Film-Tech Forum ARCHIVE: Test film index

This is topic Test film index in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=011927

Posted by Fredrik A Thulin (Member # 9115) on 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?

Posted by Steve Guttag (Member # 268) on 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.

Posted by Sam D. Chavez (Member # 1841) on 01-04-2016, 10:29 AM:

Ditto to all Guttag says here about how difficult this task is.

I had one test reel for "Annie" striped and sounded on the emulsion side and upside down with some random picture footage. It drove my Salt Lake City projectionist around the bend because he thought he was having a bad day as of course he went by the picture in threading up.

How they got the stripes to stick I don't know.

Posted by Mark Gulbrandsen (Member # 72) on 01-04-2016, 02:41 PM:

quote: Steve Guttag
The notion of sending one's "collected" test films around the glob also wouldn't excite most people.

The sheer possibility of them coming back as a "glob" is what frightens most of the people here.

Mark

Posted by Sam D. Chavez (Member # 1841) on 01-04-2016, 03:10 PM:

A quote from a Spanish poet I've used before covers the situation.

"About books there are two kinds of fools

Those that lend them

and those that return them"

Covers it well

Posted by Gordon McLeod (Member # 33) on 01-04-2016, 04:34 PM:

Sam is that a soft or hard cover [Smile]

Posted by Louis Bornwasser (Member # 3063) on 01-04-2016, 07:37 PM:

I recognize the sincerity of the original poster. While this might have been more possible back-in-the-day, it is, by now, more than futile. More like tilting st windmills.

Posted by Sam D. Chavez (Member # 1841) on 01-04-2016, 11:13 PM:

Louis,

You are always a nice guy and see the good in everything. I agree it would be a noble effort.

Posted by Steve Guttag (Member # 268) on 01-05-2016, 07:12 AM:

Don't get me wrong...I think it would be wonderful if such a reference chart could exist. I just question the notion of shipping test film around the world and using one person's less-than-stellar method of creating the reference.

If you start with measured film, you then have an offset. Lets say you have Teccon reference film...you set up your system. You then run your collection of test films and come up with the dB offset (you'll have to use an average because I can guarantee that not all tracks are going to match level-for-level with measured test film). Then, lets say someone has one of test films on your chart and wants to run a film they have no test film for, then it would be a matter of comparing the two offsets (the film you have is say +1dB and the one you want to run is -5dB...calibrate your system to -6dB via your test film and you should be pretty close.

If you wanted to be even more inventive, measure the "Dolby Level" meter for its range and come up with an approximate offset for various needle deflections (helps the CP200 folks).

Posted by Fredrik A Thulin (Member # 9115) on 02-04-2016, 08:05 AM:

Thank you all for your input. I understand that the idea is not all crazy, but unrealistic. Thanks