Tutorial DVB

Tutorial: DVB (DVB-T2 and DVB-T)

What it is. DVB-T2 and its older sibling DVB-T are the digital terrestrial television standards of Europe, the Middle East, Africa, much of Asia and Australia. One channel (1.7 to 8 MHz wide) carries several TV and radio services in one MPEG transport stream. In OnAir you tune a channel, the receiver locks, and you pick a service from a list and watch it with sound, subtitles, teletext and a programme guide. The mode detects whether the signal is T2 or T by itself.

What you need

• A radio that samples fast enough for the channel. A rough rule is 1 Msps per MHz of channel width, so an 8 MHz channel needs a little over 8 Msps. An RTL-SDR (about 2.4 Msps) is too slow for normal channels; a HackRF, Airspy, SDRplay, PlutoSDR, bladeRF, RX888 and similar work. See Supported radios and, for the commonest case, Radio: HackRF. OnAir warns next to the source when the rate is too low.
• A UHF/VHF TV antenna with a clear view of the transmitter. Height matters more than gain.
• A frequency. The frequency database lists the common channel plans: VHF channels 5 to 12 (177.5 to 226.5 MHz, 7 MHz wide) and UHF channels 21 and up (474 MHz for channel 21, 8 MHz steps: 482, 490 ... MHz). Which of them your region uses is in the band plans (see Modes, TV group) and in the Band plan list on the Scan tab.
• An antenna amplifier helps a weak signal only if it is placed at the antenna. Power it through the radio's bias-tee only if the amplifier is made for that.

Before you start

• Choose the gain with Auto-tune or Optimise gain rather than by ear. Too much gain overloads the radio and the picture breaks up exactly like with too little.
• Keep the signal away from the centre of the spectrum if you can: the radio's DC spike sits there. OnAir removes it, but a weak channel on top of it is harder.
• A mains-powered USB hub or a long USB cable near the antenna can create interference. Try a different port or cable if the noise floor looks ragged.

Step by step

1. Plug in the radio and the antenna and start OnAir.
2. In the mode list open the group TV and choose DVB (the small second line reads T2 and T, automatic).
3. Open the Scan tab. Under Band plan pick your country (or Automatic), tick detect bandwidth if you do not know the channel width, and press Start scan. The scan lists each channel with its state, number of services, SNR and load. Tick read service names (slower, ~9 s per mux) if you want names in the list.
4. Pick a channel and press Start in the top bar, or type the frequency yourself.
5. Watch the status bar for lock. When it locks, the TV tab fills its service list on the right.
6. Open the TV tab, click a service and watch. The picture and sound start; the volume, mute, Fullscreen, pop out and on top controls are under the picture.

What you should see

• A flat, rectangular block of signal in the spectrum as wide as the channel, with a clear step up from the noise.
• The status bar shows lock lamps, SNR in dB and the error counts, plus the quality score. A healthy signal typically sits well above the minimum SNR for its modulation; the exact margin is on the Receiver tab.
• Services appear in the list once the tables arrive (a few seconds). Names, the current programme (NOW/NEXT) and the logos follow.

Reading the screen

Tab	What it shows
Overview	Key numbers and plots on one page.
TV	The player, the service list, Guide (programme information from the EIT tables), Teletext, recording (Record, Show recording), pause and the live buffer (buffer, Jump to live). The PLP box chooses the data pipe of a multi-PLP T2 channel (automatic uses the first data PLP).
Receiver	Sub-views Sync, Signalling, FEC, Frame map and Channel: lock state, the L1 signalling (modulation, code rate, FFT size, guard interval), FEC counts and the channel response (echoes).
Stream	Outputs for the transport stream: file recording, UDP/RTP, a network tuner for other devices, casting (Cast the playing service...).
Scan	The channel scanner and the shared channel list.
Antenna	A direction finder that helps you aim the antenna.
History	A log of the signal over time.

Options and settings worth knowing

• Compute (CPU, GPU or Auto): where the LDPC error correction runs. The result is identical; the GPU reduces CPU load on a fast channel.
• smooth picture gaps and hardware video decoding in the TV tab's options.
• deinterlace for interlaced pictures; audio lets you pick another audio track.
• In Stream: remove null packets, record to a file, stream to a UDP address and port (optionally RTP), and share on the network to serve your channels to VLC and phones.

Troubleshooting

• No lock at all. Check the frequency and that the channel width matches. Turn on detect bandwidth in the scan. Raise the gain stepwise or press Auto-tune. Make sure the sample rate is at least the channel width plus a margin.
• Locks, but the picture breaks up. Almost always echoes or low SNR. Look at the Channel sub-view for strong echoes, move or re-aim the antenna, and use Optimise gain. OnAir also hides short dropouts automatically.
• Services list stays empty. Wait ten seconds for the tables. If a service shows a lock icon or no picture, it is encrypted; OnAir cannot decode scrambled services.
• "Rate too low" warning. Raise the sample rate in the top bar. An RTL-SDR cannot do an 8 MHz channel.
• Stutter every few seconds. The computer is not keeping up: the status bar shows dropped samples. Lower the sample rate, switch Compute to GPU, or close other heavy programs.
• Wrong picture size or no sound on one service. Try another audio track under audio. A few HEVC or AC-3 services depend on the FFmpeg inside OnAir; the player says what it cannot decode.

Tips

• Scan once, then use the channel list on the Scan tab: it is shared by the TV modes.
• Record a service from the TV tab, or the whole multiplex from Stream, and replay it later without a radio.
• T2-Lite is decoded like T2 and was checked on generated signals; a real T2-Lite broadcast is still welcome as a recording.

Legal and privacy notes

Receiving free-to-air broadcasts is legal in most countries. OnAir does not decode encrypted or scrambled services and does not try to. Check local rules before re-streaming a channel outside your home.

Under the hood

• DVB-T2 (ETSI EN 302 755): OFDM with FFT sizes up to 32k, LDPC + BCH forward error correction, rotated constellations, time interleaving, multiple PLPs, T2 frames with P1 and P2 signalling. DVB-T (ETSI EN 300 744) uses OFDM 2k/8k, convolutional code and Reed-Solomon. The receiver tells the two apart from the preamble.
• The transport stream (MPEG-2 TS) carries the usual DVB SI tables (PAT, PMT, SDT, NIT, EIT).

Related

Mode page: DVB | DVB-T2 TV with a HackRF | DVB-C | ATSC | Supported radios | Troubleshooting | FAQ