Why do Digital projectors only like 60Hz?

Discussion in 'Projectors, Screens & Video Processors' started by Jeff, Nov 12, 2001.

  1. Jeff

    Jeff
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    I thought that I would get a debate going as to why this is. It maybe that this is only true when connected to a PC. Anyway it is very annoying that us digital projector owners cant make use of the 72 and 75Hz refresh rates that allow CRT owners to decrease pan judder. The only explanation that I have read to date is that the electronics aren't fast enough to cope with higher rates and that this results in tearing. However I don't buy this because I see tearing at 50Hz as well. There appears to be a reason why 60 Hz is the magic number. A theory of mine is that they are fixed 60Hz devices but have the means to accept other rates and translate them back to 60 Hz. Anyway we have a few experts out there who maybe able to get to the bottom of this so its worth a try.

    Jeff
     
  2. Roland @ B4

    Roland @ B4
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    My thoughts are that most manufactuers gear them selves up for the American NTSC market (60Hz) and if it will do Pal (50Hz) then fine. (the same with DVD's)

    Domestic projectors are just not designed to run at 72/75Hz It's not a domestic format.

    Us CRTs, we laugh in the face of such low refresh rates.

    Let me know hen they have finished designing lamped projectors, I'll come out of my cave and have a look ;)
     
  3. Jeff

    Jeff
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    Hopefully before CRT projectors become extinct. ;)
     
  4. ROne

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    Most LCD projectors are multi-scanning devices and can support all sorts of refresh rates.

    This is not the issue, the fact is video is supplied as an overlay to the video card and the problems lies with the the projectors inability to synchronise with this overlay. The point is you can lock a normal windows screen to LCD with at 72hz etc, no problems, no tearing etc.

    However the DVD display sits on top of this display as an overylay. So all though your windows display is kicking out the correct refresh rate, the overlay is exactly what its says, an overlay and I think this is where the problem lies.

    I don't have the technical information as to why it doesn't affect CRT, but I bet its somehting to do with the CRT being a scanning device rather than a digital display.

    Maybe somebody with more in-depth knowledge of display systems could add to this.
     
  5. RichardA

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    Jeff,

    Can you confirm for me what you exactly mean by tearing-

    Is it, what we would call 'mousetoothing' - alternate 'scan lines' are offset a little on moving edges; i.e.

    ----------
    -------
    ----------
    -------
    ----------

    Or is it where the top and bottom half of the display seem to update at different times

    Thanks,

    Richard
     
  6. Jeff

    Jeff
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    Thats the one.

    Jeff
     
  7. RichardA

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    Well the only thing I can come up with is that the DLP chip is being addressed as two halves, and at the higher frame rates it's unable to update both halves instanaeously.

    In a DMD the next setting of the mirror is written to the transistors under the mirrors while the mirror is still doing it's previous 'tilt' - this means that there is a fixed amount of time that is available for the writing. I suspect that TI has decided that it can get away with reducing the number of wires to carry the data to the chip by multiplexing the data and that it needs that time to complete it's tasks transparently - If you go too fast it runs out of time!

    This wouldn't answer why this problem would be seen at 50Hz though!

    I have a strong feeling that this is the basis of the problem though - confirmation would be that an 800x600 DLP doesn't do it (the 848x600 chip is definatley not addressed as two halves)

    This is also an issue with some LCD panels, some are multiplexed in 'panels' (either top and bottom half or even 2 by 2 checker board for really high res) or in 'interlace' (alternate lines are multiplexed).

    hope this is of interest!
     
  8. ROne

    ROne
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    This seems like a good measured answer, however my LCD panel MT7, will do it only on the RGB input at whatever frequency it is fed. Even if you put 60hz in, the tear remains static.

    Then if you put 60hz in the component, you don't get any tearing whatsoever.

    60hz in RGB = tearing
    60hz in Component = no tearing.


    Could it be, you have got you windows desktop at say 75hz, and the video overlay playing on top at 50hz/60hz depending on PAL/NTSC and that the conversion to the 75hz windows desktop is not perfect?

    Maybe CRT being analogue in nature is more tolerant of the upconversion?

    Graphics cards experts we need you!
     
  9. Jeff

    Jeff
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    This is VERY interesting news indeed, with the new Radeon 8500 you can output component video. So it may be possible to get different refresh rates without tearing using component.

    Jeff
     
  10. Roland @ B4

    Roland @ B4
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    I've typed this out three times now and each time my internet connection bums out.

    I doubt your Component and RGB souces are at the same resloution. If so, not a fair comparison.

    My understanding of these digital things is that they have an in built clock that is tuned to the pannel (DLP or LCD) and optimised to the pannels best performance.

    Any incoming source has to be digitised and expanded or reduced to fit this clock speed. So not just the horizontal and Vetical pixels have to be scalled but also the number of times per second (Hz). Increasing the refresh rate will make the processor in the projector work faster. Try reducing the resolution of the PC to 640x480 at 75hz and see if that smeers

    If not I think your PJ has hits it limit on the number of calculations persecond.

    The algorithums in the component input will only be used to seeing 50 or 60hz
     
  11. jrwood

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    I have the following hardware and have experienced no tearing effects whatsoever, but I will do some testing tonight as I have'nt been looking for it. However I know what it is so I would of usually picked up on it.

    Sony VPL-CX1 XGA unit
    Dual p3-550mhz / 256mb RAM QDI Brilliant IV running WinXP
    ATI AIW 128 AGP 4x Pro
    [email protected] using ZoomPlayer for media

    James
     
  12. GaryG

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    Jeff

    I use 72 and 75 hz and have not noticed any tearing on my LT150, that's not to say it doesn't do it but it's not something that's jumped out at me, I might regret this if I suddenly start noticing it but can you give some example material that I can check it out on?
     
  13. Jeff

    Jeff
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    Gary,

    Can you check what the LT150 thinks the refresh rate is. If I use the regular display settings or the standard timmings in Powerstrip windows says that I'm using says 72Hz but the PJ still says 60Hz. If I force the 72 Hz in custom timmings then the projector accepts it.

    Jeff
     
  14. Mr.D

    Mr.D
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    Ok heres some stuff for the soup.

    As far as I know:

    The 2d overlay that is utilised for video playback on PCs ( and others) is not seperate from the desktop refresh and resolution settings. If the refresh is 72Hz the overlay will also be refreshing at the same rate . The whole reason for the 72/75 HZ refresh is to allow the overlay to repeat frames rather than interpolate ( more like simple average to be honest) . At unusual refresh rates ( ie not multiples of 24 or 25) the overlay itself can exhibit tearing and stuttering.

    Now I could be wrong about the next bit.
    I remember hearing that LCD panels have a fixed refresh rate regardless of input ( seen 60Hz mentioned but heard of 25Hz ! on some earlier panels).
     
  15. GaryG

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    Jeff

    I tried 1024 x 768 @ 60, 72 and 75, LT150 reports correct frequency. Loaded Powerstrip and tried 1024 x 768 @ 60 and 75 and 1024 x 576 @60 and 75 again LT150 reports correct frequency.
     
  16. Mark Grant

    Mark Grant
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    Jeff,

    The 60 Hz is because you have an XGA 1024x768 DLP.

    I have a Davis SVGA 800x600 DLP projector.

    It is best at 72 or 75 Hz.

    I tried an XGA DLP, a Davis DLX10 I think, it would only sync at 60 Hz without tearing.

    72 or 75 Hz was unusable because of the tearing.

    At 60 Hz I see the rainbow effect.

    At 72 or 75 Hz I dont see it, that is why I will stick with my SVGA DLP until maybe 16:9 DLP projectors are affordable, and refresh at a high rate.


    So the 60 Hz problem, as others have said, will be to do with the way the XGA DLP chip is adressed/refreshed.

    Shame really, because 60 HZ looks bad to me, so does the tearing !

    Mark G.
     
  17. Ken

    Ken
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    I use a Davis Cinema 1, with refresh rates of 72Hz & 75Hz, and no sign of tearing that I can see.
    Ken
     
  18. Comer

    Comer
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    Perhaps some of you who have come across this tearing could post some examples of movie parts where it is evident. I am currently looking at the possibility of using a HCPC with my Toshiba MT3 and I have been told that this Projector always shows tearing using the RGB in regardless of the refresh rate.

    I have tried the pj and a low spec pc with Gladiator but I could not see any tearing in any of the panning scenes I viewed.

    I want to be sure that if I do buy a HCPC that this is not something that I will start to see as time goes by.

    Thanks

    Conor
     
  19. ROne

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    Gave my HCPC a whirl again last night and my results are:

    75hz for PAL, tearing but smooth - great picture

    60hz for PAL, tearing but jerky

    50hz for PAL, virtually no tearing but jerky.

    Not many options, however when switched back to gladiator R2, the quality of the interlaced picture seemed such a let down that I would almost be prepared to put up with the tearing.

    I have also noticed some inconsitencies with peoples suggestions of refresh rates.

    I have been told 75hz for PAL because of the 25 fps divisability. And 72hz for NTSC because of 24fps divisiabilty. While this makes sense on the face of it, it doesn't acutally add up.

    The PAL refresh rate is based on the divisability of the PAL frame rate but the NTSC refresh rate is based on the divisabilty of the original FILM frame rate. Surely if its 75hz for PAL then it follows that NTSC ought to be a 60hz or 90hz signal? Or in both cases stick to the frame rate of film, a multiple of 24.

    I did a test today with some of my video equipment...

    When we film a computer screen say at 75hz, if I film the screen with my BETACAM camera in ordinary PAL at 50hz you get a scan line because the source material is not in synch with the Camera. This scan line will turn into tearing if I move the camera left-to-right fast (the equivalent of a DVD pannig shot). To get over this you can turn HYPERHAD on the camera and select a different shutter speed. So I can select the 75hz and the camera will now lock onto the monitor almost perfectly.

    I say almost perfectly, because the monitor may be putting out 75.12 hz or something and my camera may actually be 75.03 hz so there is a minute difference even though in essence that are both 75 hz. This minute difference is the best you can get, and may not exhibit tearing because your are almost in synch.

    And this is where I think the problem lies, I believe that a digtial LCD panel has got a variable (say 0.5%) tolerance of its scanning rate and to make matters worse this alters in real time, even when you have set the refresh rate.

    So I set my graphics card at 75hz (or as close to it as possible), and then set my LCD projector at 75hz. When you check the projectors signal again, it may have shifted to 75.02 then back to 75.054 and so forth. The same with the computer. So they are never exactly in synch. I hope I am clear so far.

    So in reality you have to digital devices (although they both pass through an analogue stage), thath are locking to each other at slightly different rates. Hence when I hit my sweet spot of 50hz, I some times see tearing and some times not becuase the projector is minutely altering the scanning input stage as is the graphics card.

    You then complicate matters by upsampling or downsampling the signal to 75hz or 72hz or whatever and you have even more synch errors. So you have more tearing the more you move away from the projectors internal frequency.

    The reason why I don't think CRTs have the same problem is to with the tolerance of an analogue device and perhaps the fact that it doesn't have the extra digital conversion, just a smooth signal rather than a cropped digital approximation.

    Anyway what do you think?
     
  20. Mr.D

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    film sourced NTSC ( 525/60) actually runs at 24fps with reference to the original material. 3:2 pulldown is added to raise the frame rate to 30fps but these extra frames are made up of repeated fields hence the 3:2 repeat pattern .

    Ideally you want to inverse telecine this back to 24fps and then display in multiples of the original frame rate ie 72HZ ( each frame writes to the screen 3 times) This should minimise tearing and stutter on a capable display. You should get this result from a decent deinterlacer with 3:2 pulldown detection or an HTPC properly configured.

    Video captured at 525/60 needs to be deinterlaced to 30fps as it won't contain any 3:2 pulldown frames just 60fields.

    The tearing I suspect is down to the frame buffer in the PJs failing to keep up with the higher refresh rates of the panel : normally this wouldn't be particularly noticable on a PJ showing primarily text and staic diagrams from a PC.

    Alternatively it might even be the panel having problems keeping up with the frame buffer as suggested earlier sort of.

    If its the first one then double or triple buffering in the pj would be advisable but there is nowt you can do once you've got one to fix it other than drop the refresh rate.

    If its the second one again you can do zip apart from drop the frame range and manufacturers would be advised to sort it out quickie as most of the PJs out there mostly use the same panels!!!!
     
  21. ROne

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    Thanks Mr D, you're input is always welcome.

    I'm going to do an experiment, I'm going to order myself a Key Digital RGB-YUV transcoder.

    Run the HCPC in 1280x720 60hz, this is native to my MT7 and is HDTV standard.

    I will then transcode from RGB-YUV and put in on the component of my MT7 where there has been no problem with tearing so far with other scalers and sources.

    I'm wondering if the RGB input, with it being constantly scanning and adjustable through a huge range of requencies is also part of the problem, whereas the component is either 50hz/60hz fixed.
    Maybe this is also part of the reason.

    Anway ordered today I'll let everyone no how I get on.
     
  22. Comer

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    Perhaps someone could tell of a few scenes in movies that are prone to this tearing so that some of us can determine if it is a problem for us.


    Thanks

    Conor
     
  23. Jeff

    Jeff
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    On my projector tearing is very mild and it's not predictable, I just know that if I watch a movie for a few minutes at anything other than 60Hz I will get a tear, it is also mild enough not to spot it. You also get another effect which you also get with CRTs which is less predictable stutter or wobble. The video/pan will be smooth for a couple of seconds, then you might get a stutter (like a short pause) or a wobble (unexpected movement in the video). This is where the magic 71.928 number is supposed to come in which is NTSC 59.94 / 60 X 72. With powerstrip it's very difficult to achieve this figure unless you happen to know the exact timings. With my current projector I have managed to get 71.927 and 71.929 but these didn't seem to help. My my view its better to have predictable 3/2 pulldown stutter than have something is unpredictable and far more obvious when it occurs.

    Jeff
     
  24. tryingtimes

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