Specs for Optical Sound

[carllooper]

Anyone know where one can get specs for the optical soundtrack of 35mm film?

From Wikipedia I found the following formats are the most common and can coexist together on the film:

Dolby Digital, which is stored between the perforations on the sound side;
SDDS, stored in two redundant strips along the outside edges (beyond the perforations);
DTS, in which sound data is stored on separate compact discs synchronized by a timecode track stored on the film
Analog soundtrack - I know how to do this one.

Back story:

I did some enquiries yesterday on printing digital to film and the cost threw me. To transfer 12 minutes of digital to 35mm (originally scanned from 16mm) was going to cost $1000/min. Once processing and printing was factored in the production house quoted $35,000 for those 12 minutes.

Enquiries elsewhere didn't seem to alter the figure all that much. While I'm sure there will be somewhere that can do it for far less I started thinking how I could do it myself instead.

In the early days of digital (circa 1982) when the maximum size image you could generate on a home computer screen was 320 x 240 x 16 colours, I often photographed computer animations from the screen, a frame at a time, using Super8. The computer would trigger the Super8 camera. I did experiments with multiple exposures, colour filter wheels, etc. I remember it was a lot of fun. And the results were great.

What would it be like doing the same today? What sort of result could one expect from a DIY system?

What would I need? A 35mm motion picture camera. Well, you can find one that might do the job for about $100. Amazing but true.

An important thing I'd need is a monochrome monitor. When shooting a colour screen the problem you have there is that you'll end up reproducing the individual red, green and blue pixels. You really want these to be merged, eg. shooting a mono screen three times through RGB filters.

Now flat mono screens are impossible to find until you realise what you need to search for are medical display screens. They are purpose built specifically for the medical community. They vary in resolution from low to high to much higher than your average computer screen. Cool. But they are also very expensive.

But then you find you can get them relatively cheaply second hand. I guess it's because nobody wants them. Hospitals, doctors (etc) don't buy these screens second hand. And everyone else buys colour screens.

Now one I found specified a contrast ratio of 900:1. Hmmm. What's that in stops?

A quick search using "contrast ratio to stops" gave me the the not too unexpected answer:

http://books.google.com.au/books?id=oRY ... ps&f=false

512:1 = 9 stops
1024:1 = 10 stops

That's not bad at all.

Now the film I have in mind was actually shot in B&W so I wouldn't need filters in this particular case.

To cut a long story short I found I could do my own digital to film transfer for about $1000, for the whole 12 mins. That included processing.

The only thing I hadn't considered was the soundtrack. How would I print the sound track? Are there specs around for how a soundtrack should be printed to film? With specs in hand I could write software for generating the required patterns on the screen and then exposing those patterns on the neg.

Carl

Addendum: Since writing this post, I've discovered (apart from analog) the others are all licensed systems so they aren't going to be readily available for DIY implemetation. So I'll just stick with analog, and do my own version of DTS.

[Nicholas Kovats]

Carl!

My instincts were correct and its time to accelerate multiple agendas. I would like to introduce you to another engineer by the name of John Gledhill of bitworks.org...who has been scanning my UP8 footage on his custom multi-format (R8, S8, 16mm, 35mm) sprocketless rig. He is also an expert in optical soundtrack restoration and creation.

His email his john@bitworks.org. http://www.bitworks.org/WhatCanWeDo.html

I am still working on getting you some UP8 scanned material.

I am inspired.

[richard p. t.]

It is certainly achievable to 'photograph' an optical sound track a frame at a time ... except the problem with doing so is that you get frame lines between each photograph. The optical sound track is recorded on a continuous motion camera, rather than an intermittent motion camera like a normal movie camera. Simmilarly, you can only print optical sound on continuous contact printers, rather than step contact printers. I have photographed existing optical sound recordings using a super 16 oxberry optical printer. As I say, it works, except for the 24 frame lines per second. With very precise filing, I gess you could modify the gate such that there was no frame line ...
For my part, I am not able to do the kind of software writing you are talking about. while I can record my own sound tracks on my Auricon for 16mm sound, I'd like to be able to the same in 35mm. If you do develop this software, I would love a copy, even with the frame line problem.
Would be interesting if it were possible to modify a camera such that it was continuous rather than intermittent. Then block the film gate except for a micro fine slit. Then have the sound image play on a monitor and focus the modified camera on the image such that the image falls against the sound track area of the film. Then run the camera at 24 sprockets per second while the image scrolls on the screen. That would overcome the frame line problem.
rt

[carllooper]

Would be interesting if it were possible to modify a camera such that it was continuous rather than intermittent. Then block the film gate except for a micro fine slit. Then have the sound image play on a monitor and focus the modified camera on the image such that the image falls against the sound track area of the film. Then run the camera at 24 sprockets per second while the image scrolls on the screen. That would overcome the frame line problem.
rt

Yes, the repositioning of the film in an intermittent system could cause each frame of the sound track to misalign (however small) with the next. And for sound that could be an audible problem. If it were possible, a continuous running film would be a great idea. Could use the same idea for the image as well, ie. recording both sound and image at the same time, both scrolling in sync with the film motion.

But how to modify a camera for continuous motion? Looking at my 16mm bolex for some insight - you can't just remove the claw because even though the sprocket drives are continuous the pressure plate on the gate area wants to hold onto the film, and without the counter-movement of the claw, the pressure plate would cause the film to tighten on the takeup loop. Removing the pressure plate doesn't help because there is a certain amount of friction on the opposite side that would cause it to stutter at best. And in a no-contact-at-all scenario the film would be curved with respect to the image ... unless it was wound tight against the sprocket drives, with everything else evacuated ... but then it's a too far from the focal plane. Hmmm ... by the time you had a working solution you'd have built yourself your own camera. But it could work. Tight around the sprocket drive with the front of the camera sawn off altogether, and the lens repositioned ... Depends on the architecture of the camera I guess. Looking at the bolex there's additional hacking one would have to do. But it could be done.

I wonder how much of an audible problem an intermittent system would be. I was intending to widen the gate in all directions and use the computer screen as the effective gate, ie. in a black room etc. So the only problem would be how precisely the film was advanced. If it's precise enough the only real problem is how precisely the camera/computer screen can be aligned with respect to where the image falls on the film. I'll need some sort of way of measuring where the image falls and fine tuning the camera positioning accordingly. The simplest approach would to burn a test roll in which the rig was adjusted in very fine increments, with the computer image displaying a reference number for each adjustment of the rig. The rig would need to be really solid so you were confident of exactly reproducing the required adjustment based on the selected reference number of the returned film (digitally scanned).

Ah yes. Not as simple as one first imagines.

Going Dolby or DTS starts looking a lot simpler. There you could use an intermittent system no problem. I wonder how much a license costs. Presumably you'd get the tech specs as part of the deal. I'd have no problem writing software to convert audio into bit patterns for exposure. Must chase that up.

Or for site specific installation work one could just invent one's own audio encoding/decoding system. I like the DTS idea - just encode timecode on the film and slave a computer to the projector for the sound. By which time you could start wondering why you don't just burn a BlueRay disk and forget about 35mm projection altogether.

My main interest in 35mm projection is in relation to distribution to those venues that have an existing 35mm projection system rather than setting up any site specific screening (although I like that idea as well). The last time I worked on a 35mm short I wasn't the producer (wasn't paying for it). An existing production house did all the digital to film and standard sound encoding (Dolby etc). Screened at a lot of film festivals. Couldn't really do the same thing (festival circuit) with a custom audio system! Could always try of course - sending a laptop with the reel and instructions on how to wire up the projector to the laptop and the speakers.

On that note Richard - site specific screenings. You've got a 35mm projector yes? If you're interested in doing a site specific gig sometime in the future - Melbourne area - I'd be into that. Could encode some simple timecode pluses in the analog area and have that running through a laptop for 'directing' the sound off the laptop.

[carllooper]

I am still working on getting you some UP8 scanned material.

Hey Nicholas - if your original scan was delivered as a JPEG sequence, I can use those. Don't convert them to PNG. There would be no point - it would just bloat file size for no reason. JPEGs will be just fine. The PNG suggestion was only if your original scan had been delivered in an uncompressed format. Perhaps you acquired the scan as a MOV, or AVI that used some particular compressed format anyway. If so, just spit out a few seconds worth of hi quality JPEGS from it. It'll be fine. It's just a test for the time being. We can use the test to pump up our inspiration for more detailed work.