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Digital Broadcasting > Digital Broadcasting - Pg. 172

172 Chapter 9 small delay caused by the speed of electromagnetic waves. Using digital transmission, you can transmit a burst of digital signals at a very fast rate, pause, transmit another burst, pause, and so on. At the receiver the incoming signals are stored and then released at the correct steady rate. This has several advantages: · You can multiplex transmissions, sending a burst of one program, a burst of the next, and so on, and separate these at the receiver. We will see later that six television trans- missions can be sent on one carrier frequency (using the normal 8 MHz bandwidth) in this way. You can send signals alternately instead of together. For example, the left and right stereo sound signals can be sent alternately as a stream of L, R, L, R . codes. You need not send the digital codes in the correct order, provided that you can reassemble them at the receiver. This reduces interference problems because a burst of interference will not affect all the codes of a sequence if they are sent at different times. · · n Note One odd effect is that if you watch an analog television and a digital television together, the sound and picture on the digital receiver lag behind the sound and picture on the analog receiver. This is because of the storage time in the digital receiver when the signals are being combined. n The most compelling advantage is that you can manipulate digital signals in ways that would be quite impossible with analog signals. This is why digital methods were being used in studio work well before any attempt was made to use them for broadcasting. Providing that a manipulation of digital signals can be reversed at the receiver, it has no effect on the final signal. By contrast, passing an analog signal through one filter and then through one with the opposite effect would affect the end result seriously. Finally, converting a signal to digital form can allow you to reduce redundancy. An analog television picture of a still scene uses the transmitter to send the same set of signals 25 times each second. A digital version would send one set of signals and hold it in memory until the picture changed. In addition, digital manipulation of analog signals often results in other redundant pieces of code that can be eliminated. There is one important disadvantage that made digital broadcasting seem an impossible dream until recently. Digitizing any waveform results in a set of digits that are pulses, repeating at a high speed and requiring a wide bandwidth for transmission. The CD system deals with this wide bandwidth, but it is not so simple for broadcasting because frequency allocations cannot easily be changed. The way round this is digital compression, removing redundancy in the data until the rate of sending bits can be reduced so far that a transmission will fit easily into the available bandwidth. As we have seen above, this can be done to such