Metz Slim TV manual Digital transmission, Simultaneous broadcasting network

41.Basic differences between analog terrestrial reception and DVB-T

Information for our dealers and interes- ted customers.

In order to understand how the beha- viour of DVB-T receivers differs from that of the analog terrestrial receivers used until now, two things are important: digi- tal transmission and the simultaneous broadcasting network.

Digital transmission

A DVB transmitter does not transmit only a single programme, but a set of pro- grammes – normally six to eight. The information for each programme – the picture, the sound and teletext – is con- verted into a digital format, i.e. into numerical values which are grouped together in packets. The packets of the various programmes are then transmit- ted sequentially in a so-called transport stream.

Further information which permits the receiver to detect and correct any errors which occur during transmission is added to this transport stream.

Example:

If the information 0 or 1 is to be trans- mitted, the 0 could be encoded as 000 and the 1 as 111. If one of the three digits numbers is falsified during trans- mission, i.e. the receiver receives, for example, 010 instead of 000, the recei- ver can decide that the presence of at least two zeros indicates that the inten- ded information value was 0 and, cor- respondingly, that at least two ones indi- cates the information 1.

If two of the numbers in a block are changed, the receiver can detect that an error has occurred, but any attempt to correct this would have the wrong result.

Example:

If the information 0 is encoded as 000 and is received as 101, the receiver would assume that this should have been 111 and will then decode it, incorrectly, as 1.

To sum up:

several programmes are transmitted by a single transmitter in the form of digital packets. Additional information is trans- mitted to permit complete correction of a certain number of transmission errors.

Simultaneous broadcasting network

For DVB-T, the programmes are not transmitted on a modulated carrier fre- quency; instead, one channel with a width of 8 MHz contains 8,192 individu- al carrier frequencies.

Some of the signals reaching the receiver come directly from the transmitter, others are signals which have been reflected from obstacles in the vicinity and some are even those transmitted by other transmitters. Normally, these signals reinforce each other. In extreme cases, however, if they are out of phase and have the same amplitude, the signals can cancel each other out:

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