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Easy digital broadcasting techniques based on software radio. Tips, Tricks and Experimentations.
DAB, DAB+ and T-DMB transmission
DAB, DAB+ and T-DMB European digital radio and mobile television standards share the same transmission system. It based on OFDM and use 1.5MHz of spectrum in VHF television band 3 (or L-band in SHF)
Transmitting using this technology usually requires complex and costly equipment out of reach of individuals.
However, since the democratization of software defined radio (SDR) and the development of MMBTools set made by CRC, it is now possible to run a full transmission infrastructure on a laptop using a USRP.
The transmission chain can be divided in 4:
- The encoding: encoding the sound to MPEG-2 Layer II for DAB, MPEG-4 HE-AACv2 for DAB+ or video in MPEG-4 H.264 for T-DMB.
- The Multiplexer: gather different streams, produce necessary signalling and output a single 2Mbit/s stream in ETI format (Ensemble Transport Interface)
- The Modulator: take the ETI stream and produce the complex OFDM signal ready for upconverted at the desired radio frequency.
- The RF transmission performed by the USRP using appropriate RF daughterboards (Basic TX or modified RFX400)
CRC has recently opened the source code of the multiplexer. However as the modulator depends on other developments that are not ready to open the source, it is free but closed source. The MPEG encoding is subject ot royalty payment and so CRC is selling it at the price of the royalty. However with the modular approach from these tools it is possible to interface it with other implementations. If there's no other software OFDM DAB modulator implementation at the moment, for the encoding it is possible to re-use TooLame that is an implementation of MPEG-2 Layer II.
