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This topic comprises 2 pages: 1 2
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Author
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Topic: Optical fibre as film sound pick up
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Tony Smith
Film Handler
Posts: 42
From: worcester,worcestershire, england ,UK
Registered: Sep 2012
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posted 05-07-2015 10:53 AM
Hi
Looking further into this sound issue on the Cinemeccanica Portacine, the audio pick up off the variable area sound track
is just two short lenghts of optical fiber tube, not sure what wavelenght of tube size yet, so 4 strips of optical fibe tube
two for each channel Left and Right, these just terminate in optical fibe jacks, all fitted on the original sound cell pick up plate.
This is part of the many mods that have been carried out on this projector.
The output which is then fed via single screened cable to a kit form microphone pre amp, followed by a 3 watt power amp output, all on a single audio chip, so 2 of these for stereo.
Anybody ever come across this before?
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Tony Smith
Film Handler
Posts: 42
From: worcester,worcestershire, england ,UK
Registered: Sep 2012
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posted 05-07-2015 04:05 PM
Hi Sam/Marcel
Having a closer look with a magnifier there are in fact 3 fibers
for each channel, arranged with one along side the other two which are one on top of the other,6 in total all held together by silver foil tape wraped round all 6 fibers,
I have had audio off it from a variable area stereo film sound tracks, but its very very difficult to align the fibers to film and get the light source correct.
Then when you reload the film same film its not picking up sound very well.
As the optical fibers are just held in place by twisted wire, and so close to the film they almost touch it.
Yes the light source does pass through the slit, as its not had any changes to that side, only the pick up
Tony Smith
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 05-07-2015 06:07 PM
quote: Rick Raskin
So, you are pushing analog over fiber; interesting. My experience is mostly in the digital transmission world.
Not quite actually. Many cable networks still carry analog or even analog and digital hybrid signals over miles of fiber and do a "simple" optical to electrical transformation to push it onto coax. Analog signals over fiber can travel a considerable distance over e.g. single mode fiber before they need to get repeated or regenerated, much further than over traditional copper solutions.
In this case, the fiber or apparently fibers is/are just used as a simple light pipe. That way you can position your optical sensor somewhere else.
quote: Rick Raskin
Anyone using Plastic Optical Fiber (POF)? When I was doing standards there was a push for a POF spec., but I don't think it had much steam behind it.
I've used it for temporary networks for events. POF is more pliable and generally more forgiving than glass based fiber and works fine for short haul solutions. It's not less expensive per feet though.
quote: Tony Smith
Having a closer look with a magnifier there are in fact 3 fibers
for each channel, arranged with one along side the other two which are one on top of the other,6 in total all held together by silver foil tape wraped round all 6 fibers,
I have had audio off it from a variable area stereo film sound tracks, but its very very difficult to align the fibers to film and get the light source correct
Well, I guess this convoluted setup could actually somewhat work with a variable density soundtrack, but for a variable area soundtrack... I've no doubt you could get something that *resembles* the original soundtrack, but...
The problem here is that with this three-fiber setup is that you'll probably don't cover the whole soundtrack or at least not equally well. So, you'll lose some light and as a result you'll lose audio information. What you'll lose depends on a whole load of factors.
Like Sam already mentioned, you really need to capture the light from the whole width of the soundtrack. It will be hard to find a traditional multimode fiber of that width, but a wider, single light pipe might lead to a more acceptable result.
You also want to minimize stray light leaking into the pickup location of the fibers. I'm not sure how tin foil is going to accomplish that though. ![[Wink]](wink.gif)
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Tony Smith
Film Handler
Posts: 42
From: worcester,worcestershire, england ,UK
Registered: Sep 2012
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posted 05-08-2015 04:54 AM
Hi Folks
Thanks for your replies.the optical fiber set up we have only just found, had no idea what was there untill we got as far as looking at the audio.
The whole way through this refurbishment, the project has been fighting back, not unhappy at all with this, as one learns a lot.
The plastic fibers used is very similar to Jiangsu Tx used for optical decorative lighting and such, its basic size per fiber is given as diameter, not lightspecs, is 0.75 mm about 0.029 of an inch, so 2 would give 0.058 inch,for each track, maybe slightly more as the 3 rd one may be slightly offset above the second fiber, so best case is0.087 inch.
The loss per Km is 250 dBm
So as you will understand its very very difficult to align. We have been looking at the standard Toslink plastic fiber, as called up ealier using 2 pieces of this,along side each other, the plastic fiber is about the same size as the optical jack clamp to hold the fiber
Tony Smith
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Tony Smith
Film Handler
Posts: 42
From: worcester,worcestershire, england ,UK
Registered: Sep 2012
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posted 05-08-2015 04:24 PM
http://www.film-tech.com/uploads/uploads0503/rsz_photo-0018.jpg
Hi Folks for those who would like a look at the original 6 way optical fiber audio pick up, the links there.
We have fabricated on the same plate a clamp for a TOSlink, using 3 frames of stereo variable area film, set it for the left hand track, still with its ends intact, setting up is very easy to the sound light source, as just look at the other end of the TOSLink and see the light appear, trim for brightest light, then plug the other end into a optical jack, feed that into the pre amp main amp, the sound is then excellent, TOSlink optical lead is still 1 metre with both ends still intact.
Ran some different film clips spliced as one run, reads the lot fine, so thats a result.
Tony Smith
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