Film-Tech Forum ARCHIVE: sound test film question ... ... and lamp voltages?

This is topic sound test film question ... ... and lamp voltages? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Matt Close (Member # 996) on 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?

Posted by Steve Kraus (Member # 476) on 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.

Posted by Darryl Spicer (Member # 711) on 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.

Posted by John Walsh (Member # 168) on 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.)

Posted by Gordon McLeod (Member # 33) on 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"

Posted by Darryl Spicer (Member # 711) on 04-01-2002, 09:01 AM:

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

Posted by Christopher Seo (Member # 7) on 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!

Posted by Gordon McLeod (Member # 33) on 04-02-2002, 08:43 PM:

P35-sb and it is indefinetly out of stock