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Topic: CP65 random buzz sound

Bernie Anderson Jr
Master Film Handler

Posts: 435
From: Woodbridge, New Jersey
Registered: Apr 2000

posted 12-27-2010 10:32 PM

UPDATE:

Thanks for all your help. Although, I'm thinking now it has more to do with the condition of the track on that one print and how good the CE reverse scan reads the track. I could reproduce it slightly with the same print on the second machine using a standard Exciter lamp, slit lens and Solar Cell.

Anyway, I took into consideration everyone's suggestions. I first had checked the LED alignment, although I do not have any CAT 566 test film, I could see that the LED was off center. I aligned it as best as I could using a small mirror to see the placement of the LED on the track. I think there is some 566 test film at the theatre, so I'll be very interested in trying it out.

I cleaned up some of the audio wiring.

But using Sam's suggestion, I put a Digital Multimeter at the test points on the Cat 240A and the Cat 150 and set the meter to VAC. With the Dolby Level set using the LED meters, I was at around .425 and at the Cat150 it was .475. I don't know if this is correct.

But in the end, I played all different kinds of SR film and I couldn't reproduce the sound that I was getting with this one print. I only heard it on one other trailer and that too was kind of beat.

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Glen Rich
Film Handler

Posts: 55
From: Nedlands, Western Australia
Registered: Feb 2002

posted 12-30-2010 07:53 AM

Just been sorting out a similar sound issue... which had nothing to do with any failings in the projection or sound equipment, (apart from filtering).
The low frequency buzz was being caused by the switchmode power supply in the computer/server, (these power supplies pump out a lot of electro-magnetic interference at audio frequencies), for the multimedia projectors we use for the ads.

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Richard May
Phenomenal Film Handler

Posts: 1057
From: Floral Park, NY USA
Registered: Aug 2004

posted 12-30-2010 08:13 AM

This sounds pretty much like the problem I was getting when the reverse scan readers first came out and we were installing them everywhere. We were getting the same sound from prints. On quiet scenes, there would be no distortion or buzzing. When there was dialouge or music, there would be this weird buzzing sound that went in perfect timing with the dialouge or music. We couldn't figure out what was happening until one day. 90% of the prints that were doing this were using super platters. In one of the rollers on the brain, the film would slip out and it would ride in such a way that it rubbed the soundtrack. That was what was causing it. In another situation, a different theater had a loose intermittent shoe on a simplex. The shoe bounced back and forth rubbing the soundtrack. Any little scratch or mark on the soundtrack "confuses" the reader and it comes out as that sound.

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Bernie Anderson Jr
Master Film Handler

Posts: 435
From: Woodbridge, New Jersey
Registered: Apr 2000

posted 12-30-2010 10:31 AM

That's exactly what it is. And if you look at the track closely on this one print, or any print that did it, would have a scratched sound track. When I played something with little or no wear I wouldn't get that result. But I did learn a lot from this thread, though.

My only question is still, when I use a multiple meter at the test points, I'm getting around .400 volts when the Dolby level is set to the led meters. That's not correct, right?

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 12-30-2010 10:44 AM

Assuming you are running Cat. No. 69T Dolby tone test film, it should read .270 to 300 mVolts. This is the official 50% modulation test film.

Multi-meters are all over the place on tiny audio signals.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 12-30-2010 08:04 PM

Also many cheap meters have issues with reading the true RMS voltage at other than 50/60 Hz

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Bernie Anderson Jr
Master Film Handler

Posts: 435
From: Woodbridge, New Jersey
Registered: Apr 2000

posted 12-30-2010 08:46 PM

I'm using a fluke digital multimeter though. Not really a cheap meter

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Jeremy Weigel
Phenomenal Film Handler

Posts: 1062
From: Edmond, OK, USA
Registered: Mar 2007

posted 12-30-2010 09:27 PM

Not cheap, but not necessarily able to read true RMS.

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Bernie Anderson Jr
Master Film Handler

Posts: 435
From: Woodbridge, New Jersey
Registered: Apr 2000

posted 12-31-2010 09:10 PM

So what is?

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