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When is 720p Not 720p?
Henning Hoffmann writes "HDBlog has an interesting entry about many home theater displays.
Home theater displays around the resolution of 720p (most DLP, LCD, and LCOS displays) must convert 1080i material to their native resolution for display. No surprise there. But many displays do this by discarding half of the 1080i HD signal, effectively giving 720p viewers an SD signal - not watching HD at all! "
Home theater displays around the resolution of 720p (most DLP, LCD, and LCOS displays) must convert 1080i material to their native resolution for display. No surprise there. But many displays do this by discarding half of the 1080i HD signal, effectively giving 720p viewers an SD signal - not watching HD at all! "
This sounds like the visual version of what Creative Labs has been doing for YEARS with their Sound Blaster audio cards. With most other cards if you want to record with a sample rate of 44.1 khz, you record at 44.1 khz, but even with the newer Sound Blaster cards it must be resampled to 48 khz first.
It doesn't matter if you are sampling up or down, resampling is bad, your best b
et is to find a device without it, or if it is necessary like in this case, the one that does the best conversions.
If I bought one of these displays I would be pretty pissed, but I doubt there is much that can be done about it, if you COULD do something than companies like Creative Labs would be out of business.
No surprise there. But many displays do this by discarding half of the 1080i HD signal, effectively giving 720p viewers an SD signal - not watching HD at all!
The HD signal's still there... you just have to learn how to read between the lines.
Here is tfa for you...
When is 720p not 720p?
Tom Norton, in his coverage of the Home Entertainment expo, brought something up that I was unaware of.
720p displays show native 720p signals directly, of course. They also upconvert SD signals (like DVD) up to 720p for display. And 720p displays must convert incoming 1080i signals to 720p before they can be displayed. No surprise there, this makes sense. But, Silicon Optix claims that most manufacturers do the 1080i conversion just by taking one 540 line field from each 1080i frame (which is composed of two 540 line fields) and scaling that one field up to 720p, ignoring the other field. Reason being, it takes a lot less processing power to do this than to convert the image to 1080p and scale that, which would use all the information in the original signal to derive the 720p signal. If you have a display like this, it means that you're watching 540 lines of resolution upconverted to 720p. This is not HD, just like watching a DVD upconverted to 720p is not HD. Sure, you'll get the full width of the 1080i resolution, but you're only getting half the height. While this is better than DVD, it's not HD in my mind. (Aside: Tom Norton mentions this in his review of the Screenplay 777 projector.)
If this is indeed the case, most people with 720p (or similar) projectors (and most DLP, LCD, and LCOS home theater projectors are exactly that) are not seeing what their displays are capable of. They're not, technically, even watching HD. This is crazy! How can this be? Why haven't we heard of this before? How are manufacturers getting away with it?
Over-reacting? Well, if you're an owner of a 720p (or any similar resolution) projector you're either gonna be really upset by this or you're just gonna be laisez-faire about it because there's nothing you can do and you're enjoying your projector just fine thank-you. But me, I don't even own any such projector and I'm a little ticked. But I guess I should really wait for evidence of how properly-done conversion looks in comparison before making any snap judgements. I'm sure that the people selling HQV (a processor chip that does it the RIGHT way) will set something up.
To me, this is a serious issue. Comments are welcome.
from: http://www.hdblog.net/
There's got to be a fairly straightforward formula relating inherent resolution loss when performing any noninteger upsampling, or any downsampling. Any other change in resolution must necessarily degrade the signal, yes? (Except perhaps if a clever algorithm could losslessly encode the original data in a 1.5x-upsampled version, without distorting it.)
Paleotechnologist and connoisseur of pretty shiny things.
Is there any way of telling which sets do this? This is certainly something I'd like to know before I dropped that kind of cash.
"Ask not what your country can do for you." --John F. Kennedy
A Home Theater PC with good quality parts, drivers, and decoders will preserve the 1080i signal - it will combine the 1080i field pair into a single 1080p signal, and then downconvert (ie. downscale) to 720p.
As a reference, my Athlon XP running at 2.4 GHz (aproximately equivalent to an Athlon XP 3400+) with a Geforce 6800GT and TheaterTek 2.1 software will have (little) trouble achieving this, assuming the 1080i source isn't glitchy itself.
Alternative is to use the NVIDIA DVD Decoder version 1.0.0.67 ($20 US after 30 day trial) and ZoomPlayer 4.5 beta ($20 beta or nagware) for similar results.
TheaterTek is roughly $70 US and includes updated NVIDIA DVD Decoders - too bad NVIDIA hasn't updated their official DVD decoders with the bugfixes that is present in the TheaterTek package.
When the un-washed masses can't actually tell the difference (they can't even see DCT blocking) and you can get away with selling this crap to them..
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When I upgrade to an HD idiot box, I plan on sticking with tried-and-true CRT. IMHO, you can't beat the picture quality/price, and I have yet to hear a compelling reason to fork out thousands of dollars for the trendier offerings.
"Ask not what your country can do for you." --John F. Kennedy
If the broadcast is 1080i, and your display isn't 1080i, I don't think it's logical to assume the quality of the downsampled video will be equivalent to a true 720p broadcast.
When I get around to buying a HD television (not any time soon, I do all my televisioning on my computer), it will be a true 1080i (are there 1080p televisions?) display so I'll know I'm getting the full potential of HD.
Unless I'm strapped for cash, of course, in which case I'll just suck it up and know my 720p won't be the best thing for watching 1080i content on.
On the plus side, it's important to get the facts out there for the consumer, who will likely (although not logically) assume he's/she's getting more than they really are.
Why not submit a link to the original article, rather than a link to your blog, which consists only of a link to the original article?
Otherwise, people might assume this is a shameless attempt to draw traffic to your site.
Ok, here's the skinny. 1080i is 1920x1080 @ 59.94 fields / second, meaning at any one instant in time, you're looking at a 1920x540 image made up of every other line of the picture (the odd fields, if you will.) Then, ~1/60th of a second later, you see the even fields. 720p is 1280x720 @ 60 FRAMES per second, meaning at any given instant you're looking at EVERY field of the image...not just the odd or even fields. If you were to try and take all 1080 lines from the original signal, they wouldn't really map properly to 720 at any given second because half of data would be from that same ~1/60th of a second later. Scaling the fields up is really the best way to go, at least for stuff that's been shot interlaced.
And when you use it to upsample data, it is a lossless encoding that doesn't degrade the signal (unless you deliberately throw away data - discrete Fourier transforms are also used in lossy encoders).
It's not a distortion-free transform, since high frequency signals (e.g. sharp edges) in the original image get interpreted as smooth changes and can get blurred between multiple pixels in an upsampled signal. But then again, that's exactly the sort of thing that happens when you digitize a picture in the first place - if you have a sharp black/white edge that passes through the middle of a pixel, the most accurate thing you can do is make that pixel gray.
There's got to be a fairly straightforward formula relating inherent resolution loss when performing any noninteger upsampling, or any downsampling.
Its a bit messy. Imagine 1080i image with a 1-pixel wide sloping black line that is nearly horizontal on a white background. If you throw out half the data, you create an image with a dashed-line. Gaps in the line occur where the slanting line cut across the rows that were discarded. If you upsample from 540 to 720, you will find that the remaining dashes become fattened non-uniformly. In places where the row in the 720-row image falls directly on top of the 540 row image, the line will be thin and dark. In places where the 720-row image falls midway between rows in the 540 row image, the line will be wide and less dark. The end result is the thin black uniform line is converted to a dashed line of varying thickness and darkness -- not pretty.
Even if you resample directly from 1080 to 720, you still run into problems where the 720-row image pixels fall between the 1080-row pixels. At best, you can use higher-order interpolation (e.g. cubic) to try and fit a curve through the original data and try to estimate what was in the middle of the pixels so they can be shifted half way over. But the result wil never look like an image that was taken with a 720-row camera in the first place.
Two wrongs don't make a right, but three lefts do.
Kind of hard to "do your research" when you can't find out how the conversion takes place or can't understand how it all works. This is not a consumers fault kind of thing. This kind of information is not made known unless people ask the question. Who would have thought to ask a question like this?
This is not particularly news. Some "blogger" discovers something because he never bothered to ask and screams something about the sky is falling.. I'm kind of sick of this "news" reporting. Incidentally, this same issue affects owners of most plasma and LCD tv's with native resolutions below 1920x1080 too.. depending on how you look at it as a problem or not.
Anyway, it's fairly well known that the internal scalers in many devices suck. That is why there is a market for good external scalers. If you are paranoid about watching a lot of 1080i on your 720p projector or LCD TV or Plasma, go buy a scaler. They cost about $1000 but will improve scaled display a lot.
At least if you have an external scaler you will have some options about how you convert 1080i to 720p. The article makes it sound like splitting the fields is a huge sin -- and it is if you discard one field per frame (Half field deinterlacing), but it's perfectly acceptible to scale EACH 540-line field to a seperate 720-line frame and double the framerate. This is called bob deinterlacing and is often the best for converting 1080i video to lower resolutions. If you are watching a 1080i upconvert of a film or something, though, you can have the scaler do full field deinterlacing and inverse telecine for you and see a nice 720p/24fps picture. Scalers also generally have internal audio delays for various types of audio feeds so you won't have to worry about AV sync issues either.
If you have any questions about how your device does this, you should try to find out before you buy it. Most devices don't publish how they do it, though, so your only option may be to derive it -- and that will not be an easy job.
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The results may be one of the following:
You will get a screen full of tiny, shimmering horizontal lines that shift in the center of your screen Congratulations! Your HT gear is showing a true 1080i picture You will get a full screen of gray, possibly with a line in the center Not bad, your gear is properly downscaling the signal Half your screen is black, the other is white Uh oh. Your gear is taking the easy way out and dropping half the scanlines to downconvert (Bele and Lokai) I call that the Cheron Test.Back in the day, on the Atari and Amiga, you could actually do stuff in that interrupt time. The most common thing was to swap display buffers for double buffering. This made for rock steady hardware scrolling, an effect that still lacks somewhat in today's PC's, believe it or not, as there was absolutely no tearing of the display whatsover. Just a beautiful effect.
This is my sig.