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! "

Comments

Which Models?

[goldspider]

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.

Re:Which Models?

[David Leppik]

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.

Yes. Go to a showroom and look at the displays. If you see some that have greater vertical resolution than the non-HD models, there you go. If you can't see a difference, then it doesn't make a difference.

If there is a difference you can't see but could learn to see, don't learn; it will not bring you joy, it will only make you miserable or annoying. Long ago I learned to see the FFT distortion in JPEG and MPEG images. Has it made me happy? No. I end up making the JPEGs on my website bigger than everyone else's so I won't see wrinkles on people's faces that are apparently invisible to everyone else. And I can't stand to watch satellite television on a big screen TV because of the annoying compression artifacts.

Re:Resampling

[gyg]

The only effective limitation comes from linear algebra - there are only as many degrees of freedom as there are pixels, so if you downsample, you *always* lose data, like it or not.

However, even this is not a problem in practice since in real-world pictures nearby pixels are not independent. By using an appropriate encoding dictionary such as wavelets, which zoom in on sharp edges and economize on flat surfaces, you can shrink a typical picture by something like 90% without visible quality loss.

Now since wavelets are actually continuous functions, you could then convert from wavelet representation to upsampled signal, with no information loss. I imagine this could give sharper edges as wavelets are better at egdes than Fourier.

The problem of upsampling well is very similar to making a blurry image crisp - called deconvolution. The problem in doing that is that any noise in the blurry image gets amplified, must be filtered out etc...

Moral is that clever algorithims have been around for ages - the effective limit is whether they can be done in realtime.

How to prove/disprove it easily.

[Stavr0]

Simply provide a 1080i test pattern made of alternating horizontal lines where the left half is black lines on even scanlines, white lines on odd scanlines and the right half is reversed (white-even,black-odd)

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.

CD Audio all over again.

[karlandtanya]

Remember when CDs first came out?

Oh, this was going to be great. Fidelity like you never had it before. No scratches. No groove wear. Dynamic range you won't believe. Crystal clear highs. Thunderous lows, with no rumbling feedback even if sat your player on your speaker.

Remember the little logos? AAD? ADD? DDD was the best you could have (digital recording, digital mastering, and (obviously) digital media in your hand). And a lot of hard work on the part of the engineers operating the mixing boards. It's that last part that costs time and money. Now, all the equipment is digital. So, it's all great, right? Sorry--the technology is not the limiting factor in sound quality anymore.

The limiting factor is apathy. Most people can not really hear the difference. And fewer people care.

Exactly the same thing is now happening in video.
Since we can't improve the functionality (well, we could, but you'd never notice). It's pure hype from here on out.

Now, where'd I leave that case of speaker spikes and green markers? Gotta get 'em up on ebay; David Hannum was right.

Shameless Atari / Amiga Plug

[tjstork]

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.

Re:When is 720p Not 720p?

[Ibiwan]

the DCT is lossless; it just serves to separate the block information into lower-freqency components at the top left of the block, gradient to high-freq components at the bottom right of the block. The artifacts come from a lossy process where they divide the matrix, element-wise, by a matrix with ones in most places but higher and higher numbers as you go toward the bottom right. The end result is, you throw away partial information on your higher frequencies, in a pattern that the JPEG committee's research decided people can't see. Of course, you can further raise the factors from there, which most people do, which is why too many images show the blocking.

Incidentally, the image is further compressed by

• converting the block to a linear array of data by reading in a zig-zag pattern from the top left
  
• using (lossless) run-length encoding on that bit stream (or on the data as bytes or words, not too sure on this point)
  
• compressing THAT output with a (lossless) entropy coding; I believe it's the huffman family of algorithms.

Re:For the inevitable /.ing

[Infinite Entropy]

Just wait for the thin CRT technology to come out. It uses millions of tiny electron guns instead of one, and if it fulfills its promise it will destroy plasma and LCD.