Q: When is 1080i not 1080i?
Thursday, 15th July, 2010A: …when it’s 720p!
With the huge move towards high-definition (HD) television in recent years there has been a noticeable improvement in picture quality, but there also has been an increase in the amount of confusion relating to the subject.
Nowadays it’s not just a simple matter of walking into a TV store and buying a colour TV based just on looks; consumers also need to consider just what they need the TV for, and as a consequence are faced with a barrage of hard-to-grasp numbers and other technical jargon.
It’s safe to say that TV broadcasts around the world - for the most part - are confined to but two flavours: 720p and 1080i. When we talk of ‘720p’ we talk of a screen size made up of 1280 pixels by 720 pixels. The “p” in 720p refers to “progressive scan” which means that the transmitted image is electronically created from the top to the bottom of the screeen in a single pass, refreshed at either 50 or 60 times per second, depending on where in the world you are watching it.
For those TV broadcasts which use 1080i there is a higher resolution of 1920 pixels by 1080 pixels. One would therefore think that because of a greater numbers of pixels this would generate a higher-quality picture.
But this is not necessarily the case!
The ‘i’ in 1080i refers to ‘interlaced’. As compared to progressive scan images, interlaced images are created very quickly in a two-pass process. The first pass creates every other line (as in lines 1, 3, 5, 7 and so on), while the second pass creates the skipped lines (lines 2, 4, 6, 8 etc).
In Europe and other parts of the world where electricity is provided at 50Hz, creating one full interlaced image takes 1/25th of a second, or twice as long as a progressive scan signal. So while resolution is higher, interlacing can create flicker and can also make fast moving objects appear slightly blurred. For this reason networks which transmit high-definition fast-moving sports’ programmes typically use 720p, while less-energetic nature programmes tend towards 1080i for a richer resolution.
Most newer Bang & Olufsen TVs - for example the BeoVision 9, BeoVision 4-103 and BeoVision 10-46 - can all display transmissions both at 720p and 1080i, but along with most manufacturers’ sets, convert signals to the native resolution of the set. For example, a 720p HDTV takes a 1080i signal and uses an internal processing chip to downconvert the picture to the 720p resolution. This is called scaling. It can also de-interlace the 1080i signal and display it in progressive scan mode. Manufacturers sometimes market the 720p HDTV as a 1080i HDTV, simply because it supports 1080i, albeit by scaling down the resolution.
A 1080p HDTV doesn’t reduce the resolution of a 1080i signal. It only has to de-interlace it. Therefore a 1080i picture should look slightly better on a 1080p HDTV, particularly when using larger screens. When watching a 720p broadcast on a 1080p HDTV, the signal is upconverted to the higher 1080p resolution. While this upconversion arguably makes a negligible difference in modest sized TVs, it does reduce pixel-related artifacts which can be appreciated on larger displays.
There are few, if any broadcasts in 1080p (1920 x 1080 in progressive scan mode), although this may change in the future as the format gains more appeal. However, there are already digital formats which are able to produce a true 1080p signal: Blu-ray and HD DVD players for example. Only a 1080p HDTV can display Blu-ray and HD DVD in their native, full resolution formats. Among 1080p HDTVs there are also various models with different support modes. The newer models support 1080p/60, 1080p/30 and 1080p/24, with the latter number being frames per second. The 1080p format is also preferable for gaming and PC use.
Hence, 1080i is a video or broadcast mode. There is technically no such thing as a ‘1080i HDTV’ because all HDTVs are progressive scan and not interlaced. This includes LCD and plasma flat screens as well as LCD, LCoS and DLP rear projection TVs. Only cathode ray tube (CRT) televisions were interlaced; and though there were CRT rear-projection HDTVs, the majority have now been phased out.
So why do some manufacturers promote 720p HDTV as 1080i? The answer is purely marketing. When 1080p HDTVs were first introduced, renaming 720p HDTV to ‘1080i HDTV’ was an astute move. The 1080i was priced much lower than a comparable 1080p model, with many if not most consumers not really appreciating the difference. Some industry sites now refer to ‘720p/1080i HDTV’ to make it clear that a 1080i HDTV is really a 720p HDTV.
For those consumers currently looking around for a 720p HDTV, they need to know that they should not be paying extra for a ‘1080i’ model. But by purchasing a genuine 1080p model it would still be necessary to pay the higher prices. But by doing so, you will know that you’ll be more future-proof… or until technology changes yet again and something bigger, better and clearer comes along!
