Genesis shutter angles

[richard thomas]

Hello to everyone.

 

Does anyone know that mathmatics involved when adjusting shutter angles on the Genesis camera.

 

I am working on a Genesis film in Canada and while shooting in an ice arena, we set the camera at 45 degree shutter for that "Saving Private Ryan" look. When set this way (frame rate set at 29.98 as recommended by panavision) we could not only see pulsing on the wave form monitor but on our video monitor as well.

The effect on the video monitor was a slow decrease in exposure as well as a shift in color (magenta).

 

We changed the camera frame rate to 24p and still had the problem. We thought that it might have been an

"out of phase" problem (electrical) and made sure that the camera was running on the same power source as our practical (metal halide) lights were. Still no change. When we changed to a 90 degree shutter the problem disappeared.

 

Any Ideas?

[David Mullen ASC]

Using a 45 degree shutter angle shouldn't cause HMI flicker but it could cause a drop or variation per shot in exposure (even after adjusting for the new exposure time) because you are essentially only capturing part of the 60 hz sine wave, so you may fall into sync with the peak or the valley everytime you trigger the camera.

 

With a 180 degree shutter, you capture more than two pulses per frame.

 

Now with flicker-free lights, it shouldn't be a problem.

[richard thomas]

Using a 45 degree shutter angle shouldn't cause HMI flicker but it could cause a drop or variation per shot in exposure (even after adjusting for the new exposure time) because you are essentially only capturing part of the 60 hz sine wave, so you may fall into sync with the peak or the valley everytime you trigger the camera.

 

With a 180 degree shutter, you capture more than two pulses per frame.

 

Now with flicker-free lights, it shouldn't be a problem.

 

Thank you for your response David. I am wondering however, if you might know what the calculations might be so that I might be able to figure out an alternative shutter angle to use in similiar situations.

The camera running with a 90 degree shutter produced no visible pulsing.

 

By the way, I posted the wrong frame rate. It should be 23.98.

 

Thanks again for your help.

 

Rick Thomas

[Phil Rhodes]

Hi,

 

Just let me get this straight - you have DATs (or smart slates or whatever) that will do 24-frame timecode, and you have a camera that will do 24-frame timecode, and yet you still choose to go through the ridiculous dance of working at 23.98?

 

What are you, suckers for punishment?

 

Phil

[David Mullen ASC]

I believe the reason why most HD is shot at 23.98P instead of 24P in the US is that sound post-production people work with NTSC downconversions -- even if you shot film at 24 fps, it gets slowed to 23.976 fps by the telecine when transferred to NTSC (29.97 fps) for offline post-production work.

 

Now why we can't just shoot at 24P (like we shoot film at 24 fps) and let the post people work at 23.976 fps with the NTSC downconversions and make some sort of adjustment during the mix to sync, or later when making the printmaster, I don't know. I mean, I assume this must be an issue with 24 fps film material posted in NTSC but finished back to film?

[David Cox]

David, we have a similar difficulty in PAL land (25FPS) when sending dubbing copies to the sound houses.

 

What we do is play our 24FPS "master" back at precisely 96% of its real speed to a 25FPS format such as beta SP. When this is played at 25FPS (i.e. 1 frame per second too fast or 4% too fast), the sound guys get a film that has the same running length as our 24FPS master and so can safely dub to it. It doesn't look too pretty duplicating a frame every second, but hey - they're only looking with their ears!

 

So I guess the same practise for your post guys would be to work your picture post at true 24FPS, but make NTSC copies for your sound guys with your pictures running "varispeed" at precisely 99.9% Doesn't sound like much, but it stops the sound going out of sync by another frame every 41 seconds or so.

 

David Cox

Baraka Post Production Ltd

[Phil Rhodes]

Hi,

 

I still don't get it. I mean, I get it, but it seems like such a ludicrously twisted way to work. Every sound NLE is capable of running at 24; it hardly matters what it is on tape. Do the dailies at 30fps frame for frame and ingest it faster!

 

I mean, yikes.

 

Phil

[David Mullen ASC]

Do the dailies at 30fps frame for frame and ingest it faster!

 

Then everyone watching dailies on their TV sets will see the footage sped-up... Besides, it would be 29.97 fps, not 30 fps.

[Brad Grimmett]

Hi,

 

I still don't get it. I mean, I get it, but it seems like such a ludicrously twisted way to work. Every sound NLE is capable of running at 24; it hardly matters what it is on tape. Do the dailies at 30fps frame for frame and ingest it faster!

 

I mean, yikes.

 

Phil

It's a simple change in the menu. What's the big deal? I know it seems strange, but why not help them out with such a simple thing?

[David Cox]

Hi,

 

I still don't get it. I mean, I get it, but it seems like such a ludicrously twisted way to work. Every sound NLE is capable of running at 24; it hardly matters what it is on tape. Do the dailies at 30fps frame for frame and ingest it faster!

 

I mean, yikes.

 

Phil

 

Its just a tape based interchange problem. It all works much better if us picture people make copies of films that run at the right duration for the format they are recorded to. For example, if the sound house digitised a 24FPS film at either 23.98 or 25 FPS (the speed of the tape they were supplied), you're right - if they are playing back the pictures from an NLE (and not all do) they could make the speed alteration there.

 

However, they would then end up with a soundtrack that doesn't fit the tape they were supplied. So the director can't walk away with a copy of the film with the new soundtrack unless the sound house now makes a second version of the track that is speed and pitch corrected to run at the wrong speed. If they do, now we have two soundtracks going around and sod's law, the wrong one will end up on the print and it is us picture people who get blamed for the track being out of sync!

 

So our experience is that best practise for sound / effects / musicians that require a tape of the film is to provide one that runs at the correct film duration for the natural FPS of that tape format.

 

David Cox

Baraka Post Production Ltd

[Keith Mottram]

Alternatively sod the tapes and supply the sound guys with 24fps QTs....

[David Cox]

Alternatively sod the tapes and supply the sound guys with 24fps QTs....

 

...where the sound house will take it. We find that a lot of musicians can take the QT, but a lot of the mixing studios can't. There's no timecode on QT either, so you just have to hope that everyone is starting on the same first frame as the QT.

[Keith Mottram]

...where the sound house will take it. We find that a lot of musicians can take the QT, but a lot of the mixing studios can't. There's no timecode on QT either, so you just have to hope that everyone is starting on the same first frame as the QT.

 

I have worked with some very experienced audio post houses, for both features and commercials, and they are often reluctant- until you go through the workflow. i cant really see why any modern mixing studio cant take a QT. As for TC- burn it in, most NLE's can do that on the fly. This creates the best workflow, preliminary mixing from QT's (projection is obviously studio specific) then switch to 24p HD playback for final mixing. Its a hell of a lot better than constantly frameblending and dumping down- wheres the accuracy in that? How many problems have you had with people working at different frame rates? at the end of the day i have yet to find an audio mixing studio, or anyone else for that matter, who will turn down work rather than accomidate a simple change to there workflow.

 

Keith

[Stefan Nell]

The following is a formula for working out shutter angles (Hz) for HD Cams

 

 

FPS X 360 = SUBTOTAL / Film Shutter Angle = Hertz

 

 

 

Explanation: Take the frame speed you are shooting at and multiply by 360(360 degrees). Then take the Subtotal and divide it by the Film shutter angle you want .The total will give you the (Hz) frequency.

[Delorme Jean-Marie]

hi all

last week we did test the genesis at variables shutter angles with 3 differents pan speeds at IDIFF europeen film festival.

result will be ready soon but i was framing thos test and from what i saw from 250° to 150°, over 180° don't pan to speed because you'll see noticiable kind of smear

[John Sprung]

Now why we can't just shoot at 24P (like we shoot film at 24 fps) and let the post people work at 23.976 fps with the NTSC downconversions and make some sort of adjustment during the mix to sync, or later when making the printmaster, I don't know.

 

Indeed, there is no technical reason that that wouldn't work. The only important thing is that every clocked device on the shoot -- cameras and audio recorders -- must be on the same side of the "point zero zero" vs. "point nine something" fence. Get that one wrong, and the audio has to be re-sampled before you can sync it up.

 

If you're doing a simultaneous downconversion to NTSC during production, you'd be locked into the "point nine something" world. Other than that, you can do it either way. Like driving on the left side of the road here, and on the right side in the U.K., either one works fine. But what is really important is that everybody on the same road does it the same way.

 

Here the majority of shows seem to pick point nine something. But that's custom, not necessity.

 

 

-- J.S.

[John Sprung]

Hello to everyone.

 

Does anyone know that mathmatics involved when adjusting shutter angles on the Genesis camera.

 

We thought that it might have been an

"out of phase" problem (electrical) and made sure that the camera was running on the same power source as our practical (metal halide) lights were. Still no change. When we changed to a 90 degree shutter the problem disappeared.

 

Any Ideas?

Are you sure that the Genesis was acutally synced to the line frequency, not just powered from the mains? Line regulation is nowhere near as good as what we need for sync, especially if you're on a generator. It could be anywhere in the 58 - 62 Hz. range.

 

As for the math, arcing sources make light on both the positive and negative halves of the AC cycle. Photons are all the same, not positive and negative, so you get 120 pulses of light per second. From the zero crossing, it takes a while to reach the strike voltage, and the arc drops out before it reaches the next zero. So, you have pulses of light with a humped top and black intervals between them. At 24 fps, you have 120/24 = 5 light pulses per frame. So, you have 360/5 = 72 degrees of shutter rotation from one light pulse to the next. With a 45 degree shutter opening, you'll only get part of one pulse cycle. It could be mostly the "on" part, or mostly the "off" part. If the camera and power are not locked together, the drift would cause your exposure to ramp up and down between those extremes. If they are in sync, you'd get a stable but unknown relationship each time you started. With phase lock, it would be the same every time.

 

With a 90 degree shutter, you get part of two pulse cycles, so the variation would be smaller. With a multiple of 72, such as 144 or 216, there would be no variation at all because you'd get the same amount of "on" and "off" time.

 

 

-- J.S.