Film-Tech Forum ARCHIVE: analogsound w/white light

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Posted by Allan Lyman (Member # 9449) on 08-09-2018, 04:30 AM:

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

Posted by Sam D. Chavez (Member # 1841) on 08-09-2018, 09:26 AM:

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.

Posted by Harold Hallikainen (Member # 5405) on 08-09-2018, 10:22 AM:

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

Posted by Ed Inman (Member # 2583) on 08-09-2018, 12:04 PM:

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.

Posted by Sam D. Chavez (Member # 1841) on 08-09-2018, 01:21 PM:

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.

Posted by Harold Hallikainen (Member # 5405) on 08-09-2018, 01:58 PM:

Thanks! I saw this in 16mm projectors in a small theater in the 1970s.

Posted by Stephan Shelley (Member # 8587) on 08-09-2018, 02:27 PM:

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.

Posted by Brad Miller (Member # 2) on 08-09-2018, 06:02 PM:

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".)

Posted by Jim Cassedy (Member # 4115) on 08-09-2018, 07:07 PM:

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.

Posted by Stephan Shelley (Member # 8587) on 08-09-2018, 07:20 PM:

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.

Posted by Monte L Fullmer (Member # 2797) on 08-09-2018, 07:33 PM:

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.

Posted by Brad Miller (Member # 2) on 08-09-2018, 07:50 PM:

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.

Posted by Sam D. Chavez (Member # 1841) on 08-09-2018, 07:52 PM:

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.

Posted by Allan Lyman (Member # 9449) on 08-10-2018, 02:36 AM:

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

Posted by Sam D. Chavez (Member # 1841) on 08-10-2018, 10:44 AM:

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.

Posted by Steve Guttag (Member # 268) on 08-10-2018, 06:32 PM:

Yeah, but if you have one, they'll last probably forever. Under what would be typical use today, they are not going to go down much.

Posted by Allan Lyman (Member # 9449) on 08-11-2018, 05:24 AM:

SAM
Thanks.I have one,which I will try with.

regards
Alan

Posted by Steve Roberts (Member # 9544) on 08-28-2018, 12:09 AM:

As I have no films made after 1985 I still use the tungsten exciter lamps in all 3 of my DP70's. This was advantageous to the arts cinema in Edinburgh 2 years ago as they had booked a film with an odd coloured sound track their red LED would not read very well. I happily lent David the chief two of my sound heads for the show.

The Cinemeccanica Vic 10's at the New Oxford Cinema Manchester had red coloured PEC's.