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Topic: Gain setting on Amp, Filter & Processor
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 06-26-2018 03:26 PM
The proper gain setting is when each component will "Clip" at the exact same time. That is the maximum S/N through the system. Some parts of the system you don't have access to (particularly in digital). Sometimes you need to allow for that you might do things like equalize the system so you have to allow for the fact you might have added gain to the system.
A simple rule of gain is you want to do it EARLY but not often (unlike voting ;-))
With analog getting the gain structure right keeps the noise level lowest. If you have a low signal and try to amplify it at the end, you also amplify the noise with it.
In digital, the gain is, in effect, the resolution of the digital word. It is great that you have a 24 bit system but if you only ever use 8 bits of it, well that is what you have and it too will be noisy. In earlier DSPs you paid that penalty much more than today.
So, in cinema, you generally would like your cinema processor to be at its nominal reference position (for most "classic" cinema processors, that is 300mV out when 1KHz tone at reference is playing. That reference could be as much as 20dB down from full-output.
A CP650 also has a noise-optimization mode that will improve its noise floor providing you DON'T put gain into the system on your EQ and then run the fader up past 7.0.
So, presuming you leave the CP650 at its normal shipping position move on down the line to the amplifier.
If your QSC 30D is a DSP-30...ditch it and get something else. That goes back to QSC's early DSP days (before the Basis stuff) and it wasn't the greatest. It was easily clipped and had an awkward user interface via the program called "Signal Manager."
IF the QSC 30D is the DCM-30D...well then you have another DSP box to keep an eye on. If you go to strong on your processor levels, you WILL clip its inputs. Furthermore, if you have a DCM-30D then your amplifiers should wide-open. End of story.
The Dataport inputs on the DCA line have internal resistors (varies by amplifier model) to set the gain structure on the between the DCM-30D and the DCA. They've already optimized it for you so move along.
This makes your job much easier. Just set the CP650 levels, at that point so they are right to get your reference levels (Pink noise at -20dBFS should be 85dBc at the reference position, after tuning the room).
For more interesting discussions on gain structure, check out Rane Note 135:
https://www.rane.com/note135.html
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 06-29-2018 07:30 AM
There are different ways and philosophies on how to integrate film, digital and other sources. If you were to check out the Warehouse and look up the AFI/Silver, you'll see how I integrated film and digital there...back in 2003.
i would NEVER have a CP200 feed into a CP650. There is zero benefit and one now has two volume controls. The CP650's audio path is narrower unless you put in the CAT778. With the CP200 one does have some work involved, for sure but either do it or don't. The CP200 can do a lot (and I've pushed it a bit, over the years). My guess is that people that feed a CP200 into a CP650 are seeking the CP650's ability to take serial or Ethernet commands for automating things. At the AFI/Silver, I used the Pennywise DF-1 box to also provide for serial control to the CP200 (you can do fader and format selection. Heck, we have a Crestron control there and if you dial up "77" then Crestron can set the format too.
Today, with DSP, I'd be more inclined to feed a CP200 (or whatever "film processor) into the DSP processor (e.g. QSC's Q-SYS) and let DCinema feed into it on its own path. As much as possible, I try to avoid feeding cinema processors into cinema processors. Not that I haven't done it too. In fact, I have several DCP300s (including at the AFI/Silver, now) that take a film processor feed though all future designs will be Q-SYS based, for me.
If you set your CP200 to "7.0" and feed Dolby Tone (format 66) out, your CP650 meters should be at Dolby Level (-20dBFS).
The CP650 has a 6-channel analog input, which doesn't support 5 screen channels. The only real way to support the full CP200 channel compliment is with the CAT778 card but that requires an A-D conversion to get the CP200 feed to AES form...another potential source for noise (not all A-D or D-A converters are equal...in fact, most are less than equal. Good ones are expensive).
The CP200's power supply, the PS1(B) can be a source of noise too. It really can't handle the load properly. Take an O-Scope and look at the ripple coming off of the various power rails. You WILL get hum and some extra noise out of it. We've adapted the PS1B to use switch mode power supplies and reduce the ripple to effectively zero. The difference in noise was pretty stark. Instead of a ripplely 12.5ish volts we supply +/-13.5 clean volts (and 24VDC too). Why 13.5? Because the CP200 was designed knowing its power supply would sag a bit. There are 12V relays used in it, not 15V and EVERY board has clamping diodes on it set to clamp at 15V or so. The CAT517, again knowing about the sagging voltage rails uses the +24 and -15V rails to get a greater voltage swing. They weren't expecting the full -15V (or 39V to those output chips, which is near their maximum) so we knock it down to +/-13.5V but it is a clean power.
But back to the problem at hand...you really have to get the gain structure right all of the way through and you SHOULD be able to determine where the noise contributions are happening.
Presuming you are using the analog inputs of the DCM-30D, then unplug it and see if the noise vanishes...if so, the noise is coming from the CP650 or upstream (CP200, if that is the source). If the noise is there, then look no further.
As for the gain within the DCM-30D, you have gain controls over all sections of the crossover output. It is normal to have the more sensitive sections (typically HF) turned down but if all sections have negative dB values (LF, MF and HF), then that is not right. The least efficient section (typically the LF section, but not always), will be at 0.0dB and then the others are balanced to it.
Before ANY tuning is done in ANY cinema processor, the Crossover should be optimized (get the delays right and apply whatever PEQ filters are necessary (within the capability of the unit) FIRST. Within the crossover, all three stage speakers should have the exact same settings. I have found that the "default" settings are often not correct or as good as they can be, particularly for non-QSC speakers (they are just going to put in what the other manufacturers supply them with and not all DSPs process the same so what settings apply to one DSP will likely need to be altered in a different DSP.
Then look at your speaker. You likely have something like the JBL 4632 or 4732. It need a LOT boost and tuning on its high frequencies. This is the response that the DCM-30D should be applying to a JBL 4632:
There is a LOT of boost need at 13KHz.
This dipicts a 2-way crossover and note that the HF section is -6dB yet +5dB around 13K or an 11dB boost. You'll probably hear that.
Even wired as a 3-way, the 4632 needs a serious treble boost 12dB or so by 13KHz. (about 3-4 dB/Oct) from crossover (1.2KHz) all of the way up.
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