Are you HD-Ready?

However, as any TV with a Freeview decoder built-in also sports the wholly unrelated 'Digital-ready' tick logo the propensity for further confusion in the mass-market is immense.

What resolution is HD?

There are a number of video formats that can legitimately be called HD which are, thankfully, more complementary than competitive.

The base line for HD video is a widescreen image (16:9 ratio) of 720 horizontal lines scanned in a single sweep at 50 frames per second - i.e. progressively - and thus known as 720p.

Compared to our stalwart PAL broadcast signal of 575 visible lines, 720p offers some 25 per cent better resolution and does not suffer traditional PAL interlace flicker. 720p looks good, is not too bandwidth hungry for broadcasters and is the system of choice of the European Broadcast Union.

All TVs on the market sporting the new HD-Ready logo will be able to produce a true 720p image. Sky's initial HD offering will be in 720p, most Windows Media Video HD content is available in 720p versions, and the cable companies are also nodding towards 720p.

Freeview, had it had any spare broadcast capacity whatsoever, would also agree to 720p as it is the lowest bit rate HD format. Good so far.

More than one format.

In the US and Japan a 1080 line format HD is popular with many broadcasters and indeed HDTV subscribers as it is 25 per cent higher resolution again than 720p.

The signal is interlaced from two frames of 540 lines each at 25 frames per second, meaning the bandwidth required to transmit is not massively higher than 720p HD. (It's still high however as the inverse square law kicks in - i.e. 1080 line picture contains well over twice as much data as a 720 line image).

Many large screen CRT rear projection TVs in the US are capable of displaying a 1080i HD and produce very good-looking TV images even though there is an minor interlace flicker.

The flicker issue will not be a problem with plasma, DLP or LCD TVs however as they de-interlace the picture into single progressively scanned fames as standard. The problem currently is that no plasma TV, LCD or DLP TV on the market is capable of displaying a true 1080 line image.

The good news is that a 1080i signal will not blow a gasket on your 1280 x 768 pixel TV. Every HD-ready TV is packed with an array of digital trickery that scales the image to best fit the native resolution of the panel or chipset.

The ultimate picture?

The ultimate in HD technology as it stands today is a progressively scanned 1920 x 1080 pixel image. 1080p is the native resolution of professional digital TV and film cameras thus creating a format that is, literally, exactly what the director saw as he filmed.

The concept of direct camera to TV transfer is highly appealing but the data-rate implications are huge. Thus, 1080p is simply unlikely to happen as a mainstream broadcast HD format.

But that is not to say it won't happen at all. Packaged media - i.e. HD-DVD and Blu-ray disc - could easily support a 1080p HD standard and Windows Media Video HD is similarly 1080p friendly.

For content downloaded over broadband connection to be stored and viewed later, the bandwidth issue is of less relevance and new codecs such as MPEG4 are reducing the size of media files all the time.

While true 1080 line plasma, LCD and DLP TVs are still not readily available and few broadcasters are likely to adopt the 1080p format in the foreseeable future, it is still seen as the pinnacle of HDTV technology.

Thankfully any device capable of sourcing 1080p video, such as a media PC, will be just as capable of scaling the signal down to 720p until true 1080p displays are available.

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