Tutorial ISDB T

Tutorial: ISDB-T (Japan, Brazil, South America)

What it is. ISDB-T is the digital terrestrial television standard of Japan, Brazil and most of South America, also used in the Philippines. A channel is split into 13 OFDM segments that can be grouped in up to three layers with different modulation; one segment can carry "one-seg" mobile TV for phones. In OnAir you tune the channel, the receiver reads the mode and guard interval, locks the TMCC signalling and plays the services in the TV tab.

What you need

• A radio with a sample rate of at least 6 Msps; the mode accepts 6 to 20 Msps and 8 Msps or more is more comfortable. An RTL-SDR is too slow. See Supported radios.
• A UHF antenna with a good view of the transmitter.
• A frequency. The frequency database lists the Japan and Brazil 6 MHz UHF plan, starting with channel 14 at 473.143 MHz, channel 15 at 479.143 MHz, channel 16 at 485.143 MHz (the centre carries a +1/7 MHz offset). The Band plan list on the Scan tab selects the plan for your country. Channels can be 6, 7 or 8 MHz wide (8 MHz in the Maldives), always 13 segments.

Before you start

• Make sure the sample rate comfortably exceeds the channel width. OnAir tells you the channel width it finds in the Receiver tab.
• Gain: Auto-tune or Optimise gain. Avoid overload from strong nearby transmitters.
• You do not set the transmission mode (1, 2 or 3) or the guard interval: they are detected from the cyclic prefix.

Step by step

1. In the mode list open TV and choose ISDB-T (second line Japan, Brazil).
2. Open the Scan tab, pick the Band plan and Start scan.
3. Press Start on a channel, or type the frequency.
4. Watch the lamps in the status bar: Mode, TMCC, Layers, TS, Video, Audio.
5. Open the TV tab and click a service. The one-seg service appears in the same list.

What you should see

• An OFDM block as wide as the channel, flat in the middle.
• Status line: mode and guard interval (GI), bandwidth (BW), carrier offset (CFO), SNR, packets ok, sample rate (fs) and dropped samples.
• On the Receiver tab: channel width, synchronisation, mode, frame, carrier offset, sample clock, signal to noise, the TMCC contents, and a layer table with modulation, code rate and interleaving (I) per layer, the net bit rate per layer and the total net rate in Mbit/s, and the packets ok / fixed / failed counts.

Reading the screen

Tab	What it shows
Overview	Numbers and plots at a glance.
TV	Player, services (full-seg and one-seg), Guide, record, buffer.
Receiver	The synchronisation, TMCC and layer table described above.
Stream	Transport stream out: file, UDP/RTP, network tuner.
Scan	Channel scanner and channel list.
History	Signal over time.

Options and settings

• Stream: remove null packets, record, stream to UDP, share on the network.
• The TV tab has the usual audio, deinterlace, Fullscreen, pop out and on top controls.

Troubleshooting

• Nothing locks. Check the channel raster (the +1/7 MHz offset is part of the plan), confirm the radio reaches UHF, and widen the sample rate. A wrong standard (for example DVB-T on a channel that is ISDB-T) never locks.
• Locks but packets fail. One layer may be more fragile than the others; the layer table shows which. Improve SNR with the antenna and gain.
• Channel names show placeholder characters. Names in the Japanese character set are not converted yet; the service still works.
• Wrong width detected. Make sure the radio rate is high enough for a 6, 7 or 8 MHz channel.
• Dropped samples. Lower the rate if the channel fits, or use a USB 3 port.

Tips

• Decodes real transmissions, checked with real Dektec and PlutoSDR captures (release 0.2.5).
• Hierarchical transmission means a weak signal may still give the robust layer (often the one-seg service) when the high-rate layer fails.

Legal and privacy notes

Receiving free-to-air broadcasts is legal in most countries. Encrypted services are not decoded.

Under the hood

ISDB-T (ARIB STD-B31; ABNT NBR 15601 for the Brazilian variant, SBTVD) uses band-segmented transmission OFDM (BST-OFDM): 13 segments, modes 1/2/3 (2k/4k/8k FFT), guard intervals 1/4 to 1/32, DQPSK/QPSK/16-QAM/64-QAM per layer, TMCC and AC carriers for signalling, time and frequency interleaving, a Viterbi inner code and Reed-Solomon (204,188) outer code. The receiver finds mode and guard interval from the cyclic prefix, tracks carrier and clock, reads TMCC, equalises, de-interleaves and decodes.

Related

Mode page: ISDB-T | DVB | ATSC | Supported radios | Troubleshooting | FAQ