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Topic: sound test film question ... ... and lamp voltages?

Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 03-31-2002 11:23 PM

Please, please, please excuse my lack of knowledge in this area ..... but can somebody please tell me ......

When the A-chain alignment manual calls for a 50% modulation tone, ... do they mean the Dolby Cat69T ???

I am guessing that they do, but I would like to know for sure....

Also, some say the exciter lamp should be run at 60% of it's voltage rating, others say at least 85%......... I've checked ours here, and they are all running at 5V in to 10V lamps ???????? What is right here?

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 03-31-2002 11:57 PM

Yes, Dolby Tone (Cat.69T or the tone side of the two-tracked Cat.69) represents 50% modulation.

5V on a 10V bulb seems on the low side. If you're able to make Dolby level, great, but I'd worry about the evenness of the track illumination since at such a low level the bulb filament may not be getting hot enough at the ends where it contacts the supports and would therefore not illuminate the track evenly all the way across.

In general, lower voltage yields greater bulb life which is good but unless you run the uniformity test film to verify that all is well I'd say this is too low and crank it up a bit. There is no hard and fast rule but 75-80% of rated voltage as a minimum seems reasonable; higher, obviously, if you're having a problem making Dolby level. You should still get good bulb life at these voltages.

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 04-01-2002 12:36 AM

Run your exciter lamp at 7.5v for the white light exciters. For the red led's the voltage can vary.

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 04-01-2002 07:44 AM

Yes, Steve is right. There's no specific voltage to set exciters at.
You don't want to get the correct (Dolby tone) level by having the preamp turned down, but the exciter turned too bright. This shortens the bulb life.

Conversely, you dont want the preamp turned way up, but with the exciter set at a low voltage. The bulb will last longer, but the "noise floor" may be high (the slight hiss or other noise you hear when a volume control is turned all the way up.)

I usually turn the preamp trim pots to about 70%, adjust the exciter level to get Dolby tone, then go back to the trim pots and "tweek" it. If one channel's adjustment is significantly different from the other, there's some problem (it's not mechanically aligned, bad cell, dirty optics, etc.)

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Gordon McLeod
Film God

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

posted 04-01-2002 08:33 AM

Usually 10v lamps I run at 8.5 volts
Eveness of slit illumination is criticle for matrix and NR tracking
Use the illumination test film or even better the SMPTE "snake Track"

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 04-01-2002 09:01 AM

by the way, the voltage I stated above was for a 9v exciter.

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Christopher Seo
Jedi Master Film Handler

Posts: 530
From: Los Angeles, CA
Registered: Jun 99

posted 04-02-2002 10:10 AM

Gordon,

What is the official part # for the SMPTE "Snake Track"? I checked the online SMPTE store and the only optical 35mm test film they have in stock is Buzz Track. They do have a lot of "out of stock, indefinitely" test films listed. Thanks!

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Gordon McLeod
Film God

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

posted 04-02-2002 08:43 PM

P35-sb and it is indefinetly out of stock

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