Discussion in 'Televisions' started by johnnyweston, Aug 4, 2005.
if 1080i is downscaled to 720p, wouldnt it go down half to 540p then upscaled to 720p?
That's the cheap and nasty (and poorer quality) way of doing it.
The other solution is to de-interlace 1080/50i to 1080/50p and then scaled this to 720/50p (you can replace all 50s with 60s if you are dealing with 60Hz stuff)
1080/50i should have vertical resolution roughly equal to 800/50p - so treating it as 540/50p is not a good recipe for image quality.
Apparently this doesn't stop 720p display manufacturers doing this - presumably so they don't have to include 1080line quality frame stores, high resolution de-interlacing etc...
This maybe why :
1. Some people find 720p material looks better than 1080i material on their 720p displays.
2. People use HTPCs and external scalers rather than relying on the scaling within their display (though this is more difficult with HDCP)
any displays you know of that do this downscaling the true and proper way.
'fraid not... I'm not up-to-speed on all of the displays available - and I don't think the techniques used for cross-conversion are widely publicised... (For obvious reasons - I suspect those that use 1080p internally are more likely to mention it, and those that use 540p will keep quiet...)
no point in getting a hdtv then if most the content is 1080i and is downscaled to 540p
A sweeping and very inaccuarte statement:
1080i is two sets of "frames" one containing odd and one contianing even lines 720p is 720 lines of information as fixed pixel devices are generally 720 or 768 vertical rows its more complex you can go from 1080i to 720p by dumping every 3rd line from each frame giving you 720i then deinterlacing..................
No not really, firstly he says if, and secondly he is expressing an opinion not a fact. I think it's unkown at this stage how many devices convert to 540p prior to scaleing back up to 720p.
The OP is referring to the fact that some 720p displays use the 540p "quick and dirty" solution (which removes the need for 1080i deinterlacing to 1080p)
Won't your solution of ditching every third line in a 540 line field (from a 1080i frame) to create a 360 line field (for a 720i system) cause aliasing problems (as you are ditching every 3rd line, rather than filtering and ditching a 3rd of the vertical resolution) - unless you filter vertically? If you filter vertically in the field domain prior to de-interlacing then you're going to lose more resolution than if you filter in the FRAME domain after de-interlacing?
Also - if you create a 720i line interlaced scheme and then de-interlace then if you aren't careful you'll end up with approx 500 lines of vertical resolution when you de-interlace to 720p - which is roughly the same as the result you'd get if you used 540p...
The only way of maximising quality - as broadcast converters and digital video effects devices do - is to convert from 1080i to 1080p and then scale to 720p. If you want to scale an interlaced image for maximum quality (even in the SD domain) - you need to de-interlace prior to the scaling - and re-interlace if the output domain is also interlaced.
(480i to 576i standards converter go via 480p one way, and via 576p the other for example. "Frame based" SD DVEs would convert 576i to 576p internally, process the video and then convert back to 576i for output. Lower quality "field based" devices are available, but their performance is noticably poorer)
unfortunately there are very few manufacturers who tell you how they do it. There are several methods of scaling and different ways fitting incoming signals to the pixel array on the device.
hmm most the content i get hold of is 1080i
anyone know any sets that do the true downscale to 720p?
You could just wait a while then forget about downscaling!
We do tend to follow US and the lag time is getting shorter.
deinterlace 1080i and scale to 720p is the way forward I think unfortunately only a handful of devices can do this at the moment and I mean handful.
The problem is not so much lack of resolution in 720p or 768p displays but the cheap and nasty deinterlacing solutions. Even a 1080p display such as the one you linked to may have 540p effective resolution when displaying 1080i.
If a 1080p panel deinterlaces using the field bob algorithm (as most do), it will take each 1920 x 540 field and scale it to 1920 x 1080, ignoring the next field. A motion adaptive deinterlacer on the other hand uses data from both fields when there is no movement between fields and interpolates when there is movement, retaining much more of the vertical resolution; film images have the full vertical resolution.
While the poor quality of 1080i deinterlacing may be one of the drawbacks of current generation HDTVs, what is worse still is that many of the TVs on display today don't even deinterlace 576i SD material properly. The picture quality of panels with SD motion adaptive deinterlacers is significantly better than those that use field bob.
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