Film-Tech Forum ARCHIVE: Dolby CP750 Surround Noise floor

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Posted by Tony Bandiera Jr (Member # 2365) on 09-07-2009, 04:38 PM:

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.

Posted by Mark Gulbrandsen (Member # 72) on 09-07-2009, 06:14 PM:

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

Posted by Steve Guttag (Member # 268) on 09-07-2009, 09:41 PM:

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

Posted by Tony Bandiera Jr (Member # 2365) on 09-07-2009, 10:33 PM:

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.)

Posted by Mark Gulbrandsen (Member # 72) on 09-07-2009, 11:33 PM:

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

Posted by Demetris Thoupis (Member # 833) on 09-07-2009, 11:42 PM:

Or swap the output and route the surrounds to the fronts and fronts to surrounds to see if it follows.

Posted by Brendan Penny (Member # 5041) on 09-08-2009, 04:37 AM:

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

Posted by Demetris Thoupis (Member # 833) on 09-08-2009, 07:02 AM:

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

Posted by Steve Guttag (Member # 268) on 09-08-2009, 09:16 AM:

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

Posted by Tony Bandiera Jr (Member # 2365) on 09-08-2009, 11:09 PM:

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.

Posted by Steve Guttag (Member # 268) on 09-09-2009, 12:33 AM:

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

Posted by Jarret Chessell (Member # 5077) on 09-09-2009, 02:08 PM:

Tony,

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

Posted by Demetris Thoupis (Member # 833) on 09-09-2009, 03:07 PM:

I've found the CP65 to be the most silent of them all.

Posted by Steve Guttag (Member # 268) on 09-09-2009, 03:13 PM:

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

Posted by Demetris Thoupis (Member # 833) on 09-09-2009, 03:24 PM:

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

Posted by Marco Giustini (Member # 4544) on 09-09-2009, 05:47 PM:

Is there a reason, besides money, because a 2009 (but also a 2000) 10k euro processor must be so noisy? I remember doing tests on one of the 650 of my theater (software 2.2.sth), I could easily listen the noise floor disappearing from surrounds when pushing the "mute" button.

It seems quite inadequate to me. What do you reckon?

Posted by Tony Bandiera Jr (Member # 2365) on 09-12-2009, 12:29 AM:

quote:
Tony,

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

Jarret, It would take someone like Zac Effron sleeping with me to give up my CP-65!! [Smile]

(That and a good amount of money.) It is a fully loaded 70mm ready model. IF it does get retired from my home room it will be safely preserved for some future use.

I will have another CP-65 available soon, not as loaded as mine but in excellent condition (it's being pulled for a FILM digital upgrade to a CP-650.) I could cut you a good deal for it. [Smile]

OTOH, if I don't like it, there will be a CP-500 up for grabs soon. [Smile]

Marco:

What are the gains on your amps set at? If they are wide open, it is no surprise that they are noticibly noisy.

My guess as to why the surrounds have the higher noise floor has to do with the delay line used in the surrounds..my CP-65 is ever so slightly hissy in the surrounds as compared to the stage channels (I have a CAT150F card in it.)

But NONE of the processors I have ever run into before are as bad as these CP-750's have been.

I am going back there Monday and will report on any changes, but again time constraints may prevent the channel swap test.

Posted by Jarret Chessell (Member # 5077) on 09-12-2009, 12:38 AM:

Sleep with Zac Effron and I get a free loaded CP-65?? I'll talk to Phil, I hear he's got an in to make that happen.

It's looking more and more like we'll be replacing the Smart Mod 8 with our CP-55, probably adding a 441 and 222SR/A. If only popcorn machines could make money... and Zac Effron's.

Posted by John Hawkinson (Member # 1135) on 09-12-2009, 09:17 AM:

OK, Tony, let's get quantitative. How bad is it really?

Did you look at it on a scope?

--jhawk

Posted by Tony Bandiera Jr (Member # 2365) on 09-12-2009, 01:31 PM:

John, since this was a d-cinema only install I did not bring my scope to the site. But I can say with confidence that the noise from the surrounds was AT LEAST 20-30db or more above the noise floor of the stage channels. (Which were for all perceptions dead quiet.) It was loud enough to make normal talking in that room very difficult to understand. Or to put it another way, it was audible behind the THX baffle wall behind the screen. [Smile]

This was not in any way a mild "standby" hiss.

Posted by Mark Gulbrandsen (Member # 72) on 09-13-2009, 06:09 PM:

Tony,

I asked a local tech that has several 750s in their chain and he says that you probably have a defective unit. He has no such noise level in his surround channels.

Mark

Posted by Tony Bandiera Jr (Member # 2365) on 09-14-2009, 09:57 PM:

Thanks Mark, that confirms my suspicions. That makes Dolby 0 for 2 on this job, I'll talk to my "boss" to get yet another unit.