Digital television lets the broadcaster transmit "sub-channels". Perhaps you are watching channel 27. There may be two sub-channels available, called 27.1 and 27.2. These sub-channels are only available if the broadcaster is using one of the lower-quality formats of DTV.

An example would be to broadcast in 480i format during the daytime. We could then offer our viewers two or three different programmes on sub-channels. We could also broadcast extra information or web pages with the programmes.

In the evening, we might broadcast in really high quality 1080p format. Since the signal is much higher quality, we can only transmit one programme at once.

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