Camera to RF Modulator picks up AM radio

asdrew wrote on 2/24/2010, 7:50 PM
Feel free to blast me if I should post this somewhere else, or move it. I've not been able to find this situation in searches:

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 )

So here's the thing: 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.

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 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..)

Comments

musicvid10 wrote on 2/24/2010, 9:17 PM
Try a direct box with a ground lift. What you are probably doing is creating a ground loop that is picking up (and detecting) the AM RF signal. Converting to a balanced audio signal and eliminating the ground loop should fix it.

Possibly, the AC in the nursery and the computer location are hooked up to different legs of the building power. But there are other scenarios that will create the same type of problem.

Your description is a little too complicated to visualize, so post a detailed diagram of what you are trying to do and I'll show you where to put the box(es).
asdrew wrote on 2/25/2010, 4:50 AM
Thanks for the quick reply. I've found direct boxes to be the solution in many situations, but in this one it's not the issue, nor is a ground loop (at least not yet).

When I discovered the issue, I began taking out components/variables one by one to isolate the issue. In the end this is all I have when the problem occurs:

1. RF Modulator plugged into AC power (2-prong transformer) but not connected to the coax out, only the AV cable output of the camera.

2. Camera on battery power and turned on, no other connections besides the AV to the RF modulator and headphones.

3. If I plug in just one end of a 6' cable to the 1/8 stereo mic input I get the radio, as if the cable is an antenna. I've used an adapter to connect various types of shielded cables (XLR, 1/4", 1/8",RCA) with the same results. Again the other end isn't connected to anything.

So it seems the RF Modulator is transmitting the AM signal back into the camera whenever the camera has any cable connected to mic input as the "antenna"

No other physical variables that I'm aware of, and it happens with 2 different Sony cameras.
rraud wrote on 2/25/2010, 7:57 AM
As Musicvid said "Your description is a little too complicated to visualize." A block diagram and/or photos would certainly help us get a handle on the input/output layout of the system.
AM signals normally seep in via partially shielded cables/jacks/ect. and/or lack of proper ground. (balanced or not) Off hand I can only suggest 100% shielded cables w/ metal connectors.
Try connecting a separate chassis ground wire between the mixer, camera, and RF mod. box
Did you you say that "dropping the coax and/or RF mod. does not eliminate the RF?" It's also possible camera's "plug-in" power may have to be dealt with.

asdrew wrote on 2/25/2010, 8:18 AM
Thanks for the reply. I don't see where I have the option to attach a diagram or image here, I'd be glad to do so. What am I not seeing?

As my last post indicates, I've completely taken the mixer, coax, mics and camera plug-in power out of the equation.

This is all there is:
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.
rraud wrote on 2/25/2010, 8:53 AM
Is this your signal flow? "This is all there is: AC power via 2-prong transformer > RF modulator > AV cable to camera (on battery power) > shielded cable in mic jack"

AFAIK your cannot post photos, videos or other files directly to SCS forums.
You have to post a link, for instance, to your website, an online photo gallery, Utube, ect.
see Topic Subject #1: Sticky

Try another forum, this is more software specific: I'd recommend
http://www.dvinfo.net/forum/all-things-audio/ "The Digital Video Information Network > All things audio. You have to register and be a "real person" with a "real name".
musicvid10 wrote on 2/25/2010, 10:18 AM
Just a general comment: RF modulators in general were poorly designed, threw out lots of amplified broadband interference, and were useful pretty much just for video games. It would not surprise me a bit if yours is detecting the AM signal and throwing it back on the audio shield.

If that is the case, in addition to my and rraud's thoughts, your only solution may be a different A/V delivery method to your remote location. BTW, those wireless modulator / transmitters don't work any better in my experience.
musicvid10 wrote on 2/25/2010, 10:39 AM
And just another thought: What microphone / cable are you using ? Have you tried another type?

I remember when a whole concert recording was ruined because a pair of older unbalanced EV dynamic recording mics picked up the local CW station louder than the recorded audio.
asdrew wrote on 2/25/2010, 10:52 AM
thx for the good replies, I use several forums (like most of us) but always know that Sony Creative Software users tend to be more active/technical than many others. I'll try dvinfo too - I haven't been on there in a while and forgot that they use a full name logon - no wonder I couldn't log on!

rraud, yes that is my complete signal flow for troubleshooting. Other things will be connected if this part gets resolved.

musicvid, I never got back to the point where I tried different mics since the first shot, when I immediately removed as many variables as possible. I have tried a variety of cables, they all do the same.

And yes, RF modualtors are poorly designed since they are only meant for other things - and this is a typical cheap one. I might try another one I have, but expect the same result. I did consider grabbing my father-in-laws Sony rf wireless receiver to remove the physical connection, but have the same reservations as you do.

I could always use a separate camera for the RF modulator to cable TV broadcast and another one for recording with good sound input. I'll probably do that for this Sunday, but hope to come up with a simpler setup eventually.