This is topic Dolby levels drifting ? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Glenn Conatser (Member # 970) on 05-24-2002, 10:19 PM:
Im a little curious as to why the levels and EQ settings tend to drift over time. What causes them to drift if no one plays with the settings ?Glenn
Posted by Paul G. Thompson (Member # 655) on 05-24-2002, 10:37 PM:
Bah!Not unless you have a dirty exciter lens, or a flakey exciter lamp. If you have a reverse scan, they have to be kept clean, also. A dirty lens will definately screw up the high end, giving you the impression the EQ cards have drifted.
What you said is "over a period of time" indicates (to me, at least) is your exciter lamp is deteriorating, or the lens is getting cruddy.
Whenever the exciter lamp is changed, it has to be re-targeted and then run dolby tone to re-calibrate. Don't calibrate it with a dirty lens, either. That will just add insult to injury. When you recalibrate, insure you have the emulsion to the rear! If not, you can throw in an error as high as 3db, and the NR and SRA cards will not be very happy about that.
If you have a reverse scan, they should be checked for calibration also. I do that to mine if I notice something going funky.
Posted by Glenn Conatser (Member # 970) on 05-24-2002, 11:10 PM:
Actually what i was more referring to was when someone goes through the system and checks everything and notices the levels are off and the EQ is a little off and just need to be reset. What causes the levels and EQ to drift if you know no one has gone in and adjusted any of the pots.
Posted by Randy Stankey (Member # 64) on 05-25-2002, 12:13 AM:
Over what period of time?I SUPPOSE that as speakers age they could change characteristics. Amps and processors should stay pretty much on target for a long time. I would imagine that any changes that DO take place would be rather small and would take place over a period of months, if not years.
Any changes that do take place in any time period less than that should be pretty hard to notice, especially without test equipment. If you are noticing changes in your sound system the first place I would look would be in the A-Chain. Like Paul said, dirty lenses and misaligned exciters/LEDs can cause you all sorts of grief. Get somebody to give your machine a good going over with a scope, analyzer and test film.
If you notice changes in the system but your projector is well-tuned then my guess would be that you have a tinkerer. Seal the processor covers with stickers. Use your computer or someting to print, "DO NOT REMOVE UNDER PENALTY OF DEATH!", on them. Sign them and stick them over the screws that hold the front panel on in such a way that the panel can't be removed without damaging the sticker.
Then if you catch somebody breaking one of your seals, skin them alive. 
Posted by Pete Naples (Member # 761) on 05-25-2002, 03:52 AM:
Hmmm I've thought about this often. Older analogue CP's are just that analogue devices, and like all electronics, they are bound to drift as component values change with age. The same will apply to amplifiers and also speakers being partly mechanical devices will over time, alter their response as they wear.
Now the current crop of digital CP's won't drift in their nominal output, perhaps the analogue parts of the circuitry will drift, and I very much doubt that they employ output sampling and comparison to self compensate.
'A' chains will drift quite markedly, as Paul rightly said, the lamps age, as do the LEDs or Lasers. Heat and dirt affects the lenses and SVA cells suffer over time too. Also the mechanical components of the rep-set will wear.
Posted by Don Sneed (Member # 955) on 05-25-2002, 07:54 AM:
Paul & Pete, I think you two guys nailed that one...correct with both, age, time, cool/heat of the booth plays an important factor in this and all electronic equipment....also I have seen Cat.# 69T recorded on one date, & another # 69T recorded on a different date, & the levels will not be the same, very close but not always the same, so sometime this can make things look as it difted...
Posted by Gordon McLeod (Member # 33) on 05-25-2002, 09:26 AM:
Speakers age and does the room
When new the drywall and tape and paint have very high moisture content and humidity is one of the major factors effecting sound equalization
Also all electronics drift over time digital or analogue. We all get older 
Posted by Brad Miller (Member # 2) on 05-25-2002, 10:24 AM:
I agree with everything said so far, but there was one thing not mentioned yet. Not only is there the varying differences in test films, different calibration equipment from tech to tech, equipment wandering by nature, the cleanliness of the sound optics and as Gordon pointed out the fact that the acoustics of the room DO change over time, but what are the odds that the last tech to check the system placed his microphones in exactly the same locations as the current one? Highly doubtful, and moving the mics around can give you a completely different reading! If you've really got the time to tweek, nothing beats moving the 4 mics around the room a few times and making "note cards" on the EQ settings you get at each placement and then combining them together into a real "average" for the room, then tapering the differences down just a tad to prevent "overEQing". 
Posted by Gordon McLeod (Member # 33) on 05-25-2002, 10:27 AM:
Actually the R2 software for a recert documents in the text file where you place the mics and I usually stick a calibration sticker on the processor that also contains that and other information