Digital amateur television (DATV) sends video as data, using the same DVB (Digital Video Broadcasting) standards as digital broadcast and satellite TV. Video is compressed first, so a good picture needs much less bandwidth than analog fast-scan, and error correction repairs damage on the way.
From camera to RF
Compression removes the redundancy in the video and sound. Modern coding makes a watchable picture possible in a small fraction of the data that raw video needs.
Forward error correction (FEC) adds extra bits so the receiver can repair errors. A coding rate of 3/4 means that three out of every four bits are data, and the rest, 25%, are for correction.
Modulation puts the bits onto the carrier. DVB-T amateur TV uses QPSK or QAM; denser patterns carry more bits per symbol but need a cleaner signal. See Digital modulation: FSK, PSK and QAM.
Coding rate = data bits out of all bits. Modulation = bits per symbol.
The receiver does it in reverse: demodulate, correct errors, decompress, display. Bandwidth, symbol rate and the data rate you can get from them are the subject of Bandwidth, symbol rate and data rate.
Behaviour on the air
Analog picture quality falls slowly as the signal gets weaker: snow builds up but you can still see a picture. Digital error correction hides noise completely up to a point, then fails at once.
Schematic, not measured. Digital trades graceful decay for a cleaner picture above the threshold.
This is the cliff effect. When the picture breaks into blocks or freezes, a modest improvement in signal, such as a better antenna, a shorter feed line, or a lower-rate setting, may bring it back at full quality.
In practice
Setting a lower symbol rate or a more robust coding rate narrows the signal and makes it work at lower strength, at the expense of picture quality or detail.
Compression and error correction add a small delay, so a DATV picture lags a moment behind real life.
Like analog ATV, it is line-of-sight on the UHF and microwave bands, so antenna height and aim matter; see Line of sight and the radio horizon.
Check the band plan for where wide-signal TV belongs; see Band plans.