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Author
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Topic: Test film index
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Fredrik A Thulin
Film Handler
Posts: 17
From: Gothenburg, västra götaland, sweden
Registered: Oct 2015
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posted 01-03-2016 07:23 PM
I have just installed a 70 mm equipement at a new venue, And I have encoutered the problem that magnetic stripes does not have a standardised output, as opposed to optical sound that has a reliable fixed output that does not alter between titles. I have two dolby test films for two different features. The first one i set for 100 mV, then when i tread the second test film I got a 120 mV response. With vintage filmprints, the test films are often lost and you can not set the dolby level accurately. So I got an idea. Lets create an index. that mesures all the avalible test films out there and make an excel document for everybody to use. This index will use one of the films as a reference point, and with this information, you can calculate the dolby level for any other film that has been tested. SO if I have the test film for title x,,, that is 100 mv,, and a print for title y that has been tested to deliver 150 mV, I can lower the resoponse accordingly and still reach perfect dolby level. Is this a good idea? The mesurements will have to be done on the same set up, so One person has to do it. I could do this if you send me your dolby level test films, and I will return them as soon as possible. Any thoughts?
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 01-04-2016 06:00 AM
I think you'll less than an enthusiastic response on this venture. Existing magnetic films will only dwindle in quantity. The notion of sending one's "collected" test films around the glob also wouldn't excite most people.
As for the output level, you are referring to the output of the preamplifier and based on the voltage you are stating, my guess would be a Dolby MPU and feeding a Dolby CP200 (or perhaps CP100)...that output level is not uniform across all brands or even later Dolby processors.
You have two basic eras in magnetic sound composite film and that hinges around 1983. Starting with "Jedi" the level and response (and actual track width) of magnetic film changed dramatically. As such, you will have yet another set of variables in your calculations. "Level" film is typically in the 800-1KHz range and the response of your system at those frequencies will also affect your output level.
Furthermore, though the years, I've found that the level from track to track to not be uniform either (close enough but not close enough that you could run the film backwards (shouldn't matter if running a solid tone) where the levels weren't all goofy. To make matters even more confusing, I've heard some claim (and this was back in the day) that the frame line identifier was THE reference for track location and direction while common sense would have been to use the picture on the film, if present. Some of the test films were all black while others were completely clear. Later films didn't have the identifier either so you are completely dependent on someone believing and keeping track of what is the "head" of the roll too.
I've had limited success on some titles using the slate tone on the head of some leaders in "finding" the recording level (You'll hear someone calling out the title, mix, reel number, followed by a tone).
If it is a Dolby encoded track, sometimes you can "find" the level by careful listening by switching NR in and out while paying attention to the LEVEL (not frequency response) of dialog and other strong material (Dolby NR leaves the signal alone above Dolby Level).
In any event, I think you'd be better served trying to find some genuine reference film (like Teccon or perhaps Dolby though I don't know how precise theirs was) and using that as your "blessed" reference (Teccon's was measured film and serialized) and then referencing your existing library to that in terms of dB and coming up with a chart in that manner. It still will not necessarily describe the print someone has as it may have been sounded on a different date/time or even year and the recording heads wear so where in the run your print/test film are also plays into it.
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