Film-Tech Forum ARCHIVE: 3D flash ratio

This is topic 3D flash ratio in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Marco Giustini (Member # 4544) on 12-05-2014, 04:30 AM:

Hi all,

Silly question for you. I am aware that when setting up 3D 6:2 means triple flash and 4:2 means double flash. But what does that number mean? What exactly 6:2 means?
Thanks

Posted by Marcel Birgelen (Member # 6801) on 12-05-2014, 05:20 AM:

I guess a lot of people don't really know what those numbers mean [Wink]

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The first number is the Frame Rate Multiplier and the second one how many "images" form a complete frame. For 2D this is 1 and for stereoscopic 3D it is 2.

Posted by Nerijus Marmokas (Member # 6712) on 12-05-2014, 06:14 AM:

In other words, projector flashes same frame three times, for each eye.
It goes something like this:

Posted by Marcel Birgelen (Member # 6801) on 12-05-2014, 06:36 AM:

Yeah and in your scheme, it would be 2:2, 4:2 and 6:2.

There also is a setting for the frame sequence: L->R or R->L, which isn't encoded in this N:M notation.

You need to flash the same image multiple times to get rid of noticeable flicker. Anything above about 90Hz can be safely assumed to beyond the flicker fusion threshold with normal 3D content, whereas 48Hz clearly isn't.

This scheme also doesn't account for the dark time between each image change. This dark time is required to compensate for things like transitions of the polarizer or color wheel surface and minor timing/sync issues between the components involved.

Posted by Harold Hallikainen (Member # 5405) on 12-05-2014, 03:19 PM:

On flicker fusion frequency, what effect does duty cycle have? For simplicity, let's start with 2D. We typically flash the image 2 or 3 times per frame. But, as the duty cycle approaches 100%, the dark time approaches zero. If we do 3 flashes with no dark time, it's just one flash. So, how much dark time is required between flashes so they have the desired effect of reducing perceived flicker?

Harold

Posted by Marco Giustini (Member # 4544) on 12-05-2014, 03:39 PM:

thanks everybody for the clear explanation.

Then running 3D 2:2 is basically single flashing.
Which takes to a slightly OT question: I know some XpanD units are not capable of 3D HFR. If the unit does not sync when set at 4:2 but it does at 2:2, does that mean that that unit is not HFR ready? Or maybe do XpanD work single flash (in HFR) as some older MI units?

Thanks

Posted by Marcel Birgelen (Member # 6801) on 12-05-2014, 04:36 PM:

quote: Harold Hallikainen
On flicker fusion frequency, what effect does duty cycle have? For simplicity, let's start with 2D. We typically flash the image 2 or 3 times per frame. But, as the duty cycle approaches 100%, the dark time approaches zero. If we do 3 flashes with no dark time, it's just one flash. So, how much dark time is required between flashes so they have the desired effect of reducing perceived flicker?

A DCI DLP projector runs with a duty cycle of 100% in normal 2D mode.
The next frame can be loaded into the DMDs while the current frame is being displayed and can be globally switched. There's no need for a sync time. That's also why there is no more visible strobe effect.

If you increase the duty cycle, you indeed decrease the interval with no picture on screen. At rates above 90Hz it will have not much effect on the amount of perceived flicker, but a longer duty cycle will have a positive impact on the brightness.

quote: Marco Giustini
Which takes to a slightly OT question: I know some XpanD units are not capable of 3D HFR. If the unit does not sync when set at 4:2 but it does at 2:2, does that mean that that unit is not HFR ready? Or maybe do XpanD work single flash (in HFR) as some older MI units?

As far as I know, every XpanD 3D system, even the early models, should be capable of doing at least 48 FPS single flash (2:2). All newer models are also capable of doing 48 FPS at 4:2. You might need a firmware update for those earlier models though.

Posted by Marco Giustini (Member # 4544) on 12-05-2014, 07:00 PM:

I was told the latest models, where you plug the sync cable directly in the emitter, were already HFR ready. XpanD say not-ready units can be amended with a "cost-effective procedure".
Are those units SW upgradable?

Posted by Marcel Birgelen (Member # 6801) on 12-05-2014, 07:21 PM:

Those who plug directly into the sync without the "Distribution module" are the models suited for the smaller rooms. They should support double flash in HFR out of the box.

All units should do at least single flash in HFR, without any upgrade. As far as I remember, the Double Flash upgrade for older units is indeed essentially just a software update for the distribution unit, but not a free update.

Posted by Carsten Kurz (Member # 5396) on 12-06-2014, 05:20 AM:

HFR capability does not indicate flashing capability, unfortunately. e.g. MasterImage issued a note two years ago stating that their MasterImage MI2100 system was 'fully HFR compliant'. With it came instructions on how to setup the system for 48fps operation. It then actually uses single flashing at 96Hz...

LCD shutters do not have the mechanical limitations of spinning wheels like MI (Dolby managed to get their smaller wheels rotating faster without a problem), but they may have issues with increased dark time vs ghosting, or, the transmitter/receiver circuits may have sync catch windows that do not allow large variations of the flashing rate (without software changes). But to my knowledge, all LCD shutter systems are HFR compliant or at least can be upgraded to HFR without much trouble.

- Carsten

Posted by Marco Giustini (Member # 4544) on 12-06-2014, 06:06 AM:

The unit in questions does not sync at 4:2, but it does at 2:2, without changing any other parameter.
Is there a way to confirm whether the unit is not HFR-4:2 compatible (software version, serial number...)?

Posted by Marcel Birgelen (Member # 6801) on 12-06-2014, 08:12 AM:

I think you've proven this particular unit doesn't support double flash in HFR then [Wink]

.

The unit should auto-detect the sync rate, if it doesn't, it needs an upgrade. You should just drop them an e-mail and ask for an offer for the upgrade.

Posted by Steve Guttag (Member # 268) on 12-06-2014, 04:25 PM:

when you are testing the sync...are you using 48fps target? You have to have the right frame rate coming out of the server.

Posted by Marco Giustini (Member # 4544) on 12-08-2014, 05:02 PM:

yes, HFR clip from the server.