HDTV and SDTV
Let's get one thing straight: Digital TV is not just HDTV -- it's just that HDTV is so gorgeous that it gets the lion's share of publicity. HDTV is high-resolution digital television (DTV) combined with Dolby AC-3 surround sound. To watch HDTV you need at least two things: a HD-STB and a HD-capable screen (whether it is plasma, rear projection, LCD or CRT). To get the most out of it you should also have a decent sound system (preferably capable of Dolby Digital 5.1).
The big advantage of HDTV, of course, is that it gives you widescreen format plus a higher resolution -- and therefore, sharper -- picture.
HDTV in Australia can supply images with up to 1080i (interlaced) lines of resolution -- ie, 1080 lines of 1920 pixels with 50Hz interlaced scanning. At this level, HDTV quality is technically far better than what you get from a DVD, and almost as good as the quality of high-definition discs, such as Blu-ray or HD-DVD.
When people refer to HDTV they usually also include 720p (progressive) and 720i (interlaced) formats. This is 720 lines of 1280 pixels. The progressive formats have twice the amount of vertical scan lines and are therefore of higher quality than their interlaced equivalents -- eg, 720p is clearer than 720i. The benefits of HD pictures are particularly noticeable on larger widescreens and when using high-resolution projection equipment.
HDTV can also include 576p, as "High-Definition" technically includes anything above the 576i standard of SDTV. Some stations broadcast their HDTV content at this 576p standard, so, even if you have a high-resolution TV and a HD-STB you will notice quite a difference between this content and content which is broadcast at the top-quality standard of 1080i.
For this reason, from a buyer's point of view, you need to make sure of the numbers because a screen can be marketed as a "High-Definition" and not be capable of the higher resolutions that you are really looking for.
SDTV broadcasts also provide widescreen picture format with DVD picture quality and CD-quality sound (MPEG digital stereo sound and/or Dolby Digital Sound), but at 576i resolution (576 lines x 720 pixels @ 50Hz interlaced). Most recent model analogue sets are capable of displaying 576i when connected to an STB. It is worth noting that the 576i SDTV format is almost identical to the quality of DVD movie discs, so even an SD-STB will give you crisper, clearer quality than analogue reception on your existing (older) TV.
HDTV is still not yet being broadcast all the time. The good news for those with a SD-STB is that the Federal Government requires broadcasters to provide an SD signal at all times. So anyone with an SD-STB will always be able to receive digital television even when the higher quality HD television signal is being transmitted. If you have an HD-STB you will still receive SD signals when HD signals are not available. It's also worth noting that no exclusive content is broadcast in HD. Everything that's shown in HD will be shown at the same time in SD, so you won't miss out on any programmes by skipping HD.