The material transmitted via DAB is not limited to speech, but covers the whole audible frequency range. The encoding system provides four distribution modes: mono, dual, stereo and joint stereo, the multichannel extension (4 or 5 mono channels to create an "environment" effect; is an additional modality under study and definition which provides frame encoding of 5 monophonic channels, whose speakers have to be placed according to the figure; such modality is about to be defined as standard to broadcast audio of High Definition TV programs; encoding syntax is not modified, since the two stereo channels are transmitted in a normal stereo frame and the other three, two for the surround and one central in front, in the auxiliary data portion of the signal; this maintains compatibility with previous decoders, which can extract at least the two main stereo channels from the encoded sequence) and bit rates from 32 to 192 kBit/s per mono channel in function of the desired quality. Only 48kHz is available as sampling frequency. The 48kHz frequency sampling is the only one available. 32 kHz is too limited in bandwidth (not higher than 16kHz) while 44.1kHz is less interesting because of its fractional part, which creates synchronization problems. Choice of the bit rate depends on the audio modality and differs from the table shown above for MPEG, as follows (recall that DAB uses MUSICAM encoding, limited to the sampling frequency of 48 kHz and to the bit rates listed):
The input and output signal to and from the encoding chain follows the EBU/AES interface specification [Recommendation CCIR 647], thus allowing use of the normal D/A and A/D conversion systems. Moreover, since the quality of the PCM signal gets the better the higher the input resolution, a resolution of 18 bit per samples is available.
In regard to transmitters and receivers, DAB distribution does not introduce additional costs and allows re-use of the existing FM structures. Each DAB station requires a bandwidth of about one third of existing FM and the power required at the transmitter is a lot less, allowing a better confinement of transmissions to a given geographical area with reduced costs. A good geographical confinement of broadcasts reduces interferences between stations. In such a way it is possible to use the same frequency for stations which are sufficiently far away, thus increasing exploitation of the frequency band assigned to DAB.
An experimental DAB network which can broadcast nine independent, high quality radio channels has been set up in Rennes, France. The current work under development in the DAB project aims at the construction of a MPEG_Audio decoder in a single VLSI chip.
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