Camera to RF Modulator picks up AM radio

[Andrew Marshall]

I posted this on sonycreativesoftware forums and thought it might be addressed here too, since they are more on the software side:

Background info:
I recently rigged up an old Sony DV camera to record (DV pass-through) our church services to computer, and also use the AV out to send a signal to a TV in the nursery via an RF modulator & coax cable. Works great, and there is a zero budget so I'm mostly using things I have "lying around." I've produced videos to post online & get DVD's made for shut-ins - streaming live will be phase 3 or 4.

But now a few weeks into the venture I'm getting around to exploring the different ways I might get better sound. I hope to run mics on one channel through our mixer (from main/monitor out or something) and a room mic (aux out) on the other. Then in post-production I can work with the 2 tracks and mix/reassign as needed (Sony Sound Forge & Vegas Pro). That way sermon audio is crisp but I can bring up the room mic for laughs, amens, you suck*, etc. (*that's usually my wife )

The problem:
As soon as I plug a cable into the (stereo) mic-in jack I get multiple AM station broadcasts into the headphones on the camera. If I disconnect from the RF Modulator it goes back away, and using higher quality cables (XLR, 1/4 in, RCA) does not make a difference. So at this point I can either send the coax signal to the nursery TV (very important) or input better sound into the camera. The cable is only connected on the camera end so far, so it's not any of the other equip, and it still does it when I disconnect the coax from the modulator or DV from the computer.

This is the entire setup for troubleshooting purposes:
AC power via 2-prong transformer > RF modulator > AV cable to camera (on battery power) > shielded cable in mic jack

Absolutely nothing else connected other than headphones in the camera.

I get the same results (AM radio broadcast) with a variety of cables, and no broadcast until I plug a cable into in mic input.

I recognize the underlying reasons for the issue, and it appears any wires connected to the mic jack act as an antenna. What I'm wondering is if there is a filter/gate or something I can put in the system to nix the radio signal.

I've searched around and not found anyone with this issue, running AV out and converting it to coax at the same time as connecting a mic in cable. Any ideas?

Tell me there's a 1.98 RF filter at Radio Shack! (but don't lie, this is for the church..)

[Chris Soucy]

Originally Posted by Andrew Marshall

The cable is only connected on the camera end so far

This is probably where the problem is occuring.

The camera is running on battery and is thus not earthed. All it takes is a slightly dodgy solder joint anywhere in the connection string to turn the entire set up into an exceedingly efficient radio receiver/ demodulator, with a nice big amplifer in the shape of the mic in preamps.

Try connecting the other end of the mic in cable to the sound board o/p connection. If the ground on that is actually a ground it may stop the problem in it's tracks.

Else dive down to Radio Shack and buy a 50 cent ferrite torroid. Loop the mic in cable (at the camera end) through it about half a dozen times and secure tightly with electical tape.

That should strangle any RF at birth. You may find (depending on length of cable run) that you need to do the torrroid thing at both ends.

Given the nature of RF electrickery, it may be wise to follow suit on every other cable going into or out of the system (yes, power cords too), everything BUT the co - ax taking the modulated RF to the nursery (if one of the above hasn't nailed it).

You can buy "clamp on" ferrite RF killers designed for power cords and such, but as to whether they're available at RS, dunno.

Keep us posted.


CS

[Andrew Marshall]

Thanks Chris, the ferrite may be the trick, I've used them with other issues. I'll hunt some down - now where did I see some last?... I know where I left some in a drawer 2 hours from here, lol.

As for other connections, they were all originally made but were taken out for troubleshooting. With the camera plugged into AC, mic cable connected to sound board or other devices, etc. the problem still occurred. I only removed them to reduce the variables.

[Jay Massengill]

You don't mention the specifics of the RF Modulator, is it a stand-alone modulator? You could try substituting a different one. For example you could use an old VHS VCR for this just as a test although it would probably be more likely to pick up radio than a stand-alone modulator that was working correctly.
I'd also use the shortest possible cables for all these connections and make sure all the cables have no problems with their wiring.
Are you reducing the level of your outputs from the mixer to the mic-level that the camcorder input will need?
If you must use a long audio cable run from the mixer to the camera, you may need to employ some additional items to keep the run balanced for the majority of the distance.
What to use would depend on the output connector you're using from the mixer.
For a different trouble-shooting approach than connecting and disconnecting pieces one at a time, I would probably hook it all up the way you hope for it to work and then substitute a different cable in place at each leg, one at a time. It may be a combo of items or cables causing the radio reception that will require switching something completely new into the total system to eliminate it.
Good luck with the chokes and with the whole project!

[Andrew Marshall]

Thanks Jay, I just fooled with it some more, the ferrite cores unfortunately did nothing for the situation, and I realized the AV cable had one all along already :(

The RF modulator is a rinky-dink one for hooking up a vcr to tv, and I considered trying a few different ones including VCR's - but I found out tonight that it is not the root problem.

Once I had the initial problem, I took most variables out of the equation, so only the items listed here were connected:
AC power via 2-prong transformer > RF modulator > AV cable to camera (on battery power) > shielded cable in mic jack

However, tonight I discovered that it appears to be a signal carried through the RG6 coax, it was just that it was near enough to the RF modulator (though not connected anymore) to jump into it and through the AV cable to the camera.

So now I can do this, no other wired connections:

1. Camera on tripod using battery power, headphones plugged in, no other connections (just air and my body) > no AM radio heard
2. If I then plug any cable into the mic jack of the camera (other end not connected), I can hear the AM very faintly - tried a variety of cables & types
3. If I just touch the outside of the coax connector (which is the shield), then I get multiple AM stations in the headphones, clear as a bell.
4. If I then unplug the cable from the mic jack and still hold the coax, no AM radio at all.

The coax runs several hundred feet through the facility and is good quality, just ran it 2 wks ago. It could be getting a signal from many places.

It's becoming more of a brain teaser than anything, though I still want to broadcast to closed circuit TV while capturing video with proper sound input from the sound board.

[Andrew Marshall]

I re-read Chris' post and realized that the long run of coax is not grounded anywhere. Tomorrow I'll put a ground on it along the line and see if that doesn't suck out the AM signal. That would be too simple! I had been contemplating why this isn't a problem with cable TV and was going to check with a cable tech, this might be the answer.

And I also realized that it would be a great place for one of our Florida lightning bolts to travel!

[Chris Soucy]

Been giving this some more thought.

As a matter of interest, what channel(s) is the RF modulator outputing (theoretically)?


CS

[Andrew Marshall]

I've tried both ch. 3 & 4, the only 2 options. Same results.

I'm looking forward to grounding the cable later to see what happens, I'll post back.

[Chris Soucy]

If it's the former, I think I may know where the problem lies.


CS

[Andrew Marshall]

Still not resolved, but here's the latest:

With the RF Modulator out of the picture for test purposes and only the coax cable which runs all over the facility (connected to 2 TV's along the way), I can get the AM signal with the camera on battery when I just touch the coax shield. - So that leads me to think the RF modulator might be a carrier of the AM signal, but not the cause.

But If I hook the RF modulator up and put a ground wire from the signal amplifier into the AC ground (power strip), it reduces the AM signal significantly but not completely. I can and did do all kinds of different connections but bottom line is if I connect to the ground in the power strip certain ways I can clear out about 75% of the AM signal.

The power strip is quite far from outside and/or a direct link to the ground, so I wonder if it would be better with a "clean" ground. This is all located in a balcony so running a plain ground wire to a rod or something isn't ideal for testing, but could be done permanently if I was sure it would work. I'm wondering if the ground wire on my cable box at home serves this sort of function as well as lighting arresting. Any cable techs out there?

btw I also tried a ground wire way down line on the coax into an electrical box ground wire, but it did nothing at all up in the balcony (too far I guess).

I'll keep fiddling with it, and use the built-in mic in the meantime. (remember the broadcast only happens when I try a cable in the mic-in jack)

[Chris Soucy]

Originally Posted by Andrew Marshall

the signal amplifier

What exactly are the specs for this thing?

I knew there had to be a RF amplifier somewhere in the sytem, either as part of the modulator or seperate. I take it it's a descrete standalone device?

Frequency range, gain, inbuilt filters etc etc?

The reason I'm so curious is I'm not entirely sure what you're hearing is AM radio stations.

Also, about this co - ax.

You've mentioned it feeds two tv's. How? By which I mean, have you/ they fitted in - line splitters to take feeds to correctly terminated 75ohm RF inputs?

Does all this co - ax have any other purpose? Is there an aerial and if so, how is it connected into the system?

OK, my suspicion(s).

I'm gonna guess that amp is a full band VHF amp working from channel 2 all the way up to channel 13. I'm also gonna guess it doesn't have a FM trap built in.

The two channels you mentioned (3 & 4) are @60 - 66 Mhz and 66 - 72 Mhz, respectively.

If the amp is a full band amp then it's amplifing everything between Channel 2 (54 - 60 Mhz) and channel 13 (210 - 216 Mhz).

It won't have escaped anyones notice that sitting slap bang in the middle of that range is the.............88 - 108 Mhz FM band.

Without a FM bandstop filter somewhere in the works, the slightest amount of FM that gets into that amp is going to whistle around that co - ax circuit till it hits an improperly terminated connection and set up standing waves all over the place.

You now have an exceedingly efficient FM transmitter.

You may find you need something like this on the I/P of the amplifier to can that FM before it can cause chaos:

http://www.tinlee.com/PDF/CR7%20FM%2040.pdf

On the other hand, it could all be a gigantic red herring, there's already a bandstop in place or the amp is only a single tuneable channel so couldn't possibly be the culprit.

Interesting theory tho'.


CS

[Andrew Marshall]

Thanks again Chris, I always admire persistence!

I'll check the components and give you more detailed info later.

The big thing is the radio signal still comes in when the coax is not even connected to the RF modulator or the amp. I hooked back into the amp because it had a proper ground lug on it.

Downline the coax is run through one splitter (I'll check the specs 500- 2150 MHz I think) and then to 2 normal TV's via their coax jacks. It runs through several buildings and past/alongside various wiring.

If it comes to it, I'll disconnect various places along the coax line to minimize more variables, as I have in the balcony.

The broadcasts appear to be AM talk radio, but I've not listened for station identification yet.

[Andrew Marshall]

OK you made me go over and root around again (good thing I live next door).

2 stations I heard were AM 1320 and AM 1460 (Jacksonville, FL, where I am)

I removed all splitters & cable connections, and hear radio in headphones if I:
- have a cable in the 1/8 mic jack of the camera (but not if I remove it)
AND touch the shield of the UNconnected coax (but way less radio if I also touch ground)
- either have camera on battery, or AC power
- either have AV connected to RF modulator, or not
- no other wires touching the camera
- nothing connected to coax other than the fact that it touches plenty of things in its run
- I believe only 2 cable connectors in the run of several hundred feet

fwiw the amplifier (not connected above) is:
Electroline EDA 2400 drop amp, + 7dB on 4 lines
specs say 5-42 54-1000 MHz 6kv
- but the issue happens whether it is in the system or not (& unpowered)

fwiw the cheapo RF modulator is (again, not connected above):
Radio Shack #15-1214

fwiw the only splitter besides the amp is (again, not connected above):
marked DSV 50
labeled 5-2300 MHz

I'll post a drawing next

[Chris Soucy]

Andrew, do you have a multimeter?

Can you measure both the AC & DC voltage between the live and ground input pins on that modulator, then the I/P ground to a "true" earth (or as close as you can get)?

I'm assuming it's got two inputs, one each for composite and audio, either will do.

Can you tell me whether the modulator has an internal or external power supply, and if external, what's the rated voltage?

The second experiment: can you connect a cable to the mic in (any cable that will fit, as long as you can get to the other end of it and it produces radio in the headphones).

If the headphones have radio, try shorting each audio channel live pin on the mic in cable to the ground connector.

Does either or both kill the radio?

As you haven't been using external mic's up to now (I'm implying that from earlier posts) where has your audio been coming from? The "on camera" mic?


CS

[Andrew Marshall]

Yeah, I'll check with a meter tomorrow.

So far I've been using a shotgun mic on the hot shoe. Everything's fine with all connected until I try a mic cable. No different if I connect it in a variety of ways to the sound board.

I'm leaning toward a clean ground to the shield of the coax as the key, the ground I'm using (to a power strip) is the only thing that changes the issue.

I made a wonderful drawing of me when I have the issue, all other variables out of the system. If I disconnect the mic cable OR let go of the coax, all is silent. If I touch ground 75% or so of sound is reduced.