Digital television utilises compression technology to fit multiple channels onto a single frequency. A digital terrestrial television multiplex, for example, goes out on the air on a single channel, but can carry multiple TV channels. This same compression technology can allow much more picture information to go out on a single channel instead, should the broadcaster desire.

To illustrate the point, consider the digital television situation in the United States. There, local broadcasters each operate their own multiplex. WRAL-TV the first HD channel in the USA - transmits the following services over the air on channel 5:

5.1: WRAL High Definition - a high definition feed of its local and CBS programming.
5.2: WRAL News Channel - a standard definition channel showing non-stop local news.
5.3: WRAL Standard Definition - a standard definition channel in digital format.

In technical terms, WRAL had the choice of transmitting many standard definition channels or one HD channel and only a couple of standard channels. That's because a high definition channel takes up more space (bandwidth) in a multiplex than a standard definition service because of the vastly increased amounts of data it requires. The good news is that this increased amount of data leads to a clearer, more detailed picture.

As discussed earlier, standard definition broadcasts are 576i50. The European Broadcasting Union's preferred HDTV format is 720p50. As you'll see, the notation remains the same. So, 720p50 means: 720 lines progressively scanned, refreshed fifty times per second. The entire picture is built fifty times per second. BSkyB has now launched their HD service broadcasting up to 1080i. The increased resolution results in a stunningly sharper and more detailed picture.