Digital TV means the television signal is broadcast in a digital format (such as Sky Digital). Digital TV is not necessarily high definition (HD) TV - HDTV is just one form of digital television.

There are three digital options currently available:

• Sky TV's pay-digital service via satellite.
• Telstra Clear's cable pay-TV digital network, re-broadcasting Sky in Kapiti, Wellington and Christchurch.
• Freeview digital satellite and land-based services

Picture quality

Digital TV will show on any old TV with the aid of a decoder, although they won't be able to show HDTV pictures. But what about the quality?

Unlike analogue, digital transmissions don't suffer from interference or ghosting (unless the weather is foul, when it can break up or cease altogether from "rain fade"). So all the information gets through, which can mean a potentially better-quality picture. But then the decoders convert it to an analogue signal (see Analogue vs digital below) for your TV, and this can mean some loss of quality.

Using the best-available decoder connections will mean your analogue screen can get a slightly better picture from a digital broadcast. But for the best picture quality, you need a high definition signal direct to an HDTV screen.

Analogue versus digital

Analogue TV is now old technology - it was first invented in the 1930s - and its resolution is limited by the number of horizontal scanning "lines" the TV can show.

New Zealand analogue television is broadcast using the PAL (Phase Alternate Line) video format, also used in the UK and Australia. This has a higher resolution than NTSC (National Television System Committee but also known as Never Twice the Same Colour), which is used in the US and Canada.

PAL has 625 lines, but only 576 are actually used. NTSC uses 480 lines out of a possible 525. Digital television has the potential to show up to 1080 lines (at 720 or more lines, it's called HDTV).

Many screens accept either PAL or NTSC, and VCRs or DVD players can usually play both types. So if someone sends you an NTSC tape or DVD, chances are it will play okay, unless it is the wrong zone for your DVD recorder.

Analogue realities

You'll still be able to get analogue signals for another couple of years, but the uptake of Freeview and other digital tuners in New Zealand means the government will soon set a rough date for switching off analogue completely. This date is currently expected to be some time after 2011-2012.