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Cinematography Electronics Film/Video Synchronizing Control


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It's my understanding that to eliminate roll bar from a CRT or 'Tube' TV, one can use the combination of a 23.967 framerate and a 144º shutter. This specific combination, of course, only works with NTSC (60Hz) televisions only. The other PAL (60Hz) methods obviously don't pertain to my situation. Anyway, It just so happens I will be filming a CRT TV, however I own an Arriflex SRII which has a fixed 180 degree shutter. I am able to use a speed controller to set the framerate to 23.976 but that alone won't exactly fix the problem, but rather keep the black bar in a fixed location. I want to eliminate the black bar entirely. It occurred to me that this wasn't a possibility, until I researched the problem quite a bit and actually found out there is actually something called a Film/Video Synchronizing Control Box. At my first uneducated glance I thought it was some outdated equipment which was probably used to change the framerate of cameras before we had framerate controllers. Then the size of the thing struck me and I thought "Maybe this isn't simply for only framerate" because there's no conceivable way to describe going from a giant box to change the framerate to only years later the speed controllers we all know and love that can all fit in the palm of your hand. That's just not the way technology progresses. So after more research, and finding a couple instances of people vaguely talking about how they used to use these sync boxes 'back in the day', I seemingly submitted to the fact that this thing can actually do what it says. But I still just couldn't understand how people can make the claim that this thing could sync any camera with a CRT TV when not every camera has an adjustable shutter. That was until I found this post on here from back in March of 2018. 

On 3/20/2018 at 5:29 AM, James Malamatinas said:

I can also second Robin's suggestion. We had the same requirements shooting a film last year and ended up using the sync box in the image attached, my understanding was that it uses magnetic fields from the CRT to correctly sync the image.

We were shooting 35mm Arri LT but I think the sync box should work with the SR3, we also used a speedbox though to help control the sync and I'm less sure that this would work with an SR3 but worth investigating.


Steps involved for us were:

1. Take Mag OFF!
2. Connect the Speed Box to the cam and the vid sync box (which by this point should be near the CRT)
3. Run the cam.
4. Press Sync Phase (set sync to Manual).
5. Look through the eye piece till you’ve achieved the result needed!
6. Replace mag and turn over.

The box has a red light on it which, when lit, means that the box is in sync.

post-44724-0-99376100-1521538173_thumb.jpg

This actually makes sense, given how cathode ray tubes are sensitive to magnetic fields. Now before anyone dismisses the Film/Video Synchronizing Control Box, I should probably point out a few ways I will not settle for fixing the problem.

1. No I do not want to green screen the tv, even with artificial light to suggest television glow. It never looks real.

2. I don't want anything shot digitally, especially since the cost of a digital camera is more than that of the sync box.

3. I can't afford renting another film camera that has an adjustable shutter for merely one or two shots.

I have considered number one as an option in the past for shots of the television screen only, however one also runs into the problem of the glass reflection not being there. And probably the biggest problem is my idea of a wide shot of the actor watching the television from across the room, both the television and the talent visible in the frame. Now while real television glow is nowhere near enough for a key light, it would have still been present in the frame thus casting some type of glow that would reflect near objects, even the slightest bit.

 

I went a little off track as I often do, so here are my two questions.

How does the Film/Video Synchronizing Control Box actually work?

Can someone who has used this before please explain the setup and operation in detail? If applicable to my situation I'd love to own one.

Edited by Matthew J. Walker
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  • 4 weeks later...

(Long reply, sorry)

There are two variables at play, speed and phase. Speed measures how fast you do something, while phase measures the timing difference between when multiple people do the same thing.

Imagine several cars driving down a multi-lane highway at *exactly* 55 miles per hour. They will have some physical relationship to each other, and if their speed is right, will hold that relationship forever, but their relative positions will be random.
They have a *speed* lock.

Now imagine that they all decide to line up with a master car in the left lane. The master sets the pace and all the other drivers take their eyes off the spedometer and concentrate on lining up with the master car, speeding up a bit or slowing down a bit as necessary.
They form a line across the highway, they now have *phase* lock.

When filming a traditional video monitor it's important to understand that only a small part of the screen is lit up at any given moment. A moving electron beam paints one horizontal line at a time, which starts to exponentially fade almost immediately.  It drops several stops in a few milliseconds.

So it's important that the camera shutter (which is usually full frame) opens just before the electron beam in the monitor starts the top line and closes before the beam can start the next top line. That way every spot on the CRT gets the same exposure.

You need a 144 degree shutter because this is 16.7mS at 23.976fps, and that's how long it takes for one field to be written on a CRT* at 29.97fps. 

If you used a 180 degree shutter, you'd give the beam time to get back to the top and start writing a second field before the shutter closed again. The beam would get about 1/3rd of a new field written, and those parts of the screen, being flashed twice while the camera shutter was open, would appear as a brighter band. 

If the beam finished at the bottom and immediately restarted at the top, phase wouldn't make a difference, no matter when you'd started you'd get all the CRT lines only once and you'd catch the same amount of fade-out. But that is not the case. There is significant visual dead time in the video signal - about 10% - used for things like sync. 

If you could see things in slow motion, this would look like 'write a frame from top to bottom which starts to fade immediately, wait a beat, write a frame from top to bottom which starts to fade immediately, wait a beat, etc'.

If you had a button to push for exposure, you'd time yourself to start right before a CRT frame starts, and end just before the next frame begins. 

You can see where if you got out of sync, and started in the middle, you might catch the dying-out bottom half of the outgoing frame, wait that beat, then catch the bright new top half of the incoming frame. The bottom half would loose that beat's worth of exposure, and you'd see it as darker.

This, happening 24 times a second, is what's going on if you point a camera at a CRT and allow them to have random phase. In practice dropping in at a random time gives you an 80% chance of photographing a dark band. If you don't have the speeds matched, that band will slowly roll up and down through the image.

The CE box uses timing information from the video signal itself, either from the directly from the signal feeding the monitor or from a magnetic pickup that senses the electron beam jumping to the top of the picture, as the master timing source.

Then it modulates the camera speed and phase such that the camera runs at the same basic frame *rate* as the video, and subtly modulates the camera speed up and down a bit to ensure that the camera exposure begins just before the CRT frame starts.
This is called phase lock, and there is a knob on the side of the CE box so you can adjust the timing, looking through the viewfinder and adjusting phase to roll the bars that you see till they are out of the image.

Now... All this assumes we're talking film cameras (global-ish shutter) and CRT monitors (flying spot).

In the world of electronic cameras filming electronic displays there are all kinds of wild and wooly combinations of timing on both the display and camera end. Both could be global with effective 360 degree shutters, and you're golden at all speeds. Or you could have a  rolling shutter and high-speed rolling display - in opposite directions, and your world would suck profoundly.

But at least with electronic media you get instant feedback. which is nice. 

( * It actually takes about 14mS to write the NTSC visible field, then you get to watch the last few lines fade for about 2mS before it starts over again )

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Broken down brilliantly. One last question.

On 12/6/2020 at 3:39 PM, Steve Switaj said:

The CE box uses timing information from the video signal itself, either from the directly from the signal feeding the monitor or from a magnetic pickup that senses the electron beam jumping to the top of the picture, as the master timing source.

Then it modulates the camera speed and phase such that the camera runs at the same basic frame *rate* as the video, and subtly modulates the camera speed up and down a bit to ensure that the camera exposure begins just before the CRT frame starts.
This is called phase lock, and there is a knob on the side of the CE box so you can adjust the timing, looking through the viewfinder and adjusting phase to roll the bars that you see till they are out of the image.

Here you say while running the camera, the sync box adjusts the frame rate either using a cable, or like in my case, the provided magnetic pickup while at the same time one would adjust the timing of the electron beam using the phase knob to get each shutter revolution opened and closed so that the electron beam starts and ends perfectly from the top and bottom of the screen without the beam going back to the top a second time during one exposure.

On 12/6/2020 at 3:39 PM, Steve Switaj said:

So it's important that the camera shutter (which is usually full frame) opens just before the electron beam in the monitor starts the top line and closes before the beam can start the next top line. That way every spot on the CRT gets the same exposure.

You need a 144 degree shutter because this is 16.7mS at 23.976fps, and that's how long it takes for one field to be written on a CRT* at 29.97fps. 

If you used a 180 degree shutter, you'd give the beam time to get back to the top and start writing a second field before the shutter closed again. The beam would get about 1/3rd of a new field written, and those parts of the screen, being flashed twice while the camera shutter was open, would appear as a brighter band. 

However, here you say a prerequisite of filming a CRT television is that you need a 144 degree shutter. I think I'm sort of answering my own question here but does this mean that even with the sync box, a 180 degree shutter is just simply too long of an exposure to fulfill that precise window of exposure needed to film the CRT tv right when the electron beam begins and ends without a second partial electron beam exposure? Or maybe I'm a moron considering you've written to me a proper explanatory essay on CRT televisions/monitors, yet I'm still in enough denial to ask another question. 

Edited by Matthew J. Walker
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>> However, here you say a prerequisite of filming a CRT television is that you need a 144 degree shutter.
>> I think I'm sort of answering my own question here but does this mean that even with the sync box, a
>> 180 degree shutter is just simply too long of an exposure to fulfill that precise window of exposure needed
>> to film the CRT tv right when the electron beam begins and ends without a second partial electron beam
>> exposure?

Exactly.

An NTSC signal takes 1/60 sec to write one field *

A 180 degree shutter at 24fps exposes the film for 1/48 sec **

Since the exposure time is longer than the display refresh time, this means that you end up with time to write about 1-1/3 fields on the CRT. That extra time means that 1/3 of the video screen will be flashed twice, resulting in a bright band.

A 144 degree shutter gives a 1/60 sec exposure at 24fps, this is just enough to expose the whole screen once and only once.

The periodic relationship between the camera shutter rate (1/24sec) and the NTSC video rate (1/60sec) repeats every 1 camera frame and 3 video fields ( 1/24sec = 6/120sec, 1/60sec = 2/120sec, hence the relationship is 6:2 or 3:1), so the timing between the two resets with every film frame and you start in the same relative place.

If it weren't for the dead time in the video signal that's used for vertical sync, if you had a 144 degree shutter you could just start anyplace and let them run wild relative to each other (and in fact, that's what the old telekine camera chains used for network west-coast delay used to do, but they had special long-decay CRT tubes so the dark band wasn't an issue).

The disclaimers...

* technically 480/15750 sec, but doesn't start rewriting for another 45/15750 sec due to time lost to sync signals

** films shutters aren't perfectly global, so, say, the upper left corner exposure start time skews from the lower right corner start as the shutter moves across the frame. Since you're messing with timing, this can make light or dark bars roll as the image of a small CRT moves around the film frame.

*** Again, this only applies to CRT's. Modern LED and LCD monitors are a whole different, and much less predictable situation. Many of them are totally fine at almost any reasonable exposure or framerate. Some of them are Lovecraftian hell. TEST. TEST. TEST.

Edited by Steve Switaj
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