Film-Tech Forum ARCHIVE: D-Cinema refresh rate

This is topic D-Cinema refresh rate in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Sigurd Wik (Member # 6131) on 03-04-2011, 04:54 AM:

Hi,

Oddly enough, I can't seem to find any resources on the refresh rate of D-cinema projectors. The only place I can find this subject mentioned is when reading about 3D and the refresh rate of 3 per picture per eye when using RealD on DLP projectors. Are there differences in refresh rate between the Sony, Barco, Christie projectors when screening 2D, or is the refresh rate common for all? As far as I can see, the DCI specifications don't mention a standard refresh rate either.

Sigurd

Posted by Dave Macaulay (Member # 813) on 03-04-2011, 07:01 AM:

What's a refresh rate?
The frame rate is 24.
Refresh rate for a CRT refers to the scan rate of the electron beam, with LCD screens it doesn't mean much, and with a DMD system I'm not sure what it would mean at all. A video camera recording a film screen, CRT, or LCD screen (not 100% sure of the LCD) shows artifacts like flicker or dark bars but from a cinema digital projector screen the recorded image is "perfect". So it looks like there's no exact "refresh rate".

Posted by Sigurd Wik (Member # 6131) on 03-04-2011, 03:30 PM:

Maybe I use the wrong terminology or have just misunderstood it all. With refresh rate I mean how many times each frame is flashed, in the same way as when a two or three bladed shutter flashes each frame two or three times with traditional film projection. Or doesn't it work this way?

Posted by Carsten Kurz (Member # 5396) on 03-04-2011, 04:47 PM:

No, 'flashing' is only used on DLP systems in 3D (usually 144Hz/triple-flash).
2D for cinema content is 24fps=24Hz refresh rate. The cinema projectors can be configured to do double- or triple-flash with 2D as well. It just doesn't make sense, since there is no dark-period, so you gain nothing by repeating identical frames.

Input rate in 2D is 24fps->output rate 24Hz. In 3D, input rate is 48fps, output rate is 144Hz.

All cinema projektors support native input and output rates up to 60fps/Hz. Some even mention 96 or 120Hz.

Sony does not (need to) do triple-flash in 3D.

- Carsten

Posted by Marco Giustini (Member # 4544) on 03-05-2011, 04:49 PM:

I tried to take a picture at 1/1000s of an LCD screen, and nothing happened, just a dimmer picture!
What you may see is what is called "motion blur", since when a new frame is displayed, some of the previous frame is still visible - especially on slow displays.

DLPs are so fast by nature that are virtually motion blur free.

Anyway, as said, there is no need to repeat the frame. If you are showing a still picture for example, you may show it at one frame per second, there won't be any difference on the screen.

Posted by Sigurd Wik (Member # 6131) on 03-06-2011, 04:32 AM:

I see. Thank you very much for clarifying, Carsten. Is it correct then that the tripple flashes of the DLP 3D systems is just a necessity caused by the polarizing filter wheel?

Posted by Carsten Kurz (Member # 5396) on 03-06-2011, 06:57 AM:

TripleFlash is used by all single projector 3D methods using sequential L/R separation - Dolby 3D, RealD, MasterImage, XpanD.

The reason is 24fps in 2D is no problem, however displaying L/R at 48fps would give a very annoying parallax left eye/right eye flicker. You would actually notice subjects flipping left-right-left-right, like when you flicker your eyes quickly. Hence they switch the same L/R images three times at high rate interleaved, so that flicker goes away.

There are some 3D projection methods that display left and right eye simultaneously - they do not need to do triple-flash.

- Carsten