Introduction on DAB/DAB+
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 running linux and using a USRP as RF hardware (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 baseband OFDM signal ready for upconvert at the desired radio frequency.
- The RF transmission performed by the USRP using appropriate RF daughterboards (Basic TX or modified RFX400).
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.