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Author Topic: CP55 popping during format changes Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 11-16-2003 01:51 PM                        This phenomena occurs during switching all formats, the optical changes are worse than the rest, but still noticable. Any ideas? Josh  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 11-16-2003 01:59 PM                     Seems as I had that problem once in Southsound. It has been so long ago I can't remember for sure exactly what it was. However, the format card (Cat 243) comes to mind. [ 11-16-2003, 08:23 PM: Message edited by: Paul G. Thompson ]  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 11-16-2003 09:29 PM                        any other ideas? JJ  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 11-16-2003 09:47 PM                        Josh... (unless someone chimes in here...) there's probably not much you'll be able to accomplish in there... unless you can get schematics of the cards... If the 55 switches between source opamps with unbalanced offsets, you can get that... dried-up caps... switching between outputs with mismatched impedances all can do this... not to mention the possibility the switching pulses might be inducing themselves into a gain stage somewhere in there (maybe bad caps here, too). Probably nothing you couldn't trace out... if you could get extenders & schemats... On the other hand that call to Dolby did the trick for me...  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 11-17-2003 07:15 PM                        Ok, so now what. This problem becomes really annoying when SRD is reverting to SR and back again. any other ideas? Josh  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 11-17-2003 09:35 PM                     Josh, I spoke to a tech today about your problem. He said it is normal for some older units to do that. We do have some good options to get it taken care of.  |  IP: Logged Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 11-17-2003 10:29 PM                     Can you describe the "pop" in more detail? (I've never had a CP55 produce any noise when switching formats.)  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 11-17-2003 10:59 PM                     My experience with popping noises mentioned is similar to relay contact closure noise that is emitted into the audio. That's what I experienced in Port Orchard. It was low-key, but it is still there. And Brad, I agree with you. It should not be there, period! Maybe Josh can more accurately describe what it sounds like. More than likely, I'll call Dolby Labs tomorrow. Another description is of an electro-mechanical automation unit cycling if the wiring ground interface is not that great. As you sit in an auditorium, every relay closure can be heard in the sound system. One question I forgot to ask.... Is it in all channels, or just some? That might shed some more light on the problem. Seve Guttag, where are you??  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 11-17-2003 11:17 PM                        Josh/Paul... If the box is prone to this (although maybe raising input levels might help?)... does it have the same mute function that the 65 has?... and could that be used between source switching?  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 11-17-2003 11:20 PM                     Jack, that is a very valid thought. Somehow, that slipped past my feeble mind. I wonder if it could be a gain structure problem.  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 11-18-2003 01:59 AM                        The sound is more of a snapping noise, not so much of a popping. JJ  |  IP: Logged Daryl C. W. O'Shea Film God Posts: 3977 From: Midland Ontario Canada (where Panavision & IMAX lenses come from) Registered: Jun 2002 posted 11-18-2003 05:38 AM                        quote: Maybe Josh can more accurately describe what it sounds like. More than likely, I'll call Dolby Labs tomorrow. Wouldn't it make more sense for Josh to call Dolby himself, if in fact he wants Dolby's help. As you said, Josh can more accurately describe what it sounds like. I'm sure he's perfectly capable of doing it.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-18-2003 06:28 AM                     Every....and I mean EVERY Cat. 242 produces a snapping noise as it switches between formats. It is generated in the actual analog muxes...the CD4051 (I can't remember the exact number...there are three chips in the series...one of them is an 8x1 mux and that is what is used on the Cat. 242). Some are worse than others but they all do it whether you have noticed it or not. It is a "Feature" of the card. In fact, "Dolby Snaps" are kinda of an inside joke for those in the know... For the Cat 240 and 240A though they used the same series of chip (CD4053) for projector changover...to kill the "snap" they did a form of feedback kill it off...thereby admitting that they knew of the problem. For the format card, the Cat. 242 it was not considered an issue since it would normally be switching between two sound sources and once the feature had started would remain where it was. The flaw in the plan came when the CP-65 came out and you have a DA10/20 connected and the snaps would "announce" a digital change. Dolby may have run out of real estate to put in the feedback circuits for the format card since they used all "through-hole" technology...with SMT of today...it could be more effectively addressed...not to mention there are better MUXes out there. Note on the Cat. 441 that the DG series of analog muxes were being used. If the Cat. 242 were being designed today as an analog card...probably a MUX like the DG400 series would have been given the nod...it doesn't snap as bad in its natural state and could also have its snaps severely reduced. Steve  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 11-18-2003 09:02 AM                        instead of using muxes, why didnt the build discrete VCA's to control the audio path? That would have been quiet and reliable. It sounds like a nickname, it sounds like a snack, Dolby Snaps!! Josh  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-18-2003 11:49 AM                     Previously, Dolby did use descrete FETs to do analog muxing. You don't need a true VCA persey...just bias the Gate of the FET and as you switch change the gate voltage using a simple RC circuit to control the time-constant for a cross fade. For a study guide...refer to Dolby's Cat. 108 and Cat. 201. As to the descrete thing...for a cinema application...you would consume way too much real estate for what you are doing. Remember, in a CP-55 and earlier processor, you PC Card was limited to a very definate dimension to work with. You can't just add space without adding a card (like the Cat 150D and earlier)...then you bring with you all the challenges of moving the signals around between the two...for the Cat. 150 it wasn't too bad since only one channel was on the second card. Steve  |  IP: Logged

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