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» Film-Tech Forum   » Operations   » Film Handlers' Forum   » analogsound w/white light (Page 1)

 
This topic comprises 2 pages: 1  2 
 
Author Topic: analogsound w/white light
Allan Lyman
Film Handler

Posts: 31
From: COPENHAGEN, S /copenhagen,denmark
Registered: Apr 2016


 - posted 08-09-2018 04:30 AM      Profile for Allan Lyman   Email Allan Lyman   Send New Private Message       Edit/Delete Post 
Running old movies with RED LED,does not sound to good.Several has built white LED on the projector as well.
My question is:
When rev.analog sound came out,cinemeccanica started with IR LED,before the RED LED.will those work just as well as if I where running with white LED.talking of old soundtracks offcourse.
alan

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Sam D. Chavez
Film God

Posts: 2087
From: Martinez, CA USA
Registered: Aug 2003


 - posted 08-09-2018 09:26 AM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
The original LEDs were IR. Exciter lamp light is wide spectrum but the bandwidth that gets modulated going thru the film soundtrack is 660 nanometers, the same output bandwidth produced by the IR LED.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 763
From: Denver, CO, USA
Registered: Aug 2009


 - posted 08-09-2018 10:22 AM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
A little off topic, but it surprised me to find that incandescent exciter lamps were run off high frequency AC (maybe 50 kHz, but I don't really remember) instead of DC. Was this, perhaps, an effort to have the filament "wear" evenly? I only saw a few optical sound projectors, so maybe AC exciter lamp drive was not as common as I think.

Harold

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Ed Inman
Expert Film Handler

Posts: 100
From: Jackson, Mississippi USA
Registered: Jul 2004


 - posted 08-09-2018 12:04 PM      Profile for Ed Inman   Author's Homepage   Email Ed Inman   Send New Private Message       Edit/Delete Post 
I assume exciter lamps can run off either DC or AC so long as the voltage remains the same but that AC may more likely cause a slight "buzz" in the audio. Most DC exciter power supply units have an emergency AC switch to use if the DC transformer goes out.

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Sam D. Chavez
Film God

Posts: 2087
From: Martinez, CA USA
Registered: Aug 2003


 - posted 08-09-2018 01:21 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
Drive-in's used AC exciters and the 60 Hz was hardly audible on the car speakers. There is also a time lag on a bright filament so that filters the hum as well.

As to HF exciter lamp supplies, I don't know of any that worked that way purposely, perhaps the filtering was kaput. There were quite a few HF exciters in old "portable" 16mm projectors for the purposes of saving weight.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 763
From: Denver, CO, USA
Registered: Aug 2009


 - posted 08-09-2018 01:58 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Thanks! I saw this in 16mm projectors in a small theater in the 1970s.

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Stephan Shelley
Jedi Master Film Handler

Posts: 700
From: castro valley, CA, usa
Registered: Nov 2014


 - posted 08-09-2018 02:27 PM      Profile for Stephan Shelley   Email Stephan Shelley   Send New Private Message       Edit/Delete Post 
I know PFA at UC Berkeley uses Sam's BACP red readers with good results on all kings of optical tracks. They have tried all kinds and found it to work well.

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Brad Miller
Administrator

Posts: 17687
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99


 - posted 08-09-2018 06:02 PM      Profile for Brad Miller   Author's Homepage   Email Brad Miller       Edit/Delete Post 
For older silver tracks with all other things equal, white light and IR LEDs will absolutely sound better than a red reader. Playing older prints (pre-cyan) on a red reader sounds thin and spitty.

You can at least quiet down the noise on the track by wrapping a small piece of Scotch "Magic" tape over the LED itself. You will need to slightly increase your gain levels on the sound processor, but the Magic tape will act to diffuse the light and as such any scratches, dirt and scuffs on the base side of the film will not be picked up by the camera. It really does make a HUGE difference with any print that isn't of "lab new" quality and this little upgrade does not diminish the quality of even a brand new print.

(Credit goes out the Steve Guttag for the "Magic trick".)

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Jim Cassedy
Phenomenal Film Handler

Posts: 1553
From: San Francisco, CA
Registered: Dec 2006


 - posted 08-09-2018 07:07 PM      Profile for Jim Cassedy   Email Jim Cassedy   Send New Private Message       Edit/Delete Post 
I've personally never seen a 35mm projector that used hi-frequency AC
on the exciter lamps, although I suppose someone could have built a system that did that.

On many of the older 35mm systems I used to run, the exciter supply was
DC, but if one of the exciter lamp rectifiers failed, the power supplies usually
had an "emergency switch" that ran the bulbs direct from the power supply's
AC transformer. The thick filaments in the exciter lamps tended to 'filter out'
a lot of the AC hum due to their electrical & thermal inertia.

In an empty theater you could hear the hum, although it was very low, and
was usually almost unnoticeable once the theater opened, when it got masked
by the "crowd noise"

HF AC was very common however on 16mm projectors for several decades.

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Stephan Shelley
Jedi Master Film Handler

Posts: 700
From: castro valley, CA, usa
Registered: Nov 2014


 - posted 08-09-2018 07:20 PM      Profile for Stephan Shelley   Email Stephan Shelley   Send New Private Message       Edit/Delete Post 
As far as the red readers at PFA I just know what the guys there say. The did run tests and were happy with the results. They tend to be a picky group there. At first they were wanting to have IR LEDs as well as the visible red ones as they are easy to change on the BACP design but after the tests they decided they were not necessary.

It seams to me that Kelmar came out with a HF supply shortly after SR came out. They had black face plates instead of white. They did not make them long but the claim was they were better for SR.

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Monte L Fullmer
Film God

Posts: 8318
From: Nampa, Idaho, USA
Registered: Nov 2004


 - posted 08-09-2018 07:33 PM      Profile for Monte L Fullmer   Email Monte L Fullmer   Send New Private Message       Edit/Delete Post 
Seemed that silver soundtracks found on IB Tech and B/W prints, the LED had a harder time with this than the older filament based exciter lamps and slit lens to PC.

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Brad Miller
Administrator

Posts: 17687
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99


 - posted 08-09-2018 07:50 PM      Profile for Brad Miller   Author's Homepage   Email Brad Miller       Edit/Delete Post 
Stephan, The BACP design was unique in that it was easy to swap out the red vs. IR leds...but if you didn't re-focus the lens (on many machines this required a redo of the A chain) you weren't getting all that the non-aligned led could do.

My guess is they installed and aligned for red leds, then installed the IR and didn't redo the A chain. The audible improvement isn't exactly subtle.

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Sam D. Chavez
Film God

Posts: 2087
From: Martinez, CA USA
Registered: Aug 2003


 - posted 08-09-2018 07:52 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
BACP sold a number of IR LED modules to museums and other high end screening rooms so they could be changed out from the red LEDs when running silver tracks. Mostly they got lost in the parts bin and never got used. I was at the Paramount Oakland equipped with BACP red readers and ran an archival print of Maltese Falcon and of all the films shown there in the last few years, it sound best of all.

Kelmar sold a switching power supply late into exciter lamps but it was no big deal except it held its settings better than earlier linear power supplies. That would be better for holding Dolby level.

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Allan Lyman
Film Handler

Posts: 31
From: COPENHAGEN, S /copenhagen,denmark
Registered: Apr 2016


 - posted 08-10-2018 02:36 AM      Profile for Allan Lyman   Email Allan Lyman   Send New Private Message       Edit/Delete Post 
Thanks for replies.

So Sam,using a IR LED,will it sound just as good as white light
LED or exciter lamp,or just much better than RED LED.

Alan

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Sam D. Chavez
Film God

Posts: 2087
From: Martinez, CA USA
Registered: Aug 2003


 - posted 08-10-2018 10:44 AM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
Alan,

The IR LED as designed by Dolby sounds lsimilar an exciter lamp. This is due to similar cross modulation distortion artifacts, but with the added improvement of even illumination across the entire soundtrack. When used with the reverse scan readers built to Dolby specs you get much improved channel separation, and much better HF response.

Problem is, the source of IR LEDs is exhausted.

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