Does anyone have experience with the Production Intercom MS-200 master station? It's a wired station, but we are having radio interference problems. The stage manager is tired of listening to sports radio AND the lighting designer.
Com Radio Interference
- Thread starterNoble
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Anonymous067
Active Member
Anonymous067
Active Member
The cable probably already has a shield.
It appears to be a three pin cable-can anybody confirm? I'm not an expert on this stuff, but it was seem to me that if it was three pin, and you have audio going two ways, that wouldn't give you enough pins for a balanced line, but I could be incorrect. Can anybody confirm any of this?
Just to see if the length really is the problem, try shortening the length of the run. Just connect a cable from the master station to the beltpack and see what happens with the RFI.
It appears to be a three pin cable-can anybody confirm? I'm not an expert on this stuff, but it was seem to me that if it was three pin, and you have audio going two ways, that wouldn't give you enough pins for a balanced line, but I could be incorrect. Can anybody confirm any of this?
Just to see if the length really is the problem, try shortening the length of the run. Just connect a cable from the master station to the beltpack and see what happens with the RFI.
ClearCom (including PI, which is a nice ClearCom clone) are *unbalanced* systems. The XLR cable is wired as follows: Pin 1, Ground; Pin 2, Power +30V; and Pin 3, Audio. It doesn't surprise me in the least that you're getting an AM radio station coupled into the system. You might be able to try a couple of solutions here. First, see if you can get shorter runs. Next, see if you can get longer runs into metal conduit of some kind--this will help shield the cable. Finally, you might look into some sort of custom low-pass filter on the audio line, but this might prove more complicated to construct.
Correction: Pinout.
Correction: Pinout.
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I honestly can't remember, but my gut says don't do it--wire the XLR connectors according to spec. I seem to remember quirkiness regarding grounding in ClearCom, but it's been a while since I dug into this one. Andy might know if he's lurking.
Looks like only in 6 pin systems. I've never seen one in the wild...
There should be only one "earth" for the entire building and that is at the electrical service entrance.
If it is commercial construction, which it probably is, then you should have metallic conduit bonded to a metal box bonded to the plate which contacts the connector shell, so that would provide the "earth common" connection shown. In fact, that is exactly why you do not want to tie Pin 1 direct to the shell as it ties signal ground to the conduit and can create a ground loop. If you look closely at the wiring detail provided you'll see that there is a capacitor between the signal ground and the plate, there is no direct connection. Also note that there is no corresponding connection at the Master Station.
In this case, the MS-200 instructions note:
If it is commercial construction, which it probably is, then you should have metallic conduit bonded to a metal box bonded to the plate which contacts the connector shell, so that would provide the "earth common" connection shown. In fact, that is exactly why you do not want to tie Pin 1 direct to the shell as it ties signal ground to the conduit and can create a ground loop. If you look closely at the wiring detail provided you'll see that there is a capacitor between the signal ground and the plate, there is no direct connection. Also note that there is no corresponding connection at the Master Station.
In this case, the MS-200 instructions note:
So I think it is pretty clear that the signal ground, the shield and drain wire, should not be connected to the shell.
A simple thing to minimize potential RF is also to keep the exposed conductors as short as possible at all terminations. Any conductor after where you stripped back the shield is unshielded and the wavelength of radio frequencies is pretty small so it doesn't take too much exposed length to act as an antenna.
A simple thing to minimize potential RF is also to keep the exposed conductors as short as possible at all terminations. Any conductor after where you stripped back the shield is unshielded and the wavelength of radio frequencies is pretty small so it doesn't take too much exposed length to act as an antenna.
Shorter runs aren't really possible. We don't have any hardwired XLR line to the stage, so the current run is pretty much essential. Conduit is a good theoretical solution, but everything in this place is already so kludged and the run to the stage is so long and convoluted, I can imagine that solution being... less than economical.
When you say low-pass filter, do you mean something like this? Or is it something I'd have to hack together on breadboard?
Because ClearCom is not an audio line, I doubt a filter like that would work (and the +30V might toast the filter too). Basically, I'm thinking capacitors between Audio and Ground...but something like that would be pretty kludgy in the world.
I think the most realistic advice is to use good XLR, wire it to ClearCom spec (i.e., don't tie shell and Pin 1 together), and make sure all other equipment is in good repair. Other than that, you're looking at solutions that require $$$.
I think the most realistic advice is to use good XLR, wire it to ClearCom spec (i.e., don't tie shell and Pin 1 together), and make sure all other equipment is in good repair. Other than that, you're looking at solutions that require $$$.
There is a simple way to attenuate interference from AM radios stations. Buy some ferrite toroids, such at the FT-193-J from Amidon.
https://www.amidoncorp.com/items/21
Wrap several turns of the cable around the toroid, jut a few inches before each connector that plugs into a belt pack or other active device. Each pass of the cable through the center of the donut adds attenuation of the RF signal. The toroid acts as low pass filter.
This is very effective and easy to do. This fix will also work for any kind of audio systems plagued with AM interference. By using a larger diameter toroid like the one I mentioned, you should not have to take apart the cable connector to wrap a few turns.
https://www.amidoncorp.com/items/21
Wrap several turns of the cable around the toroid, jut a few inches before each connector that plugs into a belt pack or other active device. Each pass of the cable through the center of the donut adds attenuation of the RF signal. The toroid acts as low pass filter.
This is very effective and easy to do. This fix will also work for any kind of audio systems plagued with AM interference. By using a larger diameter toroid like the one I mentioned, you should not have to take apart the cable connector to wrap a few turns.
Thank you for the responses everyone!
As a sort of epilogue; we ended up re-routing a cable run so that SL and SR were on separate runs, halving their total distances. It helped immensely. Also, the local basketball tournaments ended, and the AM sports station we were picking up stopped boosting their signal, so we only got the occasional sports update.
I'll mention the Ferrite Toroids to my boss. That sounds like a good long term solution.
As a sort of epilogue; we ended up re-routing a cable run so that SL and SR were on separate runs, halving their total distances. It helped immensely. Also, the local basketball tournaments ended, and the AM sports station we were picking up stopped boosting their signal, so we only got the occasional sports update.
I'll mention the Ferrite Toroids to my boss. That sounds like a good long term solution.
It is highly unlikely that the AM station varied their power levels for special programming. To do so would be a gross violation of FCC rules. Many AM stations vary their power from day to night and are required to do so. If things change around sunset, that's what is happening.
Also, AM interference can change depending upon the wetness of local soils, and water table height. Ground conductivity plays a big part in AM radio signal strength.
I don't know if this is an installed cable or not, but the only good solution for long runs of unbalanced com is to use high quality cables with a braided shield with at least 95% coverage.
Most install-grade cable is just a foil shield with a drain lead, than can crack and open up when flexed or bent, allowing RFI to enter.
Now I have to ask, how loud is the radio vs. the other users on the line? If it is really loud, you've got other issues besides a long cable run. PI and other ClearCom compatible systems drive pretty hot with fairly low-gain amplifiers in the headsets, so the small amount of RFI pickup by the cabling should put the noise pretty far down.
Most install-grade cable is just a foil shield with a drain lead, than can crack and open up when flexed or bent, allowing RFI to enter.
Now I have to ask, how loud is the radio vs. the other users on the line? If it is really loud, you've got other issues besides a long cable run. PI and other ClearCom compatible systems drive pretty hot with fairly low-gain amplifiers in the headsets, so the small amount of RFI pickup by the cabling should put the noise pretty far down.
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