There are two types of digital TV — standard definition (SD) and high definition (HD). Essentially, HD offers better image and sound quality – more like a cinematic experience – than SD.
SD is broadcast at nearly the same resolution as analogue TV, 576 horizontal interlaced lines per screen. Interlacing is where every alternate line is newly displayed (1, 3, 5, … then 2, 4, 6, …). The picture appears smooth because the screen is fully refreshed at least 25 times a second, which is too fast for you to see the interlacing.
However, it can mean the edges of some text on the screen appear a bit jagged and some people may notice a bit of flicker on CRT TVs. Some 100Hz TVs can reduce this problem because they have double the picture refresh rate. LCD TVs can come in 100Hz and higher models and Plasma TVs refresh at a much higher rate so flicker isn't an issue with them
SD usually comes with MPEG stereo sound at close to CD quality. HD can come with surround sound and at around DVD quality.
The Australian Government has decreed that HD can be broadcast at a number of resolutions, beginning with 576p, stepping up to 720p and finally 1080i. The ‘p’ stands for ‘progressive’, which just means the lines are refreshed in one pass, rather than interlaced.
Some commentators argue that 576p isn’t real HD, but the fact remains that it’s within the official description. Whatever the resolution, an HD picture is more stable and potentially has more detail.
Most programs will have Dolby Digital 2.0 sound, which is a slight improvement on MPEG sound. Both are better than analogue sound because neither suffers from interference. It’s possible to broadcast surround sound with both SD and HD, and if you’ve got the right equipment to play it you’ll get a very similar experience to watching a DVD with surround sound. However, not all programs are made or broadcast with surround sound — it’s usually restricted to movies or special events.
Can you see the difference?
- To actually see a difference between HD and SD you need a screen that has a resolution equal to or better than the broadcast resolution and that can display progressively.
- LCD TVs are usually at HD resolution, but some older plasma TVs can come in both SD and HD resolutions, so if you want HD you’ll need to check what the model you’re thinking of buying has. Just to confuse the issue some more, currently there are two HD screen resolutions available;
- 768 x 1366 is mainly on TVs under 100cm, but some cheaper big TVs may have this resolution. On smaller TVs (under 94cm) it's very difficult to see the difference between this and higher resolution screens when sitting at normal viewing distance.
- 1080 x 1920 is often called "True HD" because it's closer to Blu-ray, or the highest broadcast HD specification.
- Traditional CRT TVs are normally SD only, but it was possible for a while to get HD models. They’ve been largely displaced by LCD and plasma sets.
The Federal Government has introduced a labelling scheme to try to make it easier to tell whether a TV or set-top-box is capable of delivering an SD or HD broadcast. It doesn't mean the screen is necesarily HD or SD. You'll need to check this when purchasing.
There are three labels:
This label indicates it's an analogue device, which will need a digital device to make it work once the analogue signal is switched off. In most cases this means it'll be only capable of displaying an SD picture.
This label indicates that the TV or set top box can receive Standard Definition digital free-to-air shows. With Standard Definition, you will receive the same free-to-air shows you currently receive on analog TV and any extra SD digital channels. So even if its screen is HD, it can't receive the HD channels.
This label indicates that the TV or set top box can receive High Definition digital TV. With High Definition broadcasts, you are able to receive the full range of channels, including those that are unavailable in Standard Definition. However, it may have the lower 768 x 1366 resolution screen. Depending on it size, this may be an issue. Check before you buy.