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View Full Version : Which channel L/R is closest to perfs on 35mm optical sound?



Dan Hudgins
10-10-2011, 01:55 PM
I hope someone here can help me find the answer to a question about 35mm Optical Stereo sound tracks on release prints.

I've looked all over the internet and don't seem to be able to find out which audio channel Right or Left is the one that is closest to the perfs (most distant from the frame's image) on 35mm release prints.

Since people on REDUSER have been so knowledageable and helpfull in the past, I was hopeing that someone here might know, or know a link to a URL that states which of the two bi-lateral tracks closest to the perfs is the Right or Left channel.

Thanks in advance for any help toward finding this filmmaking fact...

Marc Wielage
10-10-2011, 11:09 PM
The left channel of an analog variable-area optical track is on the left side (closest to the perfs); the right channel of an optical soundtrack is just to the right of that.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1d/35mm_film_audio_macro.jpg/250px-35mm_film_audio_macro.jpg

Why does this matter? Are you trying to read an optical soundtrack, or are you trying to align an optical reader?

Dan Hudgins
10-11-2011, 03:40 PM
The left channel of an analog variable-area optical track is on the left side (closest to the perfs);
Why does this matter? Are you trying to read an optical soundtrack, or are you trying to align an optical reader?

I thought that might be the order to have left on the left with the image reading right way around, thanks for taking the time to answer as this fact seems hard to find on the internet.

I adding some new features to my DIY film recorder program and need label some prompts in the setup menu, some of those have to do with placement of the stereo analog tracks relative to the perfs.

Do you know the distance of the Left and Right track center-lines from the "left" edge of film (in your graphic above). I have measured some film but the placement of the tracks seems to vary, so I was in need of finding the right target values for the track centerlines (and maximum total width per channel). Any info. about the dimensions in 1/10000" (0.0001") would be a big help.

Marc Wielage
10-11-2011, 07:02 PM
Not a clue. I'm positive there have been many SMPTE papers over the past 50 years specifying the exact size and width of bilateral optical soundtracks:

https://www.smpte.org/standards/find

It's very hard to find optical sound alignment films anymore, and I think SMPTE has stopped making them, but you can get them from NT Audio in Los Angeles:

http://ntaudio.com/services/test-film.php

Dan Hudgins
10-12-2011, 01:21 AM
It's very hard to find optical sound alignment films anymore, and I think SMPTE has stopped making them, but you can get them from NT Audio in Los Angeles:


Thanks for the information you have been very helpful.

BTW, we have Mitchell Sound Recorder #1, before Mitchell closed we asked about when it was made, probably about 1928, and they said they never made sound recorders(!), so it may be the only one, but its definetly a Mitchell product and has the Mitchell "nut" tag on it stanped #1. The drive shaft was powered by a three phase 1200RPM sync motor. It seems later someone fitted it with a Maurer "F-Prime" galvanometer that makes a single monophonic bi-lateral track rather than the two bi-lateral tracks used for current stereo. The sound is very good quality even though it rolls off at about 9500Hz recording at 24fps, so we run it half speed at 12fps using the synchro interlock to the 35mm mag film and adjusting the playback EQ for the half speed mastering (giving up to about 18000Hz maybe). The problem is that the guy that made the light bulbs for the Maurer F-prime died, and we cannot find a source for more lamps which don't last long buring at 2.1 amps needed for electro printing. You would not by chance have any bulbs for the Maurer F-prime?

Marc Wielage
10-14-2011, 05:52 AM
The sound is very good quality even though it rolls off at about 9500Hz recording at 24fps, so we run it half speed at 12fps using the synchro interlock to the 35mm mag film and adjusting the playback EQ for the half speed mastering (giving up to about 18000Hz maybe).
I think the problem with that theory is that your low-end response at half speed will suffer, because I don't think the recorder will be able to handle much below 80Hz. Optical sound was always very dodgy in terms of linearity.

Not a clue on sources for light bulbs. I seem to recall the last-generation optical sound recorders were using LED sources, but that's from a distant memory of a SMPTE article in the mid-2000s. If you check my SMPTE link, all the papers on optical sound will be of use to you. Light bulb factories exist; heck, you can get CRT's refurbished if you want to spend the money. And there are still several major electronic tube manufacturers in full production in Russia -- all the analog audiophiles use them for their big-money tube audio amps.

Dan Hudgins
10-15-2011, 07:38 PM
I was thinking that I might be able to "light pipe" light to where the special light bulb filament was, its a pre-focused bulb with a single line filament that is focused on the optical slit. If I make a light fibre bundle in the shape of the filament, and focus a very bright light on that it might work.

I not sure how much longer 35mm print stock is going to be around, 5 years? 8 years?, 5 years ago I thought 20 years, but it seems things are going digital faster and maybe filmmaking will end altogether much sooner than was thought before.

About the bandwidth of optical sound, it goes from DC up to about 9.5KHz on the galvo, and maybe DC to 18KHz on the playback if everything is in best focus.

There is no lower frequency limit on the film part. The 35mm MAG film has a wide track and high tape speed, so with enough EQ you can get down to maybe 10Hz on the half speed playback, giving 20Hz as the bottom when the print is projected.

We have a high pass filter (about 6db under 10Hz or so) even though we are using a driver amp that goes down to DC on the galvo to protect the galvo from glitches when the preamps might get switched off or something before the galvo is un-plugged.

Thanks for your suggestions. Most readers are converted to LED now since silver track is being phased out, with the cyan dye track red LED are used so that there is no IR which would pass through the cyan dye tack and reduce the SN ratio and volume.

I'm working on a simpler and lower cost solution now for making prints with optical sound, it will be a free option for my DANCINEL.EXE (tm) program.