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Author Topic: Dolby CP750 Surround Noise floor
Tony Bandiera Jr
Film God

Posts: 3067
From: Moreland Idaho
Registered: Apr 2004


 - posted 09-07-2009 04:38 PM      Profile for Tony Bandiera Jr   Email Tony Bandiera Jr   Send New Private Message       Edit/Delete Post 
Well, the fun with D-Cinema continues..at another client's house while doing a D-Cinema upgrade, the client opted for the cheaper CP-750 instead of a CP-650.

The fun began when the first processor , when turned on and idle, had MASSIVE "pink noise" hissing in the surrounds..didn't matter what the fader setting was, what input mode or even if it was muted. It also would, while in alignment mode, output pink noise to BOTH surround channels (and bleed heavily into the front channels) when Either of the surround's PN was toggled on.

To Dolby's credit, they rushed an advance replacement to me at the jobsite, and although the alignment issues were fixed, the Surrounds (and ONLY the surrounds) STILL had a very high noise floor! I ultimately had to reduce the gain (a lot) on the surround amp and crank up the output from the processor to get rid of the excessive idle hiss. The stage channels all worked fine with the original amp gain settings (3/4 to full).

So I'm left to wonder if I've gotten another bad 750 that will fail soon or is that high noise floor on the surrounds an aftereffect of the cost cutting to make a CP-750?

I have basically stopped recommending CP-750's for private screening rooms at this point, where the standby noise is always very noticable. NONE of the 24+ CP-650's I have installed in private rooms have given me this trouble.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99


 - posted 09-07-2009 06:14 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
quote: Tony Bandiera Jr
I ultimately had to reduce the gain (a lot) on the surround amp and crank up the output from the processor to get rid of the excessive idle hiss
Althougyh I fel you still have a problem there with the unit to acheive the best dynamic range in a given system you should be doing this anyway. Dolby's like to operate at a certain level and that usually requires cutting back the amp gain. Other processors are similar. It's entirely possible to get a USL JS series processor dead quiet if one does this correctly. Seems that very few techs really care about this aspect. They just slap it in and go do the next one.

Mark

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Steve Guttag
We forgot the crackers Gromit!!!

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


 - posted 09-07-2009 09:41 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Yeah...uh...

I'm pretty emphatic about getting the lowest noise floor by getting my gain structures right...I've worked with the JS(D) series...never seen them dead quiet...EVER. Then again, I've never seen Dolbys dead quiet either though quieter than USL stuff. The stuff is normally adequate for the typical cinema where the noise floor is enough above the cinema processor to make it a non-issue. Add to that preshow audio and patron noise and the processor noise becomes even more transparent.

Now in screening rooms...that is another story. The speakers may be less than 10-feet from the listener...any noise will be very apparent.

Steve

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Tony Bandiera Jr
Film God

Posts: 3067
From: Moreland Idaho
Registered: Apr 2004


 - posted 09-07-2009 10:33 PM      Profile for Tony Bandiera Jr   Email Tony Bandiera Jr   Send New Private Message       Edit/Delete Post 
quote: Mark G
<snip> Althougyh I fel you still have a problem there with the unit
Mark:

That's my thought as well...and I must confess that in my early days as a tech I was firmly of the school of "amps wide open", mainly to prevent the kids in the cinema from being able to turn the amps up playing around. I have since changed my ways and become more deliberate in how I set up gains to minimize noise. Which usually, especially in my private rooms, results in amps around half open and the Dolby in the upper third of it's output settings. (A general statement, actual mileage may vary.)

Steve: I agree with the differences between Dolby and USL, that has been my experience too. I do like certain aspects of both brands design and will install the best one for the job.

And you are absolutely right, the noise floor of a cinema is usually high enough to mask any standby hiss from a processor, and home screening rooms are very critical in that respect.

I currently have a CP-65 in my home system (see my avatar) but am picking up a CP-500 to replace it with. [Smile] I have never, in all the years that I have worked with Dolby's stuff, ever had this much of a problem with noise.

I did not use my SPL meter to measure the level of that noise issue, but I would make a WAG that it was easily 20-30 db above the very silent stage channels!! Again, something I have never seen from any Dolby product.

If anyone is installing a CP-750 anytime soon, could you be so kind as to do a simple test?

After connecting the amps, turn all amp's gain wide open, and with the CP-750 idle (NOT in the Install mode) see if your surrounds are noisier that the stage and if so, by how much?

I just want to know if this is something that is part of the CP-750 cost-cutting design or if I have somehow gotten a second bum unit from Dolby (which would be a first for me.)

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99


 - posted 09-07-2009 11:33 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
The good thing is that older QSC's and some BGW's had the controls on the rear panel and now they are under a secret cover.

It is possible... but rather remotely... your surround channels may be experiencing parasitic oscillation. This can sound like poor S/N or a hiss in a given channel if this happens. You might want to throw a scope on the amp output to distinguish if it is indeed random noise or an oscillation.

Mark

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001


 - posted 09-07-2009 11:42 PM      Profile for Demetris Thoupis   Email Demetris Thoupis   Send New Private Message       Edit/Delete Post 
Or swap the output and route the surrounds to the fronts and fronts to surrounds to see if it follows.

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Brendan Penny
Expert Film Handler

Posts: 121
From: Bundoora, Australia
Registered: Dec 2008


 - posted 09-08-2009 04:37 AM      Profile for Brendan Penny   Email Brendan Penny   Send New Private Message       Edit/Delete Post 
Hey Demetris,

Maybe it will work better if we put a Kinoton sticker over the Dolby one and change the housing a bit it may work better??

Still waiting on my pix buddy??? [Wink]

P.s Don't get upset, I'm just messing with you. I really do want to see these pix though

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001


 - posted 09-08-2009 07:02 AM      Profile for Demetris Thoupis   Email Demetris Thoupis   Send New Private Message       Edit/Delete Post 
Brendan I did not know you were in manufacturing of the MI system (or so it seems). You could easily incorporate the MI filter wheel as an anamorphic attachment with a bit of design. There aren't a lot of projectors out there to make bases based on each one. Hang on. I am sorry but I cannot yet.
Demetris

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Steve Guttag
We forgot the crackers Gromit!!!

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


 - posted 09-08-2009 09:16 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Tony...watch out...the CP500 is NOT a quiet processor, by any stretch.

If you want to see/hear how quiet a cinema processor should be, get your hands on a Pana CSP1200...No lie, it is about 18dB quieter. Also, doing an A-chain on one with a reverse scan reader is spooky good...both Azimuth and response.

Steve

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Tony Bandiera Jr
Film God

Posts: 3067
From: Moreland Idaho
Registered: Apr 2004


 - posted 09-08-2009 11:09 PM      Profile for Tony Bandiera Jr   Email Tony Bandiera Jr   Send New Private Message       Edit/Delete Post 
LOL Steve, I have been wondering if the CP-500 would be good enough for my room. I have in the past installed about a half-dozen in various private rooms and had no complaints or issues. But it'll be interesting to see, especially in my very small room here, how noisy it is,,I am expecting it will be noisier that the CP-65 I have now.

I won't know for a very long time though, as I am working three installs right now and believe me, when I get home the last thing I'm going to want to do is work on another install. [Razz]

quote: Demetris Thoupis
Or swap the output and route the surrounds to the fronts and fronts to surrounds to see if it follows.
Very good suggestion Demetris.. time constraints have prevented me from trying that right now, but it will happen if the trouble persists. So far my gain adjustments have worked out well..but I am not comfortable with that as a solution.

quote: Mark G
<snip>It is possible... but rather remotely... your surround channels may be experiencing parasitic oscillation. This can sound like poor S/N or a hiss in a given channel if this happens. You might want to throw a scope on the amp output to distinguish if it is indeed random noise or an oscillation.
I had considered that as a possibility, but the same amp and full open gain settings were working fine with the CP-200 we pulled out. Unplugging the DB-25 from the 750 stops the noise, and powering down the 750 with the DB25 connected also stops the noise, after an extended period of spurious sqweeks and squeals from the surround channels only. The stage channels don't make any noises on shutdown.

FYI: QSC MXa amps behind the screen, fed with 9451. Installed originally in 2002. Until the 750, no problems reported by client who uses the room a lot.

Anyone have any problems with the MXa series amps having oscillation issues? It is possible the amp may be goin south.

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Steve Guttag
We forgot the crackers Gromit!!!

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


 - posted 09-09-2009 12:33 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
About the only problem I've ever had with the MX series are the pots going noisy. Note, QSC offers "Renew" kits to change out the components that age. Far cheaper than a new amp and the MX amp was certainly a good one.

Steve

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Jarret Chessell
Master Film Handler

Posts: 288
From: London, Ontario, Canada
Registered: Jan 2009


 - posted 09-09-2009 02:08 PM      Profile for Jarret Chessell   Email Jarret Chessell   Send New Private Message       Edit/Delete Post 
Tony,

When you pull out your CP-65, I'd be willing to take it off your hands at no charge! [Big Grin]

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001


 - posted 09-09-2009 03:07 PM      Profile for Demetris Thoupis   Email Demetris Thoupis   Send New Private Message       Edit/Delete Post 
I've found the CP65 to be the most silent of them all.

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Steve Guttag
We forgot the crackers Gromit!!!

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


 - posted 09-09-2009 03:13 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Of the Dolbys?...I dunno. It is not a quiet processor in my book...definite hiss going on there. In fact, the studio version of the CP65 used a modified Cat 242 that replaced the TDA series VCA with "THAT Corp" VCAs like are on the Cat 441. So here is a case opposite to Tony's where the CP65 is inherently quieter on the surrounds!

As for as all Cinema processors go...nothing really is close to the Pana...it really is on the order of 18dB quieter...which is quite a bit.

Steve

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001


 - posted 09-09-2009 03:24 PM      Profile for Demetris Thoupis   Email Demetris Thoupis   Send New Private Message       Edit/Delete Post 
But they want a silly price for it and you need externally DA20 or DTS to have digital audio. Of course on the other hand, many do claim that the Pana has the best analogue reproduction of all. Even the CP200.
Demetris

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