Film-Tech Forum ARCHIVE: RF/FM interference on sound processor

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Posted by Luciano Brigite (Member # 1104) on 01-22-2006, 10:59 PM:

Hello,
I need some help in order to solve a problem regarding RF interference in a theatre. all the equipment is there for decades and this problem rarely happened however a week ago or so it started out of nowhere and everything tried with hte sound equipment didn't work. while projector is stopped, at least 2 radio stations can be heard in the auditorium and booth monitor.disconecting the wires from soundhead lowers the interference level but doesn't get rid of it. a new wiring was placed with each pair of wires covered with a aluminium foil plus the usual wire shield,it didn't work. Everything was checked, most of the sound rack re-wired and the interference is still there. Grounding was redone also with no results.
the only thing that lowered some of the interference was ferrite blocks placed at both ends of the cable from hte soundhead to processor,one having the cable passing tru it and the other ( the same ferrite block used with dolby digital reader cables) with both,ground wire and signal cable wound some 2~3 turns each around the block. I was considering more of these blocks on hte cable but I'm not sure if it'll help further or will only add mess in the sound rack. the problem is within hte soundhead,cabling to processor and the processor. already disconected the signal cables to power amps to make sure the problem isn't with those.
I'm lost with it now..

Posted by Marin Zorica (Member # 1716) on 01-23-2006, 04:12 AM:

HM...you probably get some radio station on medium wave, with AM modulation! I have problem like that, friend of mine have recording studio and about 3 miles from his location there is 1MW AM transmitter and on every microphone input when is ON i got radio hear!

This is sometimes very hard, or even impossible to solve if transmitter is close and EM field is high!

Try to put these ferrite block on begining and end of cables, also on every cable what is going in processor!! And off course check ground and earth connection! Probably some capacitors in parallel with solar cell input will make some help, about 25pF connected in parallel with input, not more because you may change sound quality (inside pre amp connected directly parallel with inputs between ground and every signal cord).

What kind of Sound equipment do you have?

Posted by Luciano Brigite (Member # 1104) on 01-24-2006, 04:40 PM:

The interference is FM,not AM or MW. and the processor is a CP50.

Posted by Richard May (Member # 2627) on 01-24-2006, 07:18 PM:

had the same problem with a cp 50. just try to keep grounding different things on the rack.

Posted by Marin Zorica (Member # 1716) on 01-24-2006, 07:47 PM:

quote: Luciano Brigite
The interference is FM,not AM or MW. and the processor is a CP50.

Strange how could some FM station do this, it's probably very close near you place.

Same thing like I described for AM!

The base of problem is i think input transformer in CP50, because in conjunction with solar cell is like radio detector!

Posted by Gordon McLeod (Member # 33) on 01-24-2006, 09:40 PM:

very unusal for a fm signal to be demodulated in this fashion

Posted by Louis Bornwasser (Member # 3063) on 01-25-2006, 01:15 PM:

Many years ago I did a studio premiere in Atlanta. The guy wire from WSB (50 Kw) touched the front sidewalk of the theatre. There was no RF on any mag or optical input because of good grounding practices. Louis

Posted by Paul Mayer (Member # 355) on 01-25-2006, 01:58 PM:

Might be time to go through the entire audio chain, breaking, cleaning, and re-making each connection. Pull, clean, and re-seat all the cards in the CP-50. Tighten all screws, even ones you wouldn't think would affect audio. I have seen things like slightly loose cover plate and grounding screws and slightly dirty connections act like capacitors or detectors at RF frequencies, yet audio frequencies don't seem to be affected. Just my $0.02 USD.

Posted by Luciano Brigite (Member # 1104) on 01-25-2006, 07:00 PM:

I went today to the theatre and spent most of the day there.
Tried the capacitors on hte inpúts,it didn't work [Frown]

Also did what Paul said with not great results. changed again the cabling from soundhead to projector and from projector to amplifers. added ferrite rings with cable wound in it ,the interference is still htere but it lowered a little bit.
Then started pulling out cards from the processor. removing them starting at the preamp cart up to the matrix didn't affect anything. now when each equalizer was pulled, hte interference was gone from the channel as htey were pulled. tried grounding the metal shells but it didn't affected the problem,same for removing it completely.hte only thing that took the interference down a bit more was soldering ceramic capacitors on hte back of main board at the points where the signal goes into hte equalizers to ground.for the average level for the theatre it can't be head in the auditorium,but it being an art house with it's mostly dialogue olny, low level recording films, sometimes hte volume has to go over the level 7 along with it interference starts disturbing the peace again.I'm suspecting it's time to replace the entire sound rack ,first because of the interference htat is FM wich is by itself hard to get tru , second, the theatre has 2 auditoriums, both have exactly the same setup,power comes from the same distribution box yet hte hell breaks loose only in one of them. compared the wiring on both racks and it's exactly the same. even a new grounding bar was placed with new wiring and didn't do anything.

Posted by Louis Bornwasser (Member # 3063) on 01-26-2006, 01:00 PM:

If you have two identical racks; swap the entire processor. Then swap everything in turn. Is the AC power ground bonded to the frame of the rack? Electricians forget this. Louis

Posted by Rick Raskin (Member # 1561) on 01-26-2006, 02:22 PM:

These problems can be extremely difficult to find let alone eliminate. So far the suggested troubleshooting techniques are what I would try. One other thought though; In T-1 systems in the TELCO CO, it was common practice to ground interface cables at only one end of the run. It seems that grounding at both ends could set up some sort of closed loop and cause interference. I don't know if this would be valid here but what the heck, its worth a try.

Posted by Sam D. Chavez (Member # 1841) on 01-26-2006, 07:35 PM:

The thing to do is to determine where the RF is entering in a methodical way.

I would remove the optical preamp board. It's an easy first step to see if the interference stops.

Is the RF only when in a film format? How about Non sync? Does it go up and down with the fader? These are all useful clues.

It can also come back on the speaker wires. I've even had cases where the conduit or pipe the speaker wires were in was the cause. I grounded one end of the pipe and it went away.

I worked in both Sacramento, CA and Seattle, WA and both cities were known for this problem so I got lots of practice finding the cause and eliminating this problem.

Posted by Jack Ondracek (Member # 1466) on 01-27-2006, 12:58 AM:

It might be possible that the FM station you're hearing is simulcasting on a nearby AM channel.

In any case, you might consider whether your RF is coming in on the AC lines to the rack. I had this problem with several radio station consoles, and the fix was rediculously simple.

A couple of .02 disc caps between the power lines and ground would bypass the RF & the problem would vanish.

Along the lines of what Louis says, also check the grounding around your console/pedestal.

Posted by Marin Zorica (Member # 1716) on 01-27-2006, 04:12 PM:

Also i you may conciderence that some capacitors in power supply are bad. Why this could't be heared before, ok station maybee didn't transmitting then, but it would be good that you ask somebody are that station change anything in transmitting system, I mean if they didn't and before was good, that means something goes wrong with sound eqipment in that theatre, possibly as i said where power supply or capacitors what are in parallel with power supply of operational amplifiers in processor.

Posted by Luciano Brigite (Member # 1104) on 01-27-2006, 05:16 PM:

The only thing I didn't do until now was swapping boards with hte processors but will be doing so very soon.
Grounding was checked once more and yes, the rack is also grounded.so are the flexible conduits where the wires and cables come to and goes from the sound rack,grounding them or not didn't make any difference.

I don't think the cause may be the speaker wires because as I said before, when I disconnect the cables ( at the processor end) that goes to the amplifiers no interference is heard and when the connector is touched, only hum is heard.

Also tried removing boards from the processor,starting with pre amp boards and stopping at the matrix, with them all out of the cage, the interference is still there, it goes away when the equalizers are pulled out. It's also present in both film formats ( mono and stereo) also on non sync and it remains the same for about 70% of the fader, as in it doesn't change level as hte fader is advanced. only when the fader is comming to it's end, the interference also goes up.
Power supply was swapped with a spare one they have and everything remains the same so the problem isn't there

Posted by Sam D. Chavez (Member # 1841) on 01-27-2006, 08:54 PM:

So you have determined that the RF is coming into the CP50? Is the RF on all channels or just on the center or other channel(s)?

What kind of control card are you using, Cat. 114 manual card or the Cat. 113C?

Posted by Luciano Brigite (Member # 1104) on 01-28-2006, 09:50 PM:

Rf is on all the screen channels, a little more on the right channel. Left and center seems to have the same level of interference.
Control card is the electronic one ( grey buttons with leds).The older, manual card with the black buttons is still there as a spare, didn't try swapping it because at the time didn't think it'd be involved.