Film-Tech Forum ARCHIVE: Setting SPL Levels

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Topic: Setting SPL Levels

Paul Konen
Jedi Master Film Handler

Posts: 981
From: Frisco, TX. (North of Dallas)
Registered: Jun 99

posted 09-04-2001 10:01 AM

One benefit that I have received because of the DLP is having THX come out and EQ our auditoriums. While loading the DLP, they setup their R2 units and do EQ.

I am raising this subject to see what you all thought about the following.

When setting SPL levels with a CP-500 processor, the tech will set the levels straight across on the processor and use the gain on the amps to reach the proper level.

The idea is, ( I hope I say this right ), to amplify a higher signal with a lower gain rather than amplify a lower signal with a higher gain. With the gain all the way up, you not only amplify the signal but any line noise that may be present on the wire.

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Jon Bartow
Master Film Handler

Posts: 287
From: Massachusetts
Registered: Nov 1999

posted 09-04-2001 02:47 PM

While the concept has merit, in the real world it is not that practical for a couple of reasons:
- Without very expensive test equipment (like the R2 for example) one could not detect the S:N difference between "traditional" and the way the DLP guys are doing it.
- If the amps are all set at max then one could switch out a bad amp without resetting the SPL
- If the amps are all different then you are just asking for one of the amp's gain levels to be bumped by whatever (vacuuming out the vents on the amps, someone plugging in a piece of equipment into the back of the rack [How do you know that the DLP guys didn't bump one of the gain knobs when they were packing up their equipment the last time that they were there?], etc.)
If the amps are all set at Max then this eliminates that potential.

If you have a private screening room or you operate a single screen theater with projectionists (not operators) that really care and are very careful, then go ahead. But in the modern Megaplex with manager/operators and no projectionists in sight you would be just asking for trouble.

Jonathan Bartow

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Gordon McLeod
Film God

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

posted 09-04-2001 06:46 PM

Actually it doesn't take a R2 to notice the difference in fact the R2 doesn't really see the differenc e anyway
The idea is that the input to the THX crossover is to match the output buss of the processor and that is 300mv
By useing the amp gains to set the levels the system is dramatically quieter.

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 09-04-2001 07:38 PM

Adjusting an analog knob on an amp allows for more precision than the digital "steps" of a processor like the CP500 as well. Usually when I see this done, the first thing that happens is the knobs themselves come off. The adjustments on the amps can still be made this way, but you won't risk "bumping" a setting when you pull the knob off later. Then of course special caps go on so it is difficult to accidentally make an adjustment.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 09-04-2001 11:47 PM

Gord is absolutely correct on this point....This is not something the R-2 would ever see. You would need an AC millivolt meter to measure the noise levels at the amplifler output. You should set the output of the processor for 275 to 300 mv to the power amps. Doing so should give you a really good signal to noise ratio. This will also improve the apparent(and real) dynamic range of the B chain. No noise at all should be heard from the screen speakers even if you walk right up to the screen. NEVER BUT NEVER crank the amp gains all the way up and then set your processors output levels. You should do your job in a professional manner, not in a manner that is convenient to you. Reasoning like this is why movie theatres have deteriorated to the point that they have. You have to consider that some amps actually go to the plus side of odb when it comes to gains and going over 0db will only lower the signal to noise ratio and you will also loose some dynamic capability as well. Cranking gains up past 0db will get you nothing but noise that can most likely be heard in the center to rear of the autitorium!
Also use only an R-2 analyzer for the absolute best results! The extra effort and actual small expense is well worth it. NOTHING else for general motion pictue work even comes close to the results an R-2 will bring in! As Brad has conformed on this site before, Yes, I am married to my R-2. Its been 6 happy productive years now and counting.
Mark @ GTS

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 09-05-2001 02:04 AM

Mark, the R2 is obsolete, you of all people should know that. Especially with the coming of the R3.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 09-05-2001 09:53 AM

When the R-3 is available I'll be the first in line too......but I'll have to get divorced from my R-2 first.........but then I do live in Utah......and some have many wives here.
Mark @ GTS

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 09-05-2001 02:40 PM

OK I'm following this. Would I be right in saying that if one wanted to apply this method to non bi-amped systems, or to non THX cross overs, one would have to modify that 300 mV setting to match the input sensitivity of whatever piece of kit is in use???

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 09-05-2001 06:08 PM

Pete, it depends on the nominal output level of the processor and in input of the following device (amp in your case) as to what is the best S/N would be. Normally, but not always, the best S/N is obtained when the processor outputs are set to 0dB (digital) or maximum (analog) though it depends on how the output stage is designed. Some processors actually tell you which is best...Sony, for instance is best at 0dB. Panastereo recommends turning the output trims up all the way and adjusting the amplifier attenuators for proper level.

Steve

------------------
"Old projectionists never die, they just changeover!"

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 09-06-2001 03:05 AM

That makes sense, in a nutshell one is matching the output and input stages as closely as possible.

Though I guess one is stuck with adjusting the CP output if the power amps are of a fixed gain design though.

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 09-15-2001 02:32 PM

Hi,

My tech did the same. He set the SPL from amplifiers.
There is a theater in my town where all amplifiers are set to maximum. I remember that in a quite scene of a movie (or walking near a surround speaker) I heard a lot of noise...
Do you keep your Home Hi-Fi amplifier to maximum and set the CD output knob to have a good result?

Bye
Antonio

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Stefan Scholz
Expert Film Handler

Posts: 223
From: Schoenberg, Germany
Registered: Sep 1999

posted 09-16-2001 03:56 AM

Some amplifiers offer lockable gain controls on the rear, so that you could not change them by coincidance. It is not a great number,I know, but some mfgs. can add the extra attenuator at a small extra cost.
The idea is simple, as power amps have a fixed gain (therefore fixed S/N), you're setting the preamp (processor, crossovers, etc) to run at it's best S/N ratio level, which is normally on the higher side of output voltages.
Then attenuating the signal to match actual SPL in the auditorium the power amp controls come into service. Result can be a dead quiet system, compared to the hissy way with attenuated preamp and power amp attenuator at minimum.

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System Notices
Forum Watchdog / Soup Nazi

Posts: 215

Registered: Apr 2004

posted 04-04-2005 11:57 PM

It has been 1296 days since the last post.

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Tom Wienholt
Master Film Handler

Posts: 371
From: Towson, MD, USA
Registered: Dec 2002

posted 04-04-2005 11:57 PM

So what are the exact procedures for setting the outputs to 300mv? Is this the best thing to do with CP50's and 55's, set the processor outputs to 300mv and adjust the SPL's with the amps?

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Tom Wienholt
Master Film Handler

Posts: 371
From: Towson, MD, USA
Registered: Dec 2002

posted 04-06-2005 11:15 PM

Is it ok to just set the amps at maximum and adjust the processor outputs?

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