The technology of DAB

To ensure a receipt free from interference in CD-quality, at DAB is used the modulation type COFDM (Coded Orthogonal Frequency Division Multiplex). Near by the information of, for example, seven sound broadcasting programs and the different additional data will be splitted up into 1536 separate main frequencies. Because of a carrier gap of 1 KH/z results a wave band of 1,536 Mc/s. This method increases the immunity to interference seriously, as in case of interference only one part of the information will get lost. At the conventional FM-sound broadcasting the total information will be moduled on one main frequency only (frequency modulation +/- 75 KH/z). Appears a interference, with FM the whole program is disturbed (picture 1a). At DAB on the other hand the separate program information and data are not modulated in logical sequence on the 1536 carrier, but accidentally distributed on all frequencies (Frequency- and Time-Interleaving). Appears an interference (picture 1b) so will not be destroyed the information of one single program, but with a special method for correcting errors, the lost information will be corrugated and regained. The degree of an error protection can be tuned with a protection level.
As there exist a temporal tolerance (protection interval) for every single main information, it will be possible to analyse also signal reflections as a signal of benefit. Therefore reflections on buildings and mountains are utilised and so they enlarge the benefit signal and the supplied area. This characteristic makes it also possible to build up in SFN (Single Frequency Network). By that the listener is able to receive his preferred program in the whole transmitting area.

(picture 1) FM-interference

DAB-interference

Signal preparation:

First of all the analogue audio signals of the studio will be changed into a digital signal. At a stereo transmission in CD-quality it has the consequence that it comes to a data flow of 1,536 Mbit/s at a sampling rate of 48 kH/z and a release of 16 bit. As this data flow would need a to big wave band, the signal will be data compressed. All this is possible because of the as MUSICAM called method. It works with the characteristic of the human hearing, which does not perceive low sounds under a particular frequency depended threshold and low sounds in direct nearness of loud sounds of neighbouring frequencies (picture 2). With leaving out the unnecessary data, you’ll need for the same audio-quality only ca. 1/10 of the required data rate and come out with 112 – 192 kBit/s. The whole audio area will be transmitted from 0 to 20,25 kH/z, while at FM the area will be transmitted from 30 Hz to 15 kHz. This method is also used at the ADR (Astra Digital Radio) and at DVB-T (Digital Video Broadcasting).

(Bild 2)

As a link between the MUSICAM-unity and the COFDM-Coder is the Multiplexer. This combines the audio programs with the data services to a unity – the ensemble. Near by the data can be divided at your discretion. So can be send for instance instead of 7 stereo-programs, 14 mono-programs, but there could be transmitted also 42 voice channels. With an increase of the transmitted supplement data, it will have an effect on the audio-quality. Beside of these sound broadcasting programs the program independent data and program associated data will be sent. (Data rates/Multiplexer).

For manage an exactly synchronisation between the separate channels, the modulator, the multiplexer and the tuning duration of the DAB channel will be tied up to a reference frequency, which is received via GPS. The DAB-mode 1 (used in the frequency bans III on the blocks 12A, 12D and 13A) allows a maximal duration difference of 250 µs and requires a frequency exactness of 10 Hz.

After the modulator the signal of 38,9 MHz-intermediate frequency will be brought to a radio frequency (for instance in the band III, block A on 223,936 MHz) and will be intensified there. The end amplifier allows an amplification of 50 to 2000 W.

Block wiring diagram of a DAB-channel

To prevent interference with a neighbouring TV-channel, a sharpe broadcasting-output filter will be used, which muffles the signal at +/- 0,97 MHz interval from the intermediate frequency for 71 dB.

Nearer information about the transmitter stations

Overview of the transmitter stations with declaration of the coordinates, sea level, program, polarisation, height of the antenna on the masts and effective radiator power.

 
Data rates/Multiplexer

The picture above shows the signal processing for a DAB-Multiplex. The chart bellow shows the configuration of the presently transmitted ensemble (program name, data rate, protection level, capacity units).