Film-Tech Forum ARCHIVE: Optical fibre as film sound pick up

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Posted by Tony Smith (Member # 7191) on 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?

Posted by Tony Smith (Member # 7191) on 05-07-2015, 12:09 PM:

Hi
The level comming out of the repeater jack on full white light
is just 20 milli volts.

just trying to get a full look at what the pick cells that were originally fitted give out, also alignment of the optical fiber ends to the stereo tracks is not easy to say the least

Tony Smith

Posted by Sam D. Chavez (Member # 1841) on 05-07-2015, 01:20 PM:

Be nice to see some pics. Someone went to a lot of trouble. Unless the fiber covers the whole soundtrack, .084/2 inchs total width you're not picking up the entire sound track. It would sound terrible as it would pick up just the low modulations.

Posted by Marcel Birgelen (Member # 6801) on 05-07-2015, 04:03 PM:

quote: Tony Smith
is just two short lenghts of optical fiber tube, not sure what wavelenght of tube size yet, so 4 strips of optical fibe tube

I don't really follow. Are there two or four fiber strands in place?

Also, is the light from the exciter passed trough a slit before it hits the soundtrack?

quote: Sam D. Chavez
Unless the fiber covers the whole soundtrack, .084/2 inchs total width you're not picking up the entire sound track. It would sound terrible as it would pick up just the low modulations.

I guess you could use some kind of small collector lens to collect all the light and focus it on the fiber, but I think it'll be rather hard to get this right.

Posted by Tony Smith (Member # 7191) on 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

Posted by Rick Raskin (Member # 1561) on 05-07-2015, 04:35 PM:

Wouldn't you need an A to D converter to pass audio over fiber? This whole thing sounds rather strange without seeing some pictures. If you are talking about reading digital tracks I suppose it could be configured using a head that would appropriately focus the beam(s).

Posted by Richard Fowler (Member # 893) on 05-07-2015, 04:45 PM:

The light/image is being light piped by the fiber so no A to D conversion….

Posted by Tony Smith (Member # 7191) on 05-07-2015, 04:49 PM:

Hi Rick
No the optical fiber is just carring the variable light levels after brightness variations caused by passing through the variable area sound tracks, to the Optical fiber jacks, the fiber is pushed in and clamped inside the jack where at the bottom is a light sensitive cell which changes light variatons to varing DC, then up coax to the pre amp.

Tony Smith

Posted by Sam D. Chavez (Member # 1841) on 05-07-2015, 05:15 PM:

That's a real mess you have there. Or a dogs breakfast if you prefer. Not that a light pipe is is a bad idea per se, but this is not it.

Posted by Rick Raskin (Member # 1561) on 05-07-2015, 05:20 PM:

So, you are pushing analog over fiber; interesting. My experience is mostly in the digital transmission world.

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.

Posted by Marcel Birgelen (Member # 6801) on 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]

Posted by Tony Smith (Member # 7191) on 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

Posted by Marcel Birgelen (Member # 6801) on 05-08-2015, 06:35 AM:

I doubt you will ever get anywhere near decent results when using two separate fibers per track. Either you should use a fiber per track sufficiently broad enough to cover the whole audio track or you should have something in place that collects all the light and bundles it over the fiber.

With two fibers side by side, you will miss some of the light that falls in between the fibers. This will cause a massive amount of distortion.

Posted by Tony Smith (Member # 7191) on 05-08-2015, 08:32 AM:

Hi Marcel
Yes totally agree, have been measuring the wall thickness, with the selection of Toslink leads we have, and they are much thicker than the Jiangsu similar ones that were installed, even though two would be wide enough to cover the variable area sound track, nearly by only approx 0.001 of inch less.

Still looking at options

Tony Smith

Posted by Tony Smith (Member # 7191) on 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

Posted by Joel Whybrow (Member # 4189) on 10-28-2015, 11:30 PM:

Hi Tony

I'm facing a similar issue with a portacine which has had some pretty nasty mucking about. Could you possibly post a picture to how you have set this up?

How you are sending it to an amplifier by the way? Just optical to electrical conversion? I'm a bit confused!