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23.93 HD & 60Hz Generator

I am starting my first HD show and am wondering about flicker.

We are using a Cinealta F900 and shooting at 23.98 fps progressive scan.

The first thing the camera assistant mentioned to me was an odd experience with a six second flicker on another show.

Should I be worried about this? I am religious about the generator being dialed in. I use an optical frequency meter to double check. We are using magnetic ballasts for our HMI's.

I seem to remember a discussion about this at one point on the list and I can't seem to find it after looking in the archives.

Any help would be appreciated.

Andrew Gordon
Gaffer
Regina, SK
Canada



You shouldn't have any more problems with flicker when shooting 23.98P on the F900 than when shooting in film using a crystal-sync camera. The only potential problem is that the flicker on the monitor might mask any flicker in the lights (on the other hand, in film, true HMI flicker is usually invisible to the eye anyway so checking the camera and generator, etc. is standard operating procedure.)

I've never had any HMI flicker problems when shooting in HD.

David Mullen
Cinematographer / L.A.



>,,,shooting at 23.98 fps ,,,,, the camera assistant mentioned to me was an >odd experience with a six second flicker…Should I be worried about this?,,,

23.98 fps is not an HMI-Safe frame rate. You should be using electronic ballasts at this frame rate.

If you test HMI's and fluorescent sources at 23.98 fps using the F-900 and a vector scope you will see a noticeable phase fluctuation. This may be considered acceptable to some but it is *not* technically right.

David Perrault, csc



David and David,

Thanks for your input.

I will test this in light of the two differing opinions.

David Perrault, what are the bad things that can happen with the noticeable phase fluctuation? Post issues?

Thanks again.

Andrew Gordon
Gaffer
Regina, Saskatchewan
Canada



>,,,what are the bad things that can happen with the noticeable phase
> fluctuation?,,,,

I think in the case of HMI's, the issue is more a theoretical one of variation in exposure related to the synchronization of the camera's shutter and the HMI's. Lots of people seems to think it's not a problem, so maybe it isn't. Basically, a slight density variation take to take.

Or... Maybe not if you consider the (HDTV) camera locked to the HMI's in the sense that they are powered by the same clocked power source: the generator.

Or... Maybe there is a density variation if you power the camera down between takes for some reason.

In the case of fluorescent it *is* a problem. You will find there is a colour shift that is noticeable.

On the show I'm doing now we decided to shoot 24.00 and do sound at 30.00 and they are treating the HD playback in post as 23.975 and this seems to be working out.

I can provide contact information for the post house if anyone wants to follow up on this. Contact me off list for that.

David Perrault, csc



Andrew, what exactly was the assistant referring to when he talked about a "six second flicker" ? I'm having trouble wrapping my head around that one.

I've also heard conflicting opinions on whether or not 23.98 can be considered "HMI safe" - interesting to hear more.

FWIW, I recently used a couple of magnetic ballasts on a shoot with an IMX camera at 30psf, which is really 29.97 - didn't notice anything amiss there, but I can easily pull the tape and check with a waveform.

George Hupka
Director/DP, Downstream Pictures
Saskatoon, Canada



George,

The way it was described to me sounded like what David P. was talking about; a six second density change that pulsed like an out of sync HMI or floro
fixture.

Andrew Gordon
Gaffer
Regina, Saskatchewan
Canada



Just wanted to add my $.02...

I've dealt with several bad HMI flicker problems in post with 23.98p. It does happen, and it is a problem. I've come up with some nifty fixes in post that made clients very happy, but it is a serious b**ch.

The worst shots I've seen were speed ramps, where the flicker is not at a constant rate. I have nightmares about those.

Lucas Wilson

iO Film Hollywood



> David P. was talking about a six second density change that pulsed like an >out of sync HMI or floro fixture.

I have definitely run into that with the shutter in use, as Mike mentioned unless there's an engineer monitoring the signal with a scope, or a really good viewing area, it's going to be hard to monitor this on set.

There's one other thing I ran into recently with a DVCPRO camera that sounds a bit like this - but it seemed to affect only the red channel, and was very intermittent, which was driving me crazy. There was a definite, regular pulsing of 2-3 IRE, more in the blacks than in the highlights...It was clearly visible in the edit suite and on the scope, but not at all visible in the camera viewfinder, and just barely visible on an 8" field monitor.

I was using my HMIs on this shoot, so naturally I was also concerned that the lights might be the source of the problem. At any rate, I finally got to the bottom of it while setting up an interview – there was a window just out of frame, and I noticed that when I framed to include a bit of the window, the pulsing would begin... when I framed it out completely, the pulsing went away. Obviously not the fixtures!

The client had a custom camera setup, and after plowing through the menus I found that one of the things they had done was push the red flare to maximum, while the blue and green flare were only up about 30% above the factory setting.

When I reset the flare to factory defaults, the pulsing went away and never came back for the remainder of the shoot.

I mention this because I've noticed a lot of the "custom looks" floating around out there seem to pump up the flare settings - Now, I've never had this happen to me before or since, but frankly if those flare settings had been set evenly across the three channels, I don't know that I ever would have tracked down the problem - the red channel being out of whack with the others is what clued me in. (And I still don't understand why the flare adjustment would result in such a regular pulsing effect, but that did seem to be the case).

Just one more thing to look out for!

George Hupka
Director/DP, Downstream Pictures
Saskatoon, Canada



Anyone have any formulas that would help one figure out how much of a window one had? For example, at 23.98 or 29.97 and 1/48th or 1/36th, how far off does the genny have to be before you see a flicker? How can that be calculated?

Art Adams, DP
Mountain View, California - "Silicon Valley"

http://www.artadams.net/



Art Adams wrote:

> Anyone have any formulas…For example, at 23.98 or 29.97 and 1/48th…?
> how far off does the genny have to be before you see a flicker?

Try the American Cinematographer Manual. There is a whole section on
this issue for film cameras that should be applicable to 24P as well.

Dan Kneece, SOC
Los Angeles, CA



>Try the American Cinematographer Manual. There is a whole section on this
>issue for film cameras that should be applicable to 24P as well.

I'll give it a try. I guess I just need to figure out the true exposure
time for film, taking into account fps and shutter angle, and then compare
that to HD's 1/XX shutter modes.

Art Adams, DP
Mountain View, California - "Silicon Valley"

http://www.artadams.net/



>I'll give it a try…taking into account fps and shutter angle,

At f fps with shutter angle a degrees, the actual exposure time is a/(f*360) sec. Not sure if this would help you with the curves in AC manual though.

And I think the electronic shuttering of a digital camera is quite a square waveform - "off" or "on", whereas a mechanical rotating shutter is in effect a very fast fade-in fade-out device. I guess that may alter the cut-off point of what flicker is visible and what isn't.

And bear in mind that "seeing" flicker also depends on the subject matter. Uniform mid-gray areas may emphasise flicker.

Dominic Case
Atlab Australia



Thanks for all the responses.

At this point, I have ruled out the generator as I have done a test with house power in a studio. This power is very clean and bang on 60 Hz (as one would expect).

The test used a 1.2 HMI Fresnel aimed at a mid gray card.

Some things that this test revealed are:

1/ It was a 1-3 IRE fluctuation on the red channel. At times hard to see but at other times it was faintly visible. Angle of the light seemed to make some difference.

2/ Adjusting the shutter removed the problem but obviously resulted in changes the way motion was recorded.

3/ Shooting at 24.00 removed the problem.

Thanks again for all the help. Its better to know this going in than to discover it through the post house.

Andrew Gordon
Gaffer
Regina, Saskatchewan
Canada



1/ It was a 1-3 IRE fluctuation on the red channel. At times hard to see but at other times it was faintly visible.

Wow, that is way too similar to the problem I had with the DVCPRO...which was 60i, but of course that's really 59.9? (too tired to remember the number or do the math right now!) so I wonder if it could be related somehow. But adjusting the flare back to reasonable values did seem to fix my problem, and the frequency of the pulse was closer to 1 second than the 6 seconds you described, so maybe it's just an uncanny coincidence.

George Hupka
Director/DP, Downstream Pictures
Saskatoon, Canada



Dominic et al:

The GVG/Thomson (formerly Philips) cameras (HD and SD) all use both a mechanical shutter and an electronic shutter. The width/shutter angle of the mechanical shutter is corrected for, and calculated into the electronic shutter settings in menu settings. From my experience, flicker resulting from non-high speed ballasted light sources doesn't seem to be altered by the presence of a mechanical shutter. The color shifts noted by others here are to be expected with electronic imagers, depending on the integration rate of the sensor in use, and particularly in the possible case of lens flares created by such sources which could have notable effects on the black/pedestal levels of the individual color channels. Because of the combination of the nature of flaring and the frequency of the low speed ballast involved, even the Flares might not fix the problem; they might even exacerbate it.

GEORGE C. PALMER
HDPIX, INC.
HD and Digital Imaging Services
www.hdpix.com



Andrew,

I was on a recent HD shoot where we encountered a horrible HMI flicker problem. It started off unnoticeable then became progressively worst.

I was lucky that I discovering the HMI flicker while viewing the waveform monitor which clearly showed the pulsing whereas the HD monitor "flicker" easily masked the problem. The producer was satisfied that we discovered the problem on set instead of having to re-shoot an additional day.

I would advise that you have on set a waveform monitor to absolutely confirm you are not having any HMI flicker issues at the camera frame rate you have chosen to shoot at.

In my case the HMI flicker was present at all camera frame rates and did not disappear when I changed the shutter. I later confirmed that it turned out that Mole HMI fixture was defective as we swapped out ballasts and had the same problem.

Good luck,

Tony Salgado