3D DLP-Link syncs to black gaps instead of white flashes? Confused. - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 5 Old 10-28-2019, 11:20 AM - Thread Starter
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Question 3D DLP-Link syncs to black gaps instead of white flashes? Confused.

Hey everyone,

I always assumed that 3D DLP Link works by a single very short white flash after left and right image have been shown (L, R, flash, L, R, flash, ...). The glasses would distinguish between L/R by measuring the time between flashes and switch once half the time between them has passed.

Now I happen to record my BenQ TH682ST at 960fps and I don't see any evidence of a white flash at all. It turns black for ~2ms after each frame though. Left (Cyan, Blue, Green, Red, Yellow), Black, Right (Cyan, Blue, Green, Red, Yellow), Black.

This would correspond to the 60° white segment of the color wheel at 120 Hz. (1000ms/120Hz/360° * 60° = 1.4ms). So apparently the DMDs switch to black during the white segment.

But this does not explain how the glasses know which frame is left and which is right. It also should make sync harder when the content is primarily dark.

So what am I missing here? Is the white segment dark because within it there is actually a white flash being shown but it is so short that I can't capture it at 960fps? How short is it?

Is there any technical documentation / specs / paper on the internet where I could verify this?

Thanks a lot for your help in advance!
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post #2 of 5 Old 11-04-2019, 01:26 PM
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Originally Posted by Frankenst1 View Post
Hey everyone,

I always assumed that 3D DLP Link works by a single very short white flash after left and right image have been shown (L, R, flash, L, R, flash, ...). The glasses would distinguish between L/R by measuring the time between flashes and switch once half the time between them has passed.

Now I happen to record my BenQ TH682ST at 960fps and I don't see any evidence of a white flash at all. It turns black for ~2ms after each frame though. Left (Cyan, Blue, Green, Red, Yellow), Black, Right (Cyan, Blue, Green, Red, Yellow), Black.

This would correspond to the 60° white segment of the color wheel at 120 Hz. (1000ms/120Hz/360° * 60° = 1.4ms). So apparently the DMDs switch to black during the white segment.

But this does not explain how the glasses know which frame is left and which is right. It also should make sync harder when the content is primarily dark.

So what am I missing here? Is the white segment dark because within it there is actually a white flash being shown but it is so short that I can't capture it at 960fps? How short is it?

Is there any technical documentation / specs / paper on the internet where I could verify this?

Thanks a lot for your help in advance!
Are you sure you have the 3D on? The white flash is clearly noticeable without the glasses, by the white-washed picture and overall higher brightness. If the 3D is on, are your glasses DLP synched (rather than RF)? These would be my guesses for the missing white flash.
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post #3 of 5 Old 11-05-2019, 04:57 AM - Thread Starter
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Originally Posted by Roussi View Post
Are you sure you have the 3D on? The white flash is clearly noticeable without the glasses, by the white-washed picture and overall higher brightness. If the 3D is on, are your glasses DLP synched (rather than RF)? These would be my guesses for the missing white flash.
3D is on. I can see when each individual color is being projected for the left and the right frame. But there is no individual white flash. I notice reduced contrast and brighter blacks, but I don't see any flashes. Glasses are definitely DLP synched - covering the photo diode will make 3D sync fail.
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post #4 of 5 Old 11-05-2019, 04:15 PM
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3D is on. I can see when each individual color is being projected for the left and the right frame. But there is no individual white flash. I notice reduced contrast and brighter blacks, but I don't see any flashes. Glasses are definitely DLP synched - covering the photo diode will make 3D sync fail.
Interesting. I tried to find out how long the dlp-link flash is, but could not find any information out there. Apparently, it is much shorter than the white segment timing, otherwise it will encroach on the frame (the few sources I found say it occurs "between frames"). I am quite certain the projector is 144 Hz, and the wheel is 2x speed, I calculate the white segment timing to maximum 0.58 ms (if 60° - I suspect it is a bit more, to let the flash in "between frames"), compared to 1.04 ms for the camera. If the flash being as long, and possibly much shorter, it is possible the camera just cannot catch it.
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post #5 of 5 Old 11-06-2019, 04:56 AM - Thread Starter
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I thought that you get either 2x60Hz => 120 Hz (for 60Hz displays) or 6x24Hz => 144 Hz (for 24Hz displays). An uneven multiple of the display rate (i.e. 2.4x60Hz => 144Hz) would lead to color imbalances.

The camera catches the cyan and yellow wheel segments which are 30° and thus half as short as the white segment. The white flash within the color segment would have to be much much shorter for the camera to miss.
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