Film-Tech Forum ARCHIVE: Optical Subwoofer at 100Hz Bandwidth Pop

This is topic Optical Subwoofer at 100Hz Bandwidth Pop in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Danial Simmonds (Member # 4638) on 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..

Posted by Steve Guttag (Member # 268) on 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

Posted by Danial Simmonds (Member # 4638) on 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.

Posted by Tony Bandiera Jr (Member # 2365) on 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. [Big Grin]

Posted by Demetris Thoupis (Member # 833) on 03-16-2010, 03:23 PM:

As my father always says, if something is working DON'T touch it [Smile]

Posted by Tony Bandiera Jr (Member # 2365) on 03-16-2010, 03:30 PM:

Exactly!!

Posted by Sean McKinnon (Member # 612) on 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?

Posted by Danial Simmonds (Member # 4638) on 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..

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Sean McKinnon (Member # 612) on 03-18-2010, 02:24 PM:

Ahhh... if thats's the case then that makes sense now Tony!