Chief Blur Buster wrote:Mechanical Spinning-Disc Shutter To Lower LCD/LCOS Projector Persistence:
You can mechanically lower the persistence of an LCD/LCOS projector with mechanical rolling shutter. It is possible to strobe a projector for low persistence by using a spinning disc with a slot -- like a blacked-out colorwheel with a white area. Mechanical rolling shutter to lower LCD/LCOS projector persistence with non-modulatable HID source. Experiments have been done with a spinning variable-speed fan (with black blades) in front of a projector lens -- and it does produce blur-reducing effects to off-the-shelf LCD projectors.
You build a VSYNC-synchronized motor instead, with a plexiglas disc that's blacked out except for a small slot (percentage of disc black = percentage reduction of motion blur, 75% black pie = 75% reduction in LCD motion blur) -- like a spinning pie chart -- 75% black, 25% clear -- for a 75% reduction in LCD motion blur from your projector. GtG leakage between LCD refresh cycles is still a problem, creating the double-image strobe-crosstalk depending on strobe phase (timing of shutter scan relative to timing of LCD GtG scan, as seen in high-speed videos of LCD scanouts).
The mechanical rolling shutter should be as far away from the LCD GtG zone as possible, preferably 180 degrees out-of-phase of the LCD GtG scan, e.g. 1/2 screen height away from the GtG blur zone seen in high-speed videos of LCD refresh cycles (that is, the blurry "wipe effect" from top-to-bottom, seen in the non-strobed section of the high speed video of an LCD). Like a scanning backlight, except with a mechanical shutter -- you're making the fully refreshed part of the LCD visible in a rolling-shutter fashion, as the GtG zones occurs hidden by the mechanical shutter. Use potentiometers/adjustments to allow you to calibrate motor synchronization phase relative to VSYNC signal (either tapped into the cable, or tapped some appropriate solder joint).
Note, you'll be spinning the rolling shutter motor 3600RPM for 60Hz (60Hz x 60s) and 7200RPM for 120Hz (120Hz x 60s). Multiple clear zones (e.g. 4) can allow you to spin wheel slower (e.g. 1/4 speed) instead. Make vibrations are well-damped. Ripping out and modifying an existing DLP colorwheel motor+disc (one that already has clear zone, and blacking-out the colored sections with some kind of thin lightweight opaque paint/film) is also an alternative approach if you can fit the projector aperture on such a hacked spinning disc.
An additional idea -- a 120Hz projector LCD/LCOS does have the advantage of doing a 2-pass scan for a cleaner 60Hz strobed LCD -- one can scan the LCD twice in darkness to clean up the LCD GtG as much as possible -- then do a chasing strobe sometime behind the 2nd pass scan -- this can be much simpler for DIY electronics hackers than trying to create high-quality LCD overdrive tables that are strobe-optimized (this can be extremely difficult to do) -- the stuff of NVIDIA LightBoost/ULMB fame is often devilishly difficult to do, but the 2-pass LCD scan is quite simple and can help slower-responding non-overdriven LCDs to look cleaner when strobed.
For best persistence-lowering quality (less double-image effect -- strobe crosstalk) you want a projector with relatively fast LCD GTG response in milliseconds preferably far less than half a refresh cycle -- preferably one-quarter, especially if you plan to do single-pass refresh before strobe. Finding a fast responding small LCD/LCOS can be difficult to find, but it's a consideration worth keeping in mind.
You can even make this optional -- stop the spinning disc or move the disc out of the way -- to go back to full-brightness full-persistence mode, whenever you don't need a low persistence mode for your projector.
For more information about DIY strobed display experiments, see Electronics Hacking: Creating A Strobe Backlight
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