Revision as of 23:19, 8 October 2009

Opendigitalradio.org

Open and easy digital broadcasting techniques based on software radio - because digital radio transmission and development must also become democratized.

Broadcasting is sometime considered, by the new generations, as a dying media that will be soon completely superseeded by Internet. However it is a nice, efficient and green way to reach masses without intermediaries but just using spectrum directly. For these reasons and apart from the big public or private broadcasters interests, it has still an interest for local communities. In particular, it offers anonymous and free-to-air (no subscription needed) listening possibility which is of importance in poor countries or where freedom of information does not apply.

In radio, Analog transmission using AM, FM is still very popular but may be replaced in the future by digital techniques such as DAB, DAB+, T-DMB, DRM (Digital Radio Mondiale) or DRM+. In television analog transmission using PAL or SECAM is being stopped in Europe and replaced by DVB-T, DVB-T2, DVB-H (for mobile) or other digital satellite or cable transmission techniques.

Transmitting using this technology usually requires complex and costly equipment out of reach of individuals. However, since the democratization of software defined radio (SDR), this opens new field of possibilities. In the longer term if software based radio become also democratized in common receivers, it could even be possible to develop personalized broadccasting system based on free/opened technologies for example. The combination of personal (Internet,..) networks and broadcast also offer interesting possibilities.

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)

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.

• Example of live re-transmission on DAB of an Internet Radio radio station