Transmission modes
There are three different transmission modes, applicable to different ranges of radio frequency with the number of carriers and several other system parameters depending on the mode. DAB has three main modes with 1536, 768, 384 carriers and guard intervals between 246, 62 and 31 µs respectively (Table 24.2). There is a fourth mode between Mode 1 and Mode 2 with a symbol duration of 623 µs used in Canada. In each mode, the carriers occupy a total bandwidth of 1.536 MHz, they use DQPSK and time- as well as frequency interleaving. The maximum radio frequency that be used in each mode is that at which the system can overcome the Doppler effect while moving at speeds of up to 100 km/h. For mode 1, the maximum RF frequency is 375 MHz making it suitable for VHF transmission.
The total symbol duration consists of the principal symbol period and a guard interval. The latter prevents the echo of the previous symbol from interfering with the current symbol. By doing so, inter-symbol interference (ISI) is reduced to almost zero as long as the echoes from the various trans- mitters and propagation paths do not substantially exceed the guard inter- val. The maximum permissible difference in the length of the propagation path between two SFN transmitters D in meters can be calculated from the guard interval Tg and the propagation speed c:
Mode 1 is intended for terrestrial transmission, particularly using SFNs. Its comparatively long symbol duration (1.246 ms) and guard period (246 µs) makes it most appropriate for a large network of terrestrial VHF (Band III) transmitters. Mode 2 is intended principally for small to medium coverage area (e.g. local radio) using UHF L-Band. The guard interval is sufficiently long to ensure immunity from multi-path propagation, but is not really suit- able for SFN applications. Mode 3 is intended for cable and satellite trans- mission where there are no long echoes using the UHF L-Band.
For Mode 1, the available bit rate may be calculated as follows:
However, not all the bit rate is available because of redundancy for error correction, control, synchronisation and guard period resulting in a useful bit rate of about 2.3 Mbps. This can provide, for example, five stereo pro- gramme services each at 224 kbps.
DAB frames
Although DAB is essentially dedicated to the transmission of ‘audio’ serv- ice, it may also deliver other services under the banner of ‘general data’ service, which may be data for the display of extended text (e.g. the contents of the ‘Radio Times’). The partitioning of data into frames representing 24 ms periods of the application is retained but, generally, these are referred to as ‘logical frames’. It is helpful to consider each logical frame as a burst of data, because when the data for numerous services are multiplexed together they must be compressed in time, so each logical frame is transmitted in less than 24 ms and other data are transmitted between these bursts.
DAB-2
In November 2006, WorldDMB, the organisation in charge of the DAB standards, announced that the DAB system was in the process of being upgraded, and it will adopt the AAC+ audio codec to improve the effi- ciency of the system and stronger error-correction coding to improve the robustness of transmissions. This means there are two different versions of the DAB system: the older existing one, which was developed in the late 1980s, and an upgraded version, which has been dubbed ‘DAB+’ or ‘DAB version 2’. Existing DAB receivers are incompatible with the new DAB standard, but receivers that will support the new DAB standard will become available in spring 2007.
DAB TV
As can be seen from the above, DAB easily lends itself to portable and handheld receivers as it was designed with mobile reception and SFNs in mind. It was not surprising therefore that it became a favourite in the delivery of digital multimedia broadcasting. Unlike DVB which had to be modified to incorporate the requirements for portable and handheld reception, DAB from its inception was designed for mobile reception with one antenna. With DAB, data is sent in bursts that are part of a frame which lasts 24 ms followed by a null frame using time interleaving to over- come the problem of fading. Another advantage of DAB is the use of unequal error-protection (UEP) technique in which bits are protected accord- ing to their importance in the decoding process. This is very important for mobile and portable reception where hostile reception conditions cannot be avoided.
The DAB system is capable of carrying IP packets (datagrams) using IP/UDP connectionless protocol. As these packets travel unidirectionally from a service provider to many users simultaneously, it is not necessary to establish a connection between the transmitter and the user prior to the transmission of data.
A DAB-TV system, also known as DAB-IP is illustrated in Figure 24.5. Live TV is encoded and encapsulated into an IP frame to be multiplexed with the normal digital radio broadcasts. The resulting transport stream is then fed into the DAB broadcast network and transmitted as a Mode 1 OFDM VHF signal. The IP interface provides an independent platform which supports a wide range of services and applications. TV encoding employs Enhanced Packet Mode (a WorldDMD Forum standard) which enables video and other services such as an electronic program guide (EPG) that are more sensitive to errors than the native audio services, to be carried.
At the receiving end, the handset decoder extracts the required TV channel from the multiplex, decodes it and feeds it into the small-screen
display. The handset also receives and decodes the normal interactive services through the third generation (3G) platform.
As can be seen, DAB-TV shares multiplex capacity with DAB digital audio services. This enables operators to use spare capacity on DAB net- works to start offering mobile TV without waiting for a new spectrum to become available. This is the main reason for DAB-TV gaining ground over its rival DVB-H system.