quote: Steve Guttag

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That is generally due to the gain structure not being set up well.

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quote: Steve Guttag

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Where does Dolby state that

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Here's what it says on page 4-3 of the CP650 Installation Manual:

"Set all the gain controls on all power amplifiers to a known repeatable setting. The preferred setting for most amplifier gain controls is maximum.
If a different setting is required in order to optimize the noise performance of the system, the controls should be locked in position or marked clearly." (Emphasis is theirs.)

Their preferred way didn't quite work out, so now I'll ask the tech to go with Plan B. I sent an email already so he's aware -- coz he's planning to leave tomorrow.
 

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Posted by Mark Gulbrandsen (Member # 72) on 11-03-2010, 11:04 PM:
 
If memory serves me correctly Dolby wants to see 750 mv RMS at the output with volume set to 7. Adjust your amps gain accordingly then depending of course on theor actual gain.
If I am wrong then perhaps Sam could chime in on this...

Mark
 

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Posted by Tony Bandiera Jr (Member # 2365) on 11-03-2010, 11:19 PM:
 
Manny, I still would like answers to my questions, because at UC Irvine I have my CP-650 in HIB operating with the amps wide open and don't have a hiss problem. (MxA Series amps stage and sub, USA 900 for surrounds).

Did this come on suddenly? Or was it always there?
 

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Posted by Manny Knowles (Member # 1171) on 11-03-2010, 11:35 PM:
 
Tony

I only noticed it this evening but it's a construction zone so it may have been masked by all the noise. Or maybe I just never noticed it before because I wasn't looking for it.

Today our grand drape was being installed and I was having a conversation with the rigger so I was standing right on the stage. The tech came in to talk to me and waited in the 2nd row for me to finish my convo with the rigger. I looked over to him and asked "are you hearing this hissing?" He couldn't hear it from there.

So a little later I was still on stage with the rigger and my boss came into the theatre and -- standing in the 2nd row -- asked "is that amp hiss?" I went down to where he was and, sure enough, I could still hear it. It really is THAT loud. But by then our tech was gone for the day. That's why I came on here to ask the question. I needed to know if I was being unrealistic in my expectations.

Dolby DMA8 Plus --> CP650 --> QSC DCP300 --> QSC Amps (various) --> QSC 4-way loudspeakers
 

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Posted by Tony Bandiera Jr (Member # 2365) on 11-04-2010, 12:25 AM:
 
Ah,ha. the signal chain you described brings the next questions:

Does changing modes on the DCP-300 make the hiss go away? Adjusting the fader on the DCP-300, same question?
Turning off the CP-650, any effect? Fader on CP-650, any effect?

Since you essentially have two processors in the chain you need to isolate which one affects the hiss.

Odds are though that it is a gain settings issue as Steve correctly surmised.
 

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Posted by Steve Guttag (Member # 268) on 11-04-2010, 06:49 AM:
 
Please also note, the signal is in the digital domain all of the way until the output of the DCP300...the CP650 has the Cat 778 card so the output is also digital. There is only one D/A stage.

There were instructions on how the set the gain structure...they don't appear to have been followed.
 

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Posted by Manny Knowles (Member # 1171) on 11-04-2010, 03:14 PM:
 
Problem appears to be associated with DMA8 Plus.

Goes away when other formats are selected on CP650, or when the DMA8 unit is disconnected.

DMA8 is connected to external input (Format 11) Analog?
 

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Posted by Louis Bornwasser (Member # 3063) on 11-06-2010, 05:55 AM:
 
REmove snake from H. F. driver. Louis
 

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Posted by Tony Bandiera Jr (Member # 2365) on 11-08-2010, 01:01 PM:
 
Good one Louis.

Manny, the obvious answer is the DMA8 plus is bad..have it repaired or exchanged by Dolby.

Like the CP-750s I had a problem with a while back...HERE

I think Dolby got a bad lot of A/D or D/A converters recently.
 

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Posted by Manny Knowles (Member # 1171) on 11-08-2010, 02:27 PM:
 
Why the heck doesn't the DMA8 have a digital output???
 

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Posted by Steve Guttag (Member # 268) on 11-08-2010, 08:15 PM:
 
Because the DMA stands for Digital Media Adapter. Those that don't need Digital audio decoding, don't "need" it. You'll find that the vast majority of systems that use the DMA8 want it to have analog out.
 

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Posted by Manny Knowles (Member # 1171) on 11-08-2010, 08:31 PM:
 
Don't defend them, Steve!

It's still a shortcoming -- or as they say, a "feature."

A digital output would make the DMA that much more valuable, since it could then also be used as a switcher for those who have several digital sources, and only one digital input on their processor...
 

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Posted by Steve Guttag (Member # 268) on 11-09-2010, 08:05 AM:
 
It is not a matter of defending...it is a matter of fact! The DMA8's purpose is to give legacy products a means to play digital media. As such, there would have been little value to having a digital out.

Now there may be a growing desire to have a digital out processor as "legacy" products turn into "obsolete" products.

Personally, before I would worry about digital out, I'd like it if it also De-embedded audio from HD/SDI streams too. The original DMA8 had that option for SD-SDI...the Plus does not.

Ultimately, for the foreseeable future, one has to get audio into analog to finish the job. It is much more recently than the DMA8 that processors have digital audio inputs...and most only have one, for cinema, and it is taken up by the DCinema system.

Steve
 

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Posted by Manny Knowles (Member # 1171) on 11-09-2010, 06:48 PM:
 
Our setup and test screenings went well today and it looks like we're really gonna show a movie here tomorrow and Thursday.

The 4K audio connects via the digital input so there won't be any of this noise at all for the super-important show.

For Thursday, we'll just wait until the movie actually hits the screen before selecting Format 11.

Once these "preview" screenings are safely behind us, we can look more closely at gain structure.

Stay tuned!
 

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Posted by Tony Bandiera Jr (Member # 2365) on 11-14-2010, 11:22 PM:
 
So how did the screenings go?

And if the DMA8 has the bad hiss you described, did you get a replacement? All the DMA8's I have installed so far have been quiet.
 

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Posted by Eric Robinson (Member # 2966) on 01-06-2011, 02:22 PM:
 
I just read this White Paper on Gain Structure. While it doesn't apply directly to Cinema equipment, the concepts and methodology is explained clearly and easily so that the concepts could be applied with some studious effort.

Written by: Chuck McGregor

Check it out!!

Advanced System Gain Structure - PDF

Hopefully I'm not violating policies here by not cutting and pasting the 15 page document.

It would be a great addition to the WAREHOUSE >> INSTRUCTIONAL DOCUMENTS section, if the original author permits it.
 

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Posted by Steve Guttag (Member # 268) on 01-07-2011, 05:35 AM:
 
Here is an excellent document on the subject too from RANE

Note, RANE has many such good documents. They also have a pretty decent sense of humor (including in their manuals) so the read is normally not bad.

-Steve
 

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Posted by Eric Robinson (Member # 2966) on 01-07-2011, 01:54 PM:
 
From Rane White Paper - Setting Sound System Level Controls
URL of White Paper is above in Steve's post.

quote:

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Music's wide crest factor of 4-10 translates into 12-20 dB. This means that musical peaks occur 12-20 dB higher than the "average" value. This is why headroom is so important. You need 12-20 dB of headroom in each unit to avoid clipping.

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