10

Broadcasting digital video

Digital modulation

Information theory tells us that the capacity of a channel is its bandwidth multiplied by its dynamic range. Clearly the simplest method of transmitting digital binary data is by a simple amplitude modulation scheme, like that shown in Figure 10.1a. This technique is known as amplitude shift keying (ASK). This isn’t very efficient in bandwidth terms because, if we take the available channel bandwidth to be F, it’s only possible to achieve in the region of 2F bits/s as a maximum data rate. (Note that it’s important to ‘shape’ the digital pulses in order to limit the sideband energy, which would be very great with fast-edged pulses.) Furthermore, it isn’t a very efficient usage of the analogue ...

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