There are a number of differing world-wide standards for analog TV, but here in the UK, we use the PAL system. Picture and sound are transmitted on frequencies in the UHF range, typically between 600 and 850 MHz. Video and audio are transmitted on the same UHF channel, seperated by a fixed amount, usually something like 6.5MHz. Sound is transmitted using the frequency modulation, or FM, method. A complete video frame consists of 625 lines of information. In order to be able to fit this into the available bandwidth, a technique called interlacing is used, where the odd numbered lines of the image are sent first, followed by the even numbered lines.

The electronics inside a traditional TV receiver display the odd numbered lines by directing a stream of electrons^[1] at the phosphor coated interior of the screen. The phosphor glows at the points where the electron beam touches it and this glow lasts for a short period of time before fading. Before it fades, the even numbered lines are 'painted' on the inside of the tube by the electron beam. The human eye is (mostly) unable to distinguish between the odd and even lines being displayed and therefore sees only a single complete image on the screen.

Digital TV uses the same transmission frequencies as analog^[2]. The difference lies in how the video and audio information is encoded into the available bandwidth. Firstly, the video and audio are converted to streams of digital data. The format used is MPEG-2, the save method which is used to encode video onto DVDs. The streams are then mixed, or 'multiplexed', together before being transmitted on the selected UHF radio frequency. The clever bit lies in the fact that the resultant data stream takes up only a fraction of the available bandwidth when compared to a regular analog transmission. This means that it is possible to multiplex many different data streams together and transmit them all at the same time. Each UHF channel can now carry several digital TV channels at once. Even this, however, still leaves enough spare bandwidth that the broadcasters can also include audio-only channels for digital radio^[3], clever things like a real-time electronic program guide, or EPG, and also teletext style text-based information services.

What you end up with when you tune your DVB receiver to a given channel is a multiplexed MPEG-2 data stream. This data stream contains all of the digital channels being broadcast on that frequency, in a format called MPEG-TS, or MPEG Transmission Stream. Within the data, each channel is tagged so that it can be identified at the receiving end. These tags are known as Programme IDs, or PIDS for short.

[1] When television was first invented, nobody knew what electrons where, other than the fact that they could be produced by applying a volatge across 2 plates seperated by a vacuum. Since the plate where the electrons appeared to come from was the positively charged plate, or cathode, the elctrons were dubbed 'cathode rays' and hence the device used to make a picture became known as a 'cathode ray tube', or CRT, an abbreviation which survives to this day.

[2] I had been under the impression until fairly recently that there was some magic method whereby digital and analog TV channels were combined onto the same UHF channel. I couldn't for the life of me work out how this might be achieved, which is hardly surprising, as it's not the case! The 2 sets of channels use different frequencies from each other. What this does explain, however, is why digital TV seems to need far more sensitive antennae than you would seem to think. My local TV repeater is up on a hill about 2 kilometres north of me, but I'm classed as living in a 'weak' signal area. The reason for this is that it was already a fairly low powered transmitter, even before digital TV came along. In order to add the DVB multiplexes, it was necessary to either increase the input power to the transmitter, or reduce the amount of RF power being radiated on the analog channels. In my case, the latter was obviously the easier option, which explains why I now get a weaker analog signal from the same transmitter since digital TV came to my neighbourhood.

[3] Not to be confused with digital audio broadcasting or DAB - this uses similar technology to DVB, but for traditional FM radio in the 88-108MHz broadcast band.