Film-Tech Forum ARCHIVE: Farting Sound in Analog

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Posted by Manny Knowles (Member # 1171) on 09-24-2003, 12:17 PM:

One of my theatres has developed a farting sound problem in analog.

The offending noise is similar to hearing sprokets or DTS code and also the motorboating sound due to an unsecured impedence (damper) roller.

Actually, though, this noise isn't constant and it doesn't seem to be rhythmic. My first impression of it is that it is kind-of a random noise but I suspect that there will ultimately be a rhyme-and-reason once the cause is tracked down.

The noise affectly only the analog playback part of the system. Digital sound is fine -- luckily we have a DTS disc for this feature -- and the Non-Sync is fine, too.

(Something I have not yet had a chance to try is playing the film in another theatre. It could be print damage. What are the odds?)

The sound head is a Kelmar LED reverse-scan, using tension rollers before and after the "drum" roller. I've known those to lose tension when a bad splice is encountered but it never caused this kind of noise before. Still, I suppose it could be that.

I checked the lateral alignment to make sure that it wasn't sprockets and it checks out okay -- I didn't really think it was that anyway. In fact, a full alignment revealed only one problem -- a dying LED but I've never known that to cause this kind of noise.

I've no problem doing another A-chain if there is anything you think I should pay particular attention to.

I'm all ears... [thumbsup]

Posted by Paul G. Thompson (Member # 655) on 09-24-2003, 12:38 PM:

Look for an alignment problem or a "stuck" lateral guide roller. I don't know what kind of soundheads you have. If they are Century soundheads, the pivot for the floating rollers may have dislodged itself or are otherwise loose.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-24-2003, 01:57 PM:

Paul covered the mechanical stuff, so...

Does the 'noise' occur when the projector isn't running (with the LED on) and with the sound system set to analog?
Exactly how dead is this dying LED?
How's the LED's power supply?
What CP are you using?

Posted by Manny Knowles (Member # 1171) on 09-24-2003, 03:02 PM:

Paul -- thanks! I will check when I go back there. It's a Westar/Century (R3) soundhead with the Kelmar LED reverse-scan retrofit.

quote:
1. Does the 'noise' occur when the projector isn't running (with the LED on) and with the sound system set to analog?
2. Exactly how dead is this dying LED?
3. How's the LED's power supply?
4. What CP are you using?

Excellent questions, Daryl.

1. I didn't think to check that. Will let you know what happens.
2. Both the exciter (power supply) and preamp (in the processor) are cranked all the way up. I get only the low red LED with Dolby tone. I ordered LED's a few weeks back with some other stuff but that order hasn't come in yet so I can't do a change-out just now.
3. I can check the power supply tomorrow. I've had one go south on me before and what happened then was that the solder on the connector had become brittle. I lost sound intermittently. It was fixed by resoldering the connector to the board.
4. This house has a CP45 processor.

Oh! By the way...this processor has a dead power supply. It was running on backup about two weeks ago. I moved the backup power supply to the main supply (they are identical power packs) and got it working again in normal mode. A replacement power pack is on order. What are the chances that there's a bigger problem than a dead power pack? What should I look for?

Posted by John Anastasio (Member # 607) on 09-24-2003, 06:05 PM:

I've heard the "motorboating" many times in amplifiers with dying power supplies or battery powered equipment with bad or dying cells that can't supply enough current, so they do what you've described....make a farting noise. Since you mentioned the power supply problem, I thought I'd throw that in.

Posted by Randy Stankey (Member # 64) on 09-24-2003, 11:43 PM:

I can imagine what your farting noise sounds like but is it more like the sound you hear in movies and on television when, for instance, the starship Enterprise is going out of communications range of a distant planet? More of a distorted buzzing sound that resembles the "intended" signal?

If so, I'd definately say that your cell/CCD isn't getting enough signal out to the preamp. Check your LED supply and/or the supply to the CCD reader. (Often they are one unit, nowadays.) After that check the power supply in the processor to see that the preamp is getting enough voltage.

One way or another, I'd say you aren't getting enough gain from the incomming (to the processor) signal.

Posted by William T. Parr (Member # 677) on 09-25-2003, 12:02 AM:

You know the smart ass in me just can't help resist but to ask if it is a Dry Fart or a Wet Fart sound? [Wink]

Posted by Brad Miller (Member # 2) on 09-25-2003, 04:05 AM:

quote:
2. Both the exciter (power supply) and preamp (in the processor) are cranked all the way up. I get only the low red LED with Dolby tone. I ordered LED's a few weeks back with some other stuff but that order hasn't come in yet so I can't do a change-out just now.

Don't discount the led as being bad just yet. Have you tried to adjust the led's position during an A-chain? It amazes me how few people bother to do this and end up overdriving their leds (thus sending them to an early grave) instead of properly aligning the led to achieve maximum output in the first place. [Roll Eyes]

(No I am not accusing you of being "one of those people", but it's worth pointing out just to be sure that you have adjusted the led as well as for when you put the new one in.)

Posted by Manny Knowles (Member # 1171) on 09-25-2003, 08:54 AM:

Yep -- that was one of the very first things I did when I noticed I had run out of gain. [thumbsup]

Posted by Josh Jones (Member # 447) on 09-25-2003, 03:25 PM:

It also could be that some of the power supply filtration in the preamp card could have gone bad, so when you crank the gain pots wide open, the preamp begins to oscillate. This was more of a problem in tube stuff, but then againwe had an FM processor die this week that exhibited this same kind of noise.

Try and back the preamp gain down, and if the frequency of the motorboating changes, or suddenly goes away, this could be it

Hope that helps,

Josh

Posted by Bob Maar (Member # 763) on 09-25-2003, 03:51 PM:

This problem really stinks..... [bs]

Posted by Manny Knowles (Member # 1171) on 09-26-2003, 04:26 PM:

Tried one of Daryl's suggestions -- listen in SR mode with no program playing.

Everything was cool. No problem.

Played some Dolby Tone. Everything seemed okay.

Removed the Dolby Tone loop and -- presto -- the farting noise all by itself! It was a rhythmic thump-thump-thump noise, getting gradually slower...

...turns out there was a nasty glob of dirt on the edge of the impedence roller and as it passed the LED it would make a THUMP! sound.

I went into full cleaning mode and then did an A-chain checkout when I was done (in case I bumped something out of alignment). Everything's fine and the noise is gone.

Thanks to all of you for the many suggestions. [Cool]

Congratulations to Daryl for coming up with the winning answer! You can collect your Film-Tech tee shirt from Brad. [Wink]

Posted by Brad Miller (Member # 2) on 09-26-2003, 04:32 PM:

Manny, did you check to make sure the sound drum itself does not wander laterally? If it does, then your problem could come back.

I have seen this happen on Dolby digital penthouse readers when the installing tech forgets to put the spring on the shaft!

By the way, I ran out of shirts, so the one I was sending to you today I will just send to Daryl. [Razz]

Posted by Manny Knowles (Member # 1171) on 09-26-2003, 04:38 PM:

My shirt!!!! NOOOooooo!!!

I actually removed the LED bracket removed the drum completely so as to get back there for a full cleaning of the casting.

Later, when I put the drum back in, I fastened the retaining collar down three different times until the tension was how I usually like it.

Still...If the problem comes back I will remember your post and that would then be one of the first things to check...and then we'd have to figure out what to give you instead of a tee-shirt. Haha.