The Digital Signal

The ‘digital’ signal used for DTTB is very different to the familiar PAL analogue signal, hence some understanding of the differences by the installer and technician will aid in a quality installation and the explanation to others, including the consumer.

DTTB utilises a form of modulation called COFDM (Coded Orthogonal Frequency Division Multiplex) and is defined by DVB – T standards. The purpose of COFDM is to provide a vehicle for the carriage of the digital signal that is highly robust to the effects of echoes or ghosts that are created from multiple paths from the transmitter to the receive antenna.

The ‘magic’ of the COFDM has its origin from spreading the data between many carriers. Either of two modes of COFDM may be employed, with the ‘2K mode’ having 1705 carriers and the ‘8K mode’ having 6817 carriers. Australia uses the 8K mode, hence the power is spread evenly over the 6.7MHz portion of the 7MHz channel, in contrast to an analogue service with its concentration of power in the vision carrier at the lower end of the spectrum.

The difference in power distribution has important consequences of the measurement of signal level, as covered later in measurements.

Figure 1: Analogue and Digital Signal Spectrum

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