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Motion artifacts. 100Hz/50Hz TVs & cinema.

Discussion in 'Projectors, Screens & Video Processors' started by Ludae, Jan 5, 2001.

  1. Ludae

    Ludae
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    This is extracted from a thread I contributed to at uk.media.home-cinema newsgroup. As motion artifacts and 100Hz TV processing is a hot topic recently I thought I'd attempt to explain some of the issues on this forum as well.

    In article <y_X46.3306$293.386019@news3.cableinet.net>, "Chip" <nospamthanks@nospam.virgin.net> writes:

    >As I understand it, when you watch a film at the cinema, to avoid
    >unacceptable flicker they show each frame several times. Like 3 x 25 = 75
    >fps or something like that. There's no question of interpreting in-between
    >frames - you just get each frame several times.
    >
    >Why is it not possible to do this with TV's I wonder? No-one comes back
    >from the cinema complaining about horrible motion-blurring problems in MI-2.
    >(They've got enough to complain about watching *that* film!)

    The same problems of motion blur and judder exist for all repeat image display systems, and that includes cinema projection. It is most common for cinema projectors to repeat each frame only once, i.e. they up the flicker rate to 48Hz. This is about enough to produce a relatively flicker free image for the ambient lighting conditions of a typical cinema and the light reflected from the screen. There will be image types where the 48Hz flicker will or could be annoying but these are quite rare.

    I am writing an article on the subject of visual perception and 100Hz TVs as well as other related matters that will be posted at a dedicated Web site on video processing.

    So just briefly, the main problem with repeat image displays is that the human visual system naturally tracks the trajectory of objects within the image. It can very quickly assess the average speed of that moving object and know where it is likely to appear in the next instant. In the case of temporally sampled imaging systems, that 'next instant' is the presentation of the next image frame. If you imagine an object moving from left to right across the display, the eye will track that object and know where it should appear for any instant during its travel. If the temporal sampling (frame rate) is 24fps (24Hz as per the cinema) the object will, say, be at point A on the screen in 1/24th of a second and point B at the next 1/24th of a second. The eye will track the object from point A and expect it to be at point B in the next 1/24th of a second, so no blur should occur but as the frame rate is very low then judder can be disturbing at certain motion speeds.

    Now if the projector shows each frame twice i.e. at 48Hz then the moving object is flashed to the eye twice but at the same position on the screen, however, the eye will be tracking the object at 48Hz now and in the first 1/48th of a second will see the object at position A but will, in the next 1/48th of a second expect the object to be half way between points A and B. Therefore for the second 1/48th of a second the moving object is shown again at point A but the eye is now looking slightly to the right of point A. The persistance of image on the retina means that the image of the moving object at point A is seen twice on the retina but a two slightly different (depending on motion speed) positions and this is what causes unintentional motion blur and exacerbated judder perception.

    The same thing occurs in TVs. A 50Hz TV divides the movie frames into two fields and shows each one after the other in interlaced form, therefore as per th cinema projection, the moving object is shown in the same place twice, i.e. 25 (Hz) motion phases per second yet the eye will be presented with 50 images. The problem of judder and blur is made significantly worse with 100Hz TVs due to the fact that there are still only 25 motion phases per second but the eye is presented with 100 images and therefore now sees 3 repeat images of the moving object.

    Such motion artifacts will limit the viewers ability to see the spatial detail in moving parts of a image sequence, so solving the problem of judder and blur will bring the benefit of the viewer registering better spatial resolution in moving images and greater sense of depth to the image.

    There are ways to solve the problem of judder and blur.

    One is to capture the image frames at the same temporal rate as the display device (this is obviously not a practical solution for all displays in the market).

    The other is for the display device to show no repeat frames, (this is not desirable for some display types, eg CRTs due to their scanning properties) this can be done with such display technologies as DLP micro mirror devices. However, judder will still be a problem if the frame rate is too low for smooth motion as is the case with cinema film at 24fps.

    The other solution is to create new motion phases to replace the repeat images in such a display as a 100Hz TV. This is possible, but complicated due to the fact that the process requires motion in images to be accurately tracked and compensated for in the frame rate up-conversion.

    One consumer motion-compensated frame rate up-conversion solution is Philips Natural Motion processor, the latest version works very well (albeit with some occational artifacts of its own), and this type of technology is impoving very fast with the increase in processing power at consumer level and the increased integration and decrease in high speed memory costs. Some more powerful solutions are limited to the broadcast and post production markets due to cost and complexity and others are none real-time and therefore unsuitable.

    In answer to the question above as to why people don't complain about motion judder/blur at the cinema. Well, it manifests itself to a lesser extent as there is only one repeat image at the cinema but three repeat images on a 100Hz TV and the brighness/contrast of the imaging systems are quite different and last but not least, cinema goers are used to the low frame rate and the motion artifacts. They do see them but have grown accustomed to them in much the same way as Americans have grown up accustomed to 3:2 pulldown judder for movies shown on 60Hz displays. Generally British citizens notice 3:2 judder more than Americans and Americans notice 50Hz flicker more than the British.

    More on this subject, with diagrams and screen shots, in my future article. Details will be posted here on the HCCO forum when the time comes.
     
  2. SteveK

    SteveK
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    Great Info,

    So if it came down to it and you were buying a TV, and wanted Sony, would you buy 100hz or 50hz, bearing in mind DVD, SVHS and sky?
     
  3. Ludae

    Ludae
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    Well, one of the drawbacks of researching videopocessing and visual perception is that I seem to have become quite sensitive to video artifacts and motion judder/blur [​IMG]

    Whilst auditioning all the 36" widescreen TV models in the UK I became quite taken with the smooth motion and extra spatial detail afforded by Philips Natural Motion processor. So I bought the Philips 36PW9525, it's an excellent set in many other respects of course, I didn't just by it for the Natural Motion feature.

    As for a Sony 100 or 50Hz, well if it doesn't have motion compensation on the 100Hz set then I would have to go for the 50Hz model.
     
  4. Craig Crane

    Craig Crane
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    More like "100 hurts". I have never seen a good 100Hz image. The early phillips models killed the whole concept for me.



    ------------------
    Check out my HT
    http://freespace.virgin.net/woof.bark
     
  5. Mr.D

    Mr.D
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    I think we discussed this issue on an NG. I've just had a look at one of these and I'm sorry to say ...it posterises and I had a fiddle with it and stuck a couple of discs through it including VE. Its not the worst I've seen but its not much better if any than the DRC Sonys. Check out the end scene tunnel collapse in Starship Troopers with all the dust.
     
  6. Ludae

    Ludae
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    I wouldn't say that it is impossible for it to have any processing errors, but posterization was something I was particularly keen to do without and that is why I choose the Philips. I tested it against other 36" TVs and my plain analogue 50Hz TVs and found it to be the best digital chassis I could have a choice from.

    I tested it with some video I know shows up posterization on some TVs and/or has such errors in the source, and the Philips looked no different that my 50Hz TVs in that respect. In fact, on many occations I have thought that the 100Hz TV is causing artifacts but I have discovered they are the same on my other TVs and the source material or reception quality is usually to blame.

    The Philips 36PW9525 does produce artifacts, (which TVs don't) but on the whole they are not of too great a disturbance to me all things considered. I am particularly critical of video quality and could well do without any artifacts but I don't have the choice of such equipment if it exists at all!

    It is possible or course, that an individual TV has a fault or quality control oversight that can give rise to artifacts such as posterization and noise.

    In most of the shops where I saw the Philips I was not particuarly impressed with the picture, but that was due to the environment and source material. But at home, setup well in decent view conditions it seems like a completely different TV.


    [This message has been edited by Ludae (edited 19-01-2001).]
     
  7. Mr.D

    Mr.D
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    Fair enough if you are happy with it. I looked at the philips as I've hit the theoretical size limit for a non-100Hz TV at 32"

    I think I'll go down the prog scan HTPC CRT projector route when I next upgrade.
     

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