my profile | my password | search | faq & rules | forum home

» Film-Tech Forum ARCHIVE   » Operations   » Film Handlers' Forum   » Optical Subwoofer at 100Hz Bandwidth Pop

Author Topic: Optical Subwoofer at 100Hz Bandwidth Pop Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 03-16-2010 06:38 AM                        Hi, I have came across a odd problem during a service visit to a cinema far away at a village.. Their configuration is Cinemeccanica Victoria 5 CP500D ITT GU1 Sound Monitor ITT 9000 Amplifier for L/C/R ITT 20000 Amplifier for LS/RS QSC DCA 3022 Amplifier for Subwoofer I noticed the optical subwoofer bandwidth was set to 50Hz, when i changed it to 100Hz the subwoofers produce popping noise in Dolby SR, A-Type and Mono. In Digital Format everything works fine. Could it be because there is no presence of a subwoofer filter like QSC SF-3? Anyone faced similar problems?? Hope to hear from you soon..  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 03-16-2010 07:56 AM                     It isn't the problem due to lack of SF-3. Normally, when one gets a cyclic "pop" it due to an oscillation. I suspect something is wrong with the optical subwoofer circuit. Steve  |  IP: Logged Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 03-16-2010 09:35 AM                        Thanks for the feedback Steve. By optical path do you mean the processor cards such as the CAT 683? This place has 6 screens, 2 screens are running on CP650 with DCM and DCA amplifiers. The other 4 screens same configuration as i mentioned in my first post, and all 4 are facing this problem when set to 100 Hz optical bandwidth. There is no crossover in the system that is one thing i am sure of.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-16-2010 03:13 PM                     A critical question is: Is the pop a cyclic constant thing like Steve suggested, or does it occur only on program material with a lot of bass content? If it's the first, then there is some sort of electronic issue or A-Chain problem. If it happens only on program with bass content, then the problem is simple: the sub is being overdriven and/or cannot handle bass in the above-50Hz range. Since you say it happens on all screens, that is the most likely thing happening here. Set the optical bandwidth back to 50 hz, do a B-chain and leave it alone. Another question: Why did you change it in the first place? Unless there is a serious response issue, there was no reason to play with it. Sometimes the best thing to do is leave things alone.  |  IP: Logged Demetris Thoupis Phenomenal Film Handler Posts: 1240 From: Aradippou, Larnaca, Cyprus Registered: Apr 2001 posted 03-16-2010 03:23 PM                     As my father always says, if something is working DON'T touch it  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-16-2010 03:30 PM                     Exactly!!  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 03-16-2010 07:19 PM                        quote: Danial Simmonds noticed the optical subwoofer bandwidth was set to 50Hz, when i changed it to 100Hz the subwoofers produce popping noise in Dolby SR, A-Type and Mono. In Digital Format everything works fine. quote: Tony Bandiera Jr the sub is being overdriven and/or cannot handle bass in the above-50Hz range. Why would the sub be able to handle the demands of digital sound but be overdriven in analog?  |  IP: Logged Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 03-17-2010 03:05 AM                        Well when doing B-Chain, on 50Hz the level of dB is only 72 ~ 75, and cant really feel the bass in it. Also the 2 other halls with CP650 are set in 100Hz and able to achieve 80dB (10dB less than digital sub level) The popping will only occur on program material with a lot of bass content when in 100Hz mode. However looking at the speaker specifications of KCS C118H mentions a crossover frequency 80 to 100 hz and frequency range 30 - 500hz..  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-17-2010 02:23 PM                     Sean, IIRC (I don't have time at the moment to look at the Dolby manuals) I seem to recall that Digital sub is cut off around 80hz anyway. I'll look it up or I'm sure Sam or Steve will chime in. It is also possible that the level is not set correctly for digital mode. Danial, How big are the auditoriums? A single 18" sub is not gonna get you proper levels in anything but the tiniest of rooms. (No bigger than 50-100 seats MAX.) It is highly possible that your drivers in the subs are blown out. Did you check them for condition? In any event, the root cause of this is possibly undersized subs for the room, combined with changing settings that should have been (with the equipment on hand) left alone. You best option, if they cannot afford to add subs, is to go back, check and replace any blown drivers, and set the levels to achieve best possible output without overdriving the subs. And leave the optical crossover set to 50 hz.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 03-18-2010 02:24 PM                        Ahhh... if thats's the case then that makes sense now Tony!  |  IP: Logged

All times are Central (GMT -6:00)

Printer-friendly view of this topic

Hop To: Select a Forum: Category: Operations -------------------- Digital Cinema Forum Film Handlers' Forum Large Format Forum Ground Level Feature Info, Trailer Attachments & REAL Credit Offsets Category: Community -------------------- Film-Yak Film Handlers' Movie Reviews The Afterlife Bob Maar's Joke-A-Thon Category: Classified -------------------- Equipment and Supplies Wanted/For Sale Help Wanted/Seeking Employment Category: System -------------------- Software Beta Testing Forum

The Film-Tech Forums are designed for various members related to the cinema industry to express their opinions, viewpoints and testimonials on various products, services and events based upon speculation, personal knowledge and factual information through use, therefore all views represented here allow no liability upon the publishers of this web site and the owners of said views assume no liability for any ill will resulting from these postings. The posts made here are for educational as well as entertainment purposes and as such anyone viewing this portion of the website must accept these views as statements of the author of that opinion and agrees to release the authors from any and all liability.