS |
| DVB-S2 | QPSK | 1/4 | -2.3 dB | 500 kS |
| DVB-S2 | QPSK | 1/3 | -1.2 dB | 666 kS |
| DVB-S2 | QPSK | 2/5 | -0.3 dB | 800 kS |
| DVB-S2 | QPSK | 1/2 | 1.0 dB | 1000 kS |
| DVB-S2 | QPSK | 3/5 | 2.3 dB | 1200 kS |
| DVB-S2 | QPSK | 2/3 | 3.1 dB | 1333 kS |
| DVB-S2 | QPSK | 3/4 | 4.1 dB | 1500 kS |
| DVB-S2 | QPSK | 4/5 | 4.7 dB | 1600 kS |
| DVB-S2 | QPSK | 5/6 | 5.2 dB | 1666 kS |
| DVB-S2 | QPSK | 8/9 | 6.2 dB | 1778 kS |
| DVB-S2 | QPSK | 9/10 | 6.5 dB | 1800 kS |
| DVB-S2 | 8PSK | 3/5 | 5.5 dB | 1800 kS |
| DVB-S2 | 8PSK | 2/3 | 6.6 dB | 2000 kS |
| DVB-S2 | 8PSK | 3/4 | 7.9 dB | 2250 kS |
| DVB-S2 | 8PSK | 5/6 | 9.4 dB | 2500 kS |
| DVB-S2 | 8PSK | 8/9 | 10.7 dB | 2667 kS |
| DVB-S2 | 8PSK | 9/10 | 11.0 dB | 2700 kS |
| DVB-S2 | 16APSK | 2/3 | 9.0 dB | 2667 kS |
| DVB-S2 | 16APSK | 3/4 | 10.2 dB | 3000 kS |
| DVB-S2 | 16APSK | 4/5 | 11.0 dB | 3200 kS |
| DVB-S2 | 16APSK | 5/6 | 11.6 dB | 3333 kS |
| DVB-S2 | 16APSK | 8/9 | 12.9 dB | 3556 kS |
| DVB-S2 | 16APSK | 9/10 | 13.2 dB | 3600 kS |
| DVB-S2 | 32APSK | 3/4 | 12.8 dB | 3750 kS |
| DVB-S2 | 32APSK | 4/5 | 13.7 dB | 4000 kS |
| DVB-S2 | 32APSK | 5/6 | 14.3 dB | 4167 kS |
| DVB-S2 | 32APSK | 8/9 | 15.7 dB | 4444 kS |
| DVB-S2 | 32APSK | 9/10 | 16.1 dB | 4500 kS |