3d frame rate on DLP (XPR) ? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 3 Old 11-16-2019, 04:52 PM - Thread Starter
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3d frame rate on DLP (XPR) ?

I am confused on the actual per-eye frame-rate of DLP projectors, especially current XPR DLP projectors that have FullHD resolution but can shift pixels four times to display 4k. Eg: Optoma UHD52alv or Benq W2700

Supposedly, these XPR DLP projectors can run the color wheel up to 7200 RPM, or 120 rotations/sec. Given how they all have two times the colors, e.g.: RGBRGB, this gives a maximum frame rate of 240 Hz. The maximum published frame rate, i guess when connecting a PC ? is typically specified as 120 Hz though for the projector (2D), and also for the 3D glasses i have only seen number such as 120 Hz, or maybe 144 Hz.

My first question is:

When its said that the glasses switch with 120 Hz, what does that mean ? 60 frames for the let eye plus 60 frames for the right eye = 120 (single-eye) frames ? Or 120 frames for the left plus 120 for the right = 240 (single-eye) frames total per second ?

My second question is:

How exactly is FullHD BD 24 fps displayed ?

If the glasses show 60+60=120 Hz, then this would only work with some Judder, e.g.: 120/24 = 5, so it would require a cadence like:

stereo-frame 1: L R L R L
stereo-frame 2: R L R L R
... and so on.

Aka: Every stereo-frame would have 5 single-eye frame, and for such a frame one eye would get 50 % more brightness than the other eye. So i kinda can't believe that would look good.

The other option is to simply reduce the wheel speed to 96 Hz.

Or the glasses really switch 120+120=240 times, which would also work nicely.

So... What is it ?

Supposedly some of the color wheels make so much noise that its possible to hear if they switch down in speed... Maybe that helps to figure out ?
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post #2 of 3 Old 11-18-2019, 07:32 AM
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120hz DLP 3D is 60hz per eye--Left eye clear when Right black and vice versa. Most 3D projectors now do 144hz to avoid judder since 144/24= 6, therefore each eye is open an even number of times per sec=72hz. Don't confuse frames per second with hertz. With 72hz per eye, each eye opens/closes 3 times for each frame of a 24fps movie, creating a smoother sync.

4k projectors with XPR make a sound when the XPR kicks in and vibrates the chip to produce a 4K image and stops vibrating when in 3D mode to produce a 1080p image. I think that's the noise you are talking about. The color wheel is always maintained at a constant speed that I know of on a DLP projector.
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post #3 of 3 Old 11-18-2019, 08:02 PM - Thread Starter
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Originally Posted by 3DBob View Post
120hz DLP 3D is 60hz per eye--Left eye clear when Right black and vice versa. Most 3D projectors now do 144hz to avoid judder since 144/24= 6, therefore each eye is open an even number of times per sec=72hz. Don't confuse frames per second with hertz. With 72hz per eye, each eye opens/closes 3 times for each frame of a 24fps movie, creating a smoother sync.

4k projectors with XPR make a sound when the XPR kicks in and vibrates the chip to produce a 4K image and stops vibrating when in 3D mode to produce a 1080p image. I think that's the noise you are talking about. The color wheel is always maintained at a constant speed that I know of on a DLP projector.
Thanks, Bob, but can not bring together 144Hz and constant color wheel speed and no judder.

There is plenty of mentioning of newer projectors using 144Hz for 3D, and DLP glasses support up to that frequency, so lets go with that. That would be an improvement over the 96 Hz i have on my 5 year old Sony LCOS projector.

With 144 Hz the color wheel would have to rotate at a minimum of 60 (seconds) * 144 "frame/sec" = 8640 RPM if each displayed "frame" was a full rotation, or 8640/2 = 4320 RPM if each displayed "frame" was just one RGB(S) color wheel sequence (half a rotation).

If you make the calculation of how many RPMs you would need for other 3D (720p60) or any 2D resolution (24Hz, 50Hz, 60Hz), you end up either with judder, or with different RPM of the color wheel. Eg: If you have 144 "frame" / sec, you couldn't show 60 fps videos without judder (or 50Hz).

Hence i am hoping that the color wheel is actually spinning at a speed depending on the video mode, or else i am expecting judder in the video.

Whether or not i would actually be able to hear a difference for the different color wheel speeds, i don't know (yet). You may be right that most mentions of undesired noise from XPR projectors only comes with the pixel shift actuators.

Of course, i would very much hope for different RPMs of the color wheel,
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