Tutorial DVB C

Tutorial: DVB-C (digital cable TV)

What it is. DVB-C is the digital cable television standard (ITU-T J.83 annex A in Europe, annex C in Japan), with 16 to 256-QAM in 8 MHz channels. It runs over a cable network, not over the air. In OnAir you connect the radio to a cable outlet, the receiver measures the symbol rate and QAM order by itself, and you watch the services in the TV tab.

What you need

• A radio that samples at 7 Msps or more (the mode asks for 10 Msps). An RTL-SDR is too slow. See Supported radios.
• A cable feed, not an antenna. Connect the radio to the cable TV outlet through a DC block and an attenuator, as the app itself warns. The signal on a cable is much stronger than over the air.
• A frequency. The default is 346 MHz; the tuner's channel list follows the European cable raster, 8 MHz channels from 114 to 862 MHz, and the mode table covers 47 to 1002 MHz. Which channels carry digital TV depends on your operator.

Before you start

• Attenuate. Cable signals are strong; overload makes the constellation smear. Start with minimum gain and add an attenuator if the signal still looks flattened.
• Check that your provider uses free-to-air DVB-C. Many operators scramble most services; OnAir cannot decode them.
• Do not connect the radio to anything that carries a DC supply (power inserter on the line).

Step by step

1. Connect the cable outlet to the radio through a DC block and an attenuator.
2. In the mode list open TV and choose DVB-C (second line Cable).
3. In the tuner on the left: Channel lets you pick a centre frequency from the raster. Leave Symbol rate and QAM on auto to start with. Tick Annex C only for Japanese 6 MHz systems.
4. Press Start. Watch the lamps in the status bar: IQ, Carrier, Timing, Equaliser, RS, TS lock.
5. Open the TV tab and click a service.

What you should see

• A flat 8 MHz block with steep edges. The constellation in the Receiver tab becomes a clean square grid (16, 32, 64, 128 or 256 points).
• Status line: state, QAM order, symbol rate (SR), MER in dB, carrier offset (CFO), Reed-Solomon ok/bad counts, sample rate and dropped samples. MER above the threshold for the QAM order gives a clean picture: higher QAM needs higher MER.

Reading the screen

• Symbol rate choices: auto, 6.952, 6.900, 6.875, 6.111 and, for annex C, 5.361 and 5.274 Msym/s. Auto measures it from the spectrum; a manual rate that does not lock gives way to the measured one.
• QAM: auto, 16, 32, 64, 128, 256. The receiver tries blind first.
• Receiver tab: sync, signalling, FEC, frame map, channel (see DVB for the sub-views).

Options and settings

• Annex C: roll-off 0.13 instead of 0.15 (ITU-T J.83 annex C, Japan).
• Stream: record the transport stream, send it by UDP or serve it on the network.
• The test signal (synthetic) offers SNR, carrier offset, clock offset (ppm) and echo with its delay so you can see how the receiver copes.

Troubleshooting

• No carrier. Check the cable, check the attenuator and the channel frequency. Operators often have analogue or other channels on neighbouring frequencies.
• Carrier but no equaliser lock. Symbol rate wrong. Put Symbol rate back to auto.
• High RS failure with a clean grid. The wrong annex, or a damaged cable with echoes.
• Distorted constellation. Overload. More attenuation, less gain.
• Services listed but no picture. Encrypted by the operator.

Tips

• Checked with generated signals and impairments; recordings of real signals are welcome (see Signal samples).
• A splitter and an attenuator are your friends.

Legal and privacy notes

Cable networks are private infrastructure. Only connect to a cable you have the right to use. OnAir does not decode scrambled services and does not try to.

Under the hood

DVB-C (ETSI EN 300 429) / ITU-T J.83 annex A and C: single-carrier QAM with square-root raised cosine filtering (roll-off 0.15, annex C 0.13), Reed-Solomon (204,188), Forney convolutional interleaving and energy dispersal. The receiver measures the symbol rate from the spectrum, runs a matched filter and timing loop, determines the QAM order blind, and equalises adaptively.

Related

Mode page: DVB-C | DVB | Signal samples | Supported radios | Troubleshooting