Revision as of 23:44, 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 radio spectrum directly (despite the fact that the use spectrum is subject to licence). 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 listening and is free-to-air (no subscription needed) 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 these technologies usually requires complex and costly equipment out of reach of individuals. However, since the democratization of software defined radio (SDR), this opens a new field of possibilities for experimentation, micro networks, etc. In the longer term also if software based radio become also democratized in common receivers, it could even be possible to develop personalized broadccasting system based on free/open technologies for example. The combination of personal (Internet,..) networks and broadcast also offer interesting possibilities.

So this website will focus on tips, tricks and techniques to produce digital broadcast signals. Most of the content is based on development from researh centers, groups or individuals that did the "real work". The objective of this website is to follow developments, integrate them and show applications.

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 modulation and use 1.5MHz of spectrum in VHF television band 3 (or L-band in SHF). A transmitter is broadcasting a set of programs, called a multiplex. The system has been designed for mobile use and is robust up to 300km/h.

With the development of MMBTools set made by CRC, it is now possible to run a full transmission infrastructure on a laptop using a USRP (with gnuradio) or any other similar device.

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, it is free but not opened. The MPEG encoding is subject to royalty payment and so CRC is selling it at the price of the royalty.

However thank to 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. Using another software modulator is also possible (gnuradio based ?).

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

Digital Radio Mondiale (DRM, DRM+)

Under construction

Dream open software receiver and transmitter

Spark free DRM/DRM+ transmitter

DVB-T

under construction (soft DVB ?)