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» Film-Tech Forum ARCHIVE   » Operations   » Film Handlers' Forum   » SMART afterburner anomaly.

   
Author Topic: SMART afterburner anomaly.
Jason Miller
Expert Film Handler

Posts: 241
From: Little Rock, AR,
Registered: Mar 2004


 - posted 04-06-2004 08:27 PM      Profile for Jason Miller     Send New Private Message       Edit/Delete Post 
Has anyone ran across an afterburner that added a radio station signal to the line? when switched on, the sub and the left surround would send a clear signal to the amps. I have turned it off for now.. but was wondering if anyone has ever ran into this? chokes didnt help either (in case you were gonna suggest it).

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99


 - posted 04-06-2004 09:10 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Never heard of that one but what processor/digital player is involved and where is it inserted in the signal flow
Probably a ground loop or open ground issue
There is an error in the Pinouts regarding the grounds on the output DB25 not all of the ones listed are actually connected to ground if memory serves

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Jason Miller
Expert Film Handler

Posts: 241
From: Little Rock, AR,
Registered: Mar 2004


 - posted 04-06-2004 09:16 PM      Profile for Jason Miller     Send New Private Message       Edit/Delete Post 
It has a Dolby CP-65 processor (with a DTS 6D digital connected to the external input).

the afterburner is between the processor and the amps.

the weird this is.. this radio signal only started appearing on the channels within the last 2weeks. The theatre didnt change anything. perhaps the radio station did.. but Im not sure what they could have done to cause it.

Im positive its the afterburner though. I will check grounds, but the afterburner doesnt use the DB25 connections, it uses terminal strips.

this wouldnt be so bad if the radio station played 80s rock... heh heh

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Jack Ondracek
Film God

Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002


 - posted 04-06-2004 10:30 PM      Profile for Jack Ondracek   Author's Homepage   Email Jack Ondracek   Send New Private Message       Edit/Delete Post 
Jason, does that station have its towers nearby?... just curious.

Try lifting the ground on the power going to the afterburner. You might also check (or ask Smart) to see if the power supply has any bypass caps on it.

Otherwise, I'd just start lifting grounds & lines until I found whether the problem is coming into the afterburner or being picked up within it.

RF is sneaky... good luck!

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Thomas Procyk
Phenomenal Film Handler

Posts: 1842
From: Royal Palm Beach, FL, USA
Registered: Feb 2002


 - posted 04-07-2004 12:21 AM      Profile for Thomas Procyk   Email Thomas Procyk   Send New Private Message       Edit/Delete Post 
Hmmm. This gets me thinking. I worked at a theater that used the "Afterburner" thingies and we got complaints about "music coming from the left side when there's nothing going on in the movie" from a customer. Upon investigation, it never happened again even after standing in the theater for 15 minutes, so we just chalked it up to a crazy customer.

Wonder if that's what happened? It was a new install so they were still tweaking stuff the first few weeks of opening. It never happened again.

=TMP=

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Oscar Neundorfer
Master Film Handler

Posts: 275
From: Senoia, GA
Registered: May 2000


 - posted 04-07-2004 07:00 AM      Profile for Oscar Neundorfer   Author's Homepage   Email Oscar Neundorfer   Send New Private Message       Edit/Delete Post 
Undesired radio reception such as you are getting usually comes from some non-linearity in the signal path which causes rectification and therefore detection of the radio broadcast with the result that the radio audio gets added to the desired signal. This non-linearity in many cases results from a poor connection such as a loose wire which has some type of corrosion. This can create a diode effect which detects the modulated radio signal.

Also, if a radio signal is strong enough, then the transistors in the IC's can be driven so hard that they overload, also causing a diode action.

Grounding is a big issue in these cases because rf signals can easily get into the electronic equipment due to poor or improper grounding. Shields on audio cables are important for this reason. They can help to keep rf energy from entering if properly used.

I have seen cases where conduit connections were poor and allowed rf to get rectified and somehow sneak into the audio path. When conduit is first installed, all is OK, but eventually the individual pieces of conduit are no longer well connected to each other.

I know of one theater here which has a 50kW AM transmitter out in the parking lot a couple of hundred feet away. I cannot personally verify this, but I was told that every piece of conduit was welded together, and anywhere it contacted other structural metal, it was welded to the metal at that point to eliminate degradation of connections over time. Believe me, the theater is in a VERY strong rf field.

So, check ALL connections, not just on the Afterburner, but throughout the entire rack for tightness. Look carefully at any crimp connections you might have as these can be especially troublesome. Also make absolutely certain that the rack AC ground is intact both at the rack end and at the distribution panel end. I have seen problems in situations where a ground wire coming into the rack was wire-nutted to other ground wires. This is a big no-no. Wire nuts do NOT make good ground connections. Look in the rack AC junction box if there is one, and make certain the AC ground wire is properly terminated to the rack and to the power distribution in the rack.

Try running a ground wire from the Afterburner output ground to the rack ground.

When it comes to rf interference, you need to experiment with all types of things. It can be very tricky to eliminate, but usually it can be dealt with.

If you have another Afterburner in another house, consider temporarily swapping it out to see if the problem goes away. SMART will be happy to send you another Afterburner on a repair/exchange basis if the Afterburner proves to be at fault.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002


 - posted 06-22-2004 08:22 AM      Profile for Michael Schaffer   Author's Homepage   Email Michael Schaffer   Send New Private Message       Edit/Delete Post 
How does a poor conduit connection allow RF to influence the audio signal? The signal doesn`t flow through the conduit connections, or what am I missing here?

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Jesse Skeen
Phenomenal Film Handler

Posts: 1517
From: Sacramento, CA
Registered: Aug 2000


 - posted 06-22-2004 01:17 PM      Profile for Jesse Skeen   Email Jesse Skeen   Send New Private Message       Edit/Delete Post 
Honestly, do these things actually do enough good to justify having them hooked up? I worked at a 16-screener that had these, and in most of the auditoriums they produced a random clicking from the left surround, later found to be caused by the platter motors. They caused background hissing in a couple screens too. As soon as I disconnected them, the sound in all theatres was 200% better!
Besides that, I don't believe in compressing dynamic range- if it's too loud, turn the volume down, but don't mess with the sound mix!

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Steve Guttag
We forgot the crackers Gromit!!!

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


 - posted 06-22-2004 04:16 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Michael, for good RF suppression, the shielding body needs to be grounded at both ends...unlike for audio supression where traditionally one-end is grounded. Often when one is trying to shield RF and not create ground loops...one end will be connected directly to ground and the other end connected to the ground via a capacitior. This will allow the RF noise to drain but block any low frequency grounding problems.

As to the need/desire for the Afterburner or other dynamic range altering device...

It is NOT a simple matter of turning the volume down or up. There is a practical dynamic range of a theatre. The lower limit is set by the ambient noise of the room with all sources contributed including air handlers and the audience itself. If the recorded sound is below this threshold, then it will be lost as if it was never recorded. Clearly this is not desired.

While better designs in HVAC systems can be done to reduce ambient noise, the audience factor is not as easy to contend with. We sell them popcorn and candy and drinks...those sounds should be tolerated and effectively raise a theatre's noise floor such that most NC-30 theatres never really achieve that once the people are in it unless it is a small screening room without refreshments.

I have long maintained that a better solution would be to generate a random steady-state noise floor to raise a theatre's noise level such that people noises are effectively masked...then either mix films to that level or compress the audio such that the quieter sounds are not lost in the noise.

At the other end of the spectrum are the high levels. Presumming you meet any of the digital soundtrack specifications...each stage speaker and surround channel can be playing up to 103dBc. Add to that the sub channel playing up to 10dB louder and then sum them together and you get a VERY loud theatre.

The limits to this are your system...can your system really handle that much? Next, can your auditorium really contain that much (in multiplexes). If your inter-theatre walls are not built to STC-75 or higher, then the answer is no. Typical theatre construction has demising walls that measure from FSTC-55 to FSTC-65. Odds are, if you play your system at spec, it can be heard in the adjacent auditorium. Note, I've measured walls that are even worse than that. If all you have is a double-stagger stud drywall with insullation between and a couple layers of gyp board on each side...you are maybe at STC-60 depending on other factors. If you want to get to desired STC-75 or better, there better be a masonry septum between the two theatres in addition to the two asymetrical wall systems.

Another important limiting factor on the upper end is the tolerence of the audience to the volume. If it is too loud, it is too loud. I've done enough studio screenings to know that they want the level just too loud for most patrons and even distinquished guests. I've been in the booth when a guest came to the booth screaming to turn it down whilst the studio reps held firm to the level they screened it at and refused a volume change.

So why can't you just turn the volume down? Because you have not changed the dynamic range. You just sent the low level signals into the noise floor and the dialogue to the point it is too soft. Dialogue should sound natural an comfortable. It is one reference point even the layperson can relate to. If you turn the volume down, you turn the dialogue down with it.

With a top-end compressor/limiter you can leave the dialogue where it belongs and only restrict the offensive high level (to the customer or from damaging equipment or coming through the walls).

This is not an endorsement of the SMART product but I do accept the concept of the device. If properly designed and properly installed, such a device could alliviate problems encounted in some film/theatre combinations.

I have been known to use a compressor and frequency band-pass filters on subwoofers only to keep the sub channel from pounding through a poorly designed theatre wall (theatre designed in the 70s...trying to cope with digital soundtracks). The result, you get the subwoofer in the viewing theatre but are not listening to it in the next theatre. Is this as good as having a good wall and tuning everything to spec, of course not but one has to work with the hand they are dealt until such time as the cards are re-dealt.

Steve

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99


 - posted 06-22-2004 08:59 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
If you are getting random noise from the unit it is usually the installation technique as with most extra devices

Ioan Allen once commented that after SR dolby needed to make the dolby popcorn NR system [Big Grin]

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John Anastasio
Master Film Handler

Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000


 - posted 06-23-2004 05:46 AM      Profile for John Anastasio   Author's Homepage   Email John Anastasio   Send New Private Message       Edit/Delete Post 
It's not hard to induce RFI almost anywhere you have the slightest corrosion in an audio circuit, especially if it's near an input. Remember, you're dealing with microvolts and the nearby 50Kw radio station can induce that very easily. It doesn't take that much power, either. There were plenty of stories back in the 1970s when Citizens Band Radio was all the rage of audiences or congregations suddenly being entertained by the colorful language of nearby truckers. Any wire can be an antenna.

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Jack Ondracek
Film God

Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002


 - posted 06-23-2004 11:56 AM      Profile for Jack Ondracek   Author's Homepage   Email Jack Ondracek   Send New Private Message       Edit/Delete Post 
I have one of those "poorly designed" buildings... 4 screens & I have Afterburners in all of them. Adjusted carefully, they don't kick in until the audio really does get loud. They basically work as Steve describes. If you're sensitive to such things, you can tell that a theatre with the Afterburner doesn't have the dynamic range that it would without it. But the sound is still clean, the dialog is above the noise floor and I have stopped worrying about bleed-through... it's gone.

Can't say about the surround clicking problem. I don't have it here... nor do I have any extra amplifier noise. Sounds like your installation might have had the amp pots all the way up, and the processor volume dropped to match. That would cause most of what you described... and is why most professional sound systems were (at least when I learned the art) designed around standard signal levels throughout.

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