8mm Forum: Dolby OPU optical preamp = lousy sound?

»	8mm Forum » 16mm Forum » Dolby OPU optical preamp = lousy sound?

UBBFriend: Email this page to someone!

Author

Topic: Dolby OPU optical preamp = lousy sound?

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted February 29, 2008 12:18 PM

I may have to impose on Brad's help to get this on the main forum. But I'll start with you good folks.

A friend in the telecine business just did some work for me on a Rank Cintel of older vintage (e.g. 1980s) that looks absolutely wonderful. But the optical sound heads (one for 16mm, another for 35mm) go through a Dolby OPU 1 Optical Preamp (which contains an Output Amplifier Module 137A card) from which the output level seems strangely low, and the noise level is intense. Audio signal-to-noise is 30-35dB, with a screamingly annoying white noise across the entire spectrum. Not good.

Options:
1) find manuals for this old equipment and find something to adjust. Believe me, anything that could be adjusted was attempted. But not knowing what we're looking for is not good.
2) find manuals and determine that something is faulty
3) replace this head amp with something else. Suggestions?

Anyone who can help us with this will earn our undying gratitude.

| IP: Logged

David Kilderry
Jedi Master Film Handler

Posts: 963
From: Melbourne, Victoria, Australia
Registered: Feb 2006

posted March 01, 2008 05:06 AM

Bill,

Is the sound bad just on 16mm transfer? If so how is it on 35mm? They would share the same circuitry I imagine. I trust there are no cards to swap out? Almost every 35mm Dolby cinema processor comes with a few spare cards for trouble shooting.

How are the optics on the 16mm head? If it has been a long time since they were used check for dust and deposits on the cell and in the slit optics.

If it were a cinema processor rather than a Rack Cintel I could probably help you.

David

| IP: Logged

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted March 01, 2008 11:34 AM

Great questions, David.

Yes, the sound stinks on 35mm too, indicating the problem is circuitry-related. Can't tell you the condition of the cells right off, though I think the engineering guys probably looked things over by visual inspection. Definitely worth checking. They do sound only occasionally and this problem has been with them for years.

The fact that this Dolby unit isn't a normal rack processor makes me wonder if it wasn't a custom job for Rank. So who has a schematic or alignment manual, I wonder?

| IP: Logged

John Whittle
Jedi Master Film Handler

Posts: 791
From: Northridge, CA USA
Registered: Jun 2003

posted March 03, 2008 12:01 PM

Rank used to have an office in So Calif, I think in Newhall or somewhere around Six Flags. If they don't have a schematic, they should be able to get one and considering the number of telecines in So Calif, they also probably have the guy who installed it originally.

It almost sounds like the Dolby circuity is bad. BTW Dolby has an office in Burbank by Warner Bros.

John

| IP: Logged

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted March 04, 2008 03:32 PM

John, I'm checking with the vendor on that. Thanks for the tips very much.

| IP: Logged

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted March 09, 2008 07:32 PM

Another workaround would be to do a separate audio dub from a projector and line it up with the Rank transfer in software. But that would require a projector with the direct-drive motor so that there are no speed variations. Is there such a thing?

| IP: Logged

John Whittle
Jedi Master Film Handler

Posts: 791
From: Northridge, CA USA
Registered: Jun 2003

posted March 09, 2008 07:37 PM

In theory a projector with a synchronous motor would transport the film at 24 fps and you could record off of that. The problem with telecine is that you aren't running 24 fps but 23.97 and you'll wind up being out of sync by a noticeable amount.

What we'd do commercially is have time code on the dubb master and then do an audio laydown of the track onto the silent master from telecine which would also have timecode on it. One synch up and it'll run all the way through.

There are plenty of commercial sound houses that can transfer your tracks to the media you want or even layback the sound from your print onto the telecine master be it digibeta or whatever.

John

| IP: Logged

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted March 09, 2008 07:43 PM

All extremely good points; I'm just trying to save additional $$$ after the initial expense of the Rank transfers. Yes, 24 to 23.976 would require some attention, but if the speed is absolutely constant, this is no problem: digital resampling is relatively easy for me to do, and highly accurate to boot. Thus, I am VERY interested to know what 16mm hardware is around that runs via synchronous motor, since the cost of outsourcing this audio problem could instead buy a LOT of hardware!

I have nearly achieved an acceptable result of digitally de-noising the audio; but there's nothing like having it clean right out of the box.

| IP: Logged

John Whittle
Jedi Master Film Handler

Posts: 791
From: Northridge, CA USA
Registered: Jun 2003

posted March 10, 2008 09:36 AM

Magnasync as well as Westrex, RCA and Magnetech all made optical sound readers. Generally these mouned in a rack under the mag heads of a regular dubber and allowed playback of an optical sound track. Another possibility is to just rig a synch motor onto a 16mm projector and use the output.

But before going to that extreme, why not just replace the pre-amp on your soundhead and by-pass all the dolby circuity? Leave the card in place for the exciter lamp if that's where it's power is sourced and just use a good low noice mic pre-amp. Take the solar cell and load it with a 4.7K resistor and feed it into the mic input and you should be pretty close. You might need to pad it down it bit depending on the solar cell and the exciter lamp but then you could do it in one pass.

Working from a DA88 tape or other master material allowed us to layback both a domestic dolby prologic and an international constant level M&E prologic so one master tape could be used for all sources. As second pass as PAL produced the other necessary master.

John

| IP: Logged

Jean-Marc Toussaint
Film God

Posts: 2392
From: France
Registered: Oct 2004

posted March 10, 2008 09:56 AM

Bill, as far as I remember, there's one of the Bauer 16mm machines that can run with a pulse-sync unit such as those made by Pedro.

If you can record the sound, import the file into an editing software surch as Final Cut Pro and set the speed there (a 24fps file ran at 99.9% speed will get you very close to 23.976 fps).

It's "semi low-tech" (is there such a thing like med-tech?). I'm currently experimenting (retracking a super 8 feature) with a strange file that is a Fake PAL dub of an NTSC vid. It takes a little time but it works.

--------------------
The Grindcave Cinema Website

| IP: Logged

Bill Brandenstein
Phenomenal Film Handler

Posts: 1632
From: California
Registered: Aug 2007

posted March 10, 2008 10:41 PM

My thanks to both of you -- some very helpful ideas here. John, bypassing the preamp is a great idea, and I didn't know what value to get for a resistor. So that's a definite possibility, though it would take another run through the Rank to accomplish the job. (Or what about replacing the preamp? What would I look for?) Jean-Marc, I have a lower-end Pedro box already, and might be able to use it to do the job. Achieving 23.976 that way is easy. Just need a projector...

Thank you, gentlemen, this is truly helpful.

| IP: Logged

John Whittle
Jedi Master Film Handler

Posts: 791
From: Northridge, CA USA
Registered: Jun 2003

posted March 11, 2008 09:10 AM

Bill,

Just find the solar cell in the sound head (forget the circuit cards. Connect a shielded lead from the solar cell and run it to an external pre-amp with a 4.7k resistor across the input. That leaves all your original circuity in place and you can reconnect the solar cell if it doesn't work out for you. Any circuity necessary for the exciter lamp will still be in place.

You might even put in a jack on the sound head so you can plug in an external cable with one of those jacks that disconnects the internal output when you plug in the external lead.

Just use a shielded lead since there are tons of stray fields all over that Rank.

John

| IP: Logged

All times are Central

Printer-friendly view of this topic

UBB.classic^TM 6.3.1.2