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Liquid Crystal Display ripple

  1. Jun 5, 2012 #1
    Hi there,

    I'm measuring the voltage on the output of an amplified photodiode circuit that is capturing light emitted by an Liquid Crystal Display (LCD) projector and there is a periodic voltage drop which I don't know the reason. The projector is projecting an all-white image.

    I've been reading that LCD displays must be AC powered because if they are powered using DC there will be electrochemical reactions that will make them to fail.

    I can see the AC modulation on the signal and that is ok for me. What I cannot explain is the origin of the periodic voltage drop/ripple.

    As an attachment you can see the plot of this signal.

    Any help would be greatly appreciated.

    Thank you very much in advance!
     

    Attached Files:

  2. jcsd
  3. Jun 5, 2012 #2

    sophiecentaur

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    The photo diode output drops out at about 4ms intervals, according to your graph. That implies there is a flicker of the screen brightness about 250 times per second. This implies to me that the screen refresh rate is around 250Hz. There is a short period when the screen is 'off', called the blanking interval. Old CRT displays needed this to turn down the brightness of the spot as the scanning electron beam returned back to the top of the screen every 1/50 (or 1/60) s and this blanking feature seems still to be present with your display - despite not having the scanning electron beam. (There was also a blank period at the end of each scanning line on old TVs but it doesn't seem to be there on your LCD display)

    The fast frame repetition rate is common for new displays as computers use all sorts of screen refresh rates. 250Hz does seem quite fast though - look at the options on your computer monitor control panel.
     
  4. Jun 6, 2012 #3
    Hi sophiecentaur/all,

    thank you very much for your reply.

    But do LCD displays also have that blanking interval internally or does it come from my computer graphics board that send the signal that way, independently it is connected to a CRT or to a LCD display?

    I've been through my monitor options and I couldn't find any info regarding a frequency value close to 250 Hz. The only one I see is "screen refresh rate" that is set to 60 Hertz that is still far.

    Thank you very much in advance! :approve:
     
  5. Jun 6, 2012 #4

    sophiecentaur

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    I have no idea where this 250Hz could be coming from. Can you select a different screen refresh rate on your computer and see if that 250Hz rate changes? At least that could imply a connection with the video feed.
    My other thought is that it could be something to do with the multiplexing of the addressing of the LCD array. Perhaps there is some optical feedback involved?? I'm just kite flying here.

    What do you get when you blank off most of the screen and just look at a small area? When you wave your hand across the screen, do you get the same 'strobe' effect as when you do it in front of an ordinary LCD display?
     
  6. Jun 6, 2012 #5
    I've selected a different refresh rate and the 250 Hz rate did not change...

    when I blank off most of the screen and just look at a small area the same thing happens...

    when I wave my hand in front of the screen I do not see the "strobe" effect. Thus, I infer that the 3 images for RGB are dispayed at the same time, what makes sense since this projector does not have a color wheel.

    Any more ideas?
     
  7. Jun 6, 2012 #6

    sophiecentaur

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    Interesting! No, I wouldn't have expected a sequential colour display either.
    If the flicker is really that high, the strobe effect would be much reduced and it may be deliberate to reduce the flicker for people sitting close - which they often tend to do, despite the glubby resolution. Peripheral vision is very sensitive to flicker.
    I googled "flicker" and it seems to be a problem with data projectors. Modern thought is definitely in favour of high refresh rates - particularly with high def systems. Perhaps that's what it's all about. Did you Google your particular model and find out its detailed spec?
     
  8. Jun 6, 2012 #7
    One more thing:
    sometimes the frequency of this ripple kind of duplicates but when that happens nothing has changed to cause that.

    I've googled for my particular model but I could not find any info that could help me with this. I'd like to know the vertical and horizontal sync range but that information is either not applicable, unpublished, or unknown.

    Does anyone knows if this V-sync and H-sync are applicable to LCD projectors?

    By the way, my projector is an Epson EB-W7.

    Thank you very much in advance.
     
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