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Old September 23rd, 2008, 05:08 PM   #1
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Picking Up Radio Signal on Sound Board RCA Cable

Last week I shot a multi-cam event for a school administration fall communication update to the parents. I decided to try something new on what would be a relatively stable and completely uninspiring shoot. I brought out the alienware and the shuttle computers and recorded live direct to the PC using Vegas, as well as recording to tape (just in case).

There were only two problems with the shoot....

1) The stupid "getting audio from venue PA system into cam with out the hum & hiss" problem
and
2) Accidentally moving the alienware and bumping loose the 1394 cable interrupting the stream for a few seconds.

This is the first time I noticed that I was picking up a radio station broadcast on my audio feed from the mixing board. The board headphones didn't have the signal, but phones plugged into the cam revealed the DJs talking, and some music, etc. Gahhh. Pretty much ruined my feed.

I couldn't get a line or mic out and as usual, the operators were not highly trained. They know which input was the speaker's lapel, and which was house speakers. And that was it. I tried and tried to get an AUX line on the system out to my cam's Beachtec, but it was useless. Nothing but buzz and hum and no audio signal. I obviously didn't know how to rout the mixer's output to that AUX bus.

But I could get a signal from the monitor headphone jack. I hated to do it because I knew the problem I was going to face. And sure enough. I got a signal, and could adjust the volume, but it was a dirty signal AND it was picking up the radio signal more than likely due to my cable linkage. I was using...

0) Sound board headphone jack
1) 1/4" to 1/8" adapter
2) 1/8" to RCA pair
3) 20ft RCA stereo pair cable
4) RCA pair to 1/8"
5) to Beachtek

So somewhere in that patchwork was where the radio signal was getting picked up. Now you will immediately ask "why didn't I use my 20' XLR cable that was sitting right there" and I would say.... because I don't have the adapter(s) to turn 1/4 stereo to mono XLR female (yes I know B&H has them and the next gig I book, I'm buying one for the $15 or whatever the cost).

And even if I did have it, would that eliminate the horribly dirty feed from the mixer's headphone jack? Not likely. It seems like Headphone jacks are way to hot (probably to drive the speakers in studio headphones) so I need some sort of limiter (yes the beachtek has built in ones, but they are not the best) and some sort of frequency filter to cut out the low end buzz / hum, if possible.

I've looked on here before (and through B&H), but I'm either not using the right search terms, or I keep missing it. Does any such device exist to clean up the audio signal? The only ones I've found use TRS jacks (must be a 1/4" jack that guitars & amps use).
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Old September 23rd, 2008, 05:37 PM   #2
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First, probably be a good idea to get in there ahead of time to see what's going on in terms of your audio feed. You had a mixing board. Just knowing that, you were most likely to have a clean feed to use. A little planning, and you would have been easily on your way!

I'll agree, the headphone amp was not a clean way to go.. that is an issue in itself.

Next, your cabling was poor. Frankenstein'ing adapters is a way for rf noise to seep into your line. On top of that, you're using RCA cables which are unbalanced. To make matters worse, you were using 20 ft of it.

All you really needed was proper XLR or TRS cables and a little planning before hand.
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Old September 23rd, 2008, 06:59 PM   #3
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All you really needed was proper XLR or TRS cables and a little planning before hand.
So is a TRS the same as a 1/4" stereo cable? I assume a TRS is actually a mono cable that provides power (like for amps, etc). Is it shielded to prevent picking up the radio?

If I have the 1/8"mini jack to XLR adapter (with the 1/4to1/8 adapter in front of it) would that eliminate the chance of picking up the radio station?

The 20' cable was mostly needed to get the audio from the mixer to the cam. I could have probably made do with a 15' cable, but anything short would involve hopping over the cable.

Even if I could get the AUX8 bus to get a feed from the lapel input (this was a massive ~30 input digital mixer) wouldn't that AUX output have been a line levels instead of mic levels? I'm not sure if just bringing the gain down on that would be enough (though feel free to correct me if I'm wrong).

Secondly, on other location shoots I cannot be guaranteed a mixer that I am familiar enough with in order to fiddle with the mixer to find an AUX output, let alone be allowed to get one (especially if the audio tec doesn't know their stuff and won't allow me to poke around behind the mixer). That leaves me resorting to the headphone jack again and putting up with a dirty signal.
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Old September 23rd, 2008, 07:23 PM   #4
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you could of taken a XLR Y cable and split the main outs. most boards have 1/4" and XLR outs so maybe you could of used one of them if they were empty. you also could of used the TAPE outs too.
I've used a headphone jack in an emergency. works if the output is turned down and camera set to LINE level. not the best, but it works. most likely it was the RCA cable that did you in.

while not cheap, a audio isolation transformer is a life saver for these gigs as it will eliminate ground loops with the board. imagine doing a truck gig taking 12 feeds from the house board and putting a transformer on every connection.... but it saved the day

time to get a set of decent adapters.
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Old September 23rd, 2008, 07:50 PM   #5
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The unanswered question in your posts was if there is anything that can clean up the audio.

I will start by disclaiming: I have not "personally" used this product and do not own it (yet) but the online demo of "SOUNDSOAP 2" was impressive indeed. May not solve all your problems but could help... the lower end version is $129, and the Pro version is $400.... but it amazed me what it could do. (I found it by looking for a way to counter the noise of a 16mm film camera... and when I saw what it could do there I was hooked....their website has a great little video that demonstrates its capabilities... I joked recently on a shoot there is no such thing as an airplane noise filter, but by gosh, maybe there is !)

Google it and check it out....
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Old September 23rd, 2008, 08:20 PM   #6
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I'll disagree with some of what's written above.

The basic problem is unbalanced cabling. "Converting to XLR", suprisingly, doesn't neccessarily get you into the world of balanced signals. For that, you need to have a balanced output and balanced cabling to your beachtek. And most beachteks can handle line levels.

If you can get a proper balanced XLR or TRS output from the board and can keep that balanced to your beachtek that's it - you're done.

If you have to use an unbalanced output you have two "best practices" choices. Keep the cable short, no more than 6', or, use a passive or active direct box aka. DI to balance the output... again with a very short unbalanced run from the board output to the DI.

Once you're truly balanced, you can run a few hundred feet.

TRS, like XLR, is just a connector with 3 conductors. The magic happens in the board output and the beachtek input circuits. The 3 conductors are signal, ground, and -signal. When RF strikes the cable, it will tend to affect the signal and -signal oppositely. The balanced input magically sums the two signals in such a way that the RF mostly goes away.

Just a note, balanced circuitry and cabling is half the long-cable solution. The other half is low impedance. A high-impedance output -> input will tend to lose some of the high frequencies over longer distances. Probably more than you wanted to know, the rule of thumb is 6' of high-impedance or a couple hundred feet of low impedance will not significantly degrade the higher freqs.

Of course there are exceptions to that, too...

A search on this forum on the term direct box or di will turn up more information - you're not the first person to experience this.
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Old September 23rd, 2008, 08:22 PM   #7
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Originally Posted by Chris Swanberg View Post
The unanswered question in your posts was if there is anything that can clean up the audio.

I will start by disclaiming: I have not "personally" used this product and do not own it (yet) but the online demo of "SOUNDSOAP 2" was impressive indeed. May not solve all your problems but could help... the lower end version is $129, and the Pro version is $400.... but it amazed me what it could do. (I found it by looking for a way to counter the noise of a 16mm film camera... and when I saw what it could do there I was hooked....their website has a great little video that demonstrates its capabilities... I joked recently on a shoot there is no such thing as an airplane noise filter, but by gosh, maybe there is !)

Google it and check it out....
Software solution would be nice, but I'm trying to eliminate as much POST work as possible (since that takes me much longer to perform). I will keep that in mind though.

I do have Sound Forge and the NR plugin & the VASST audio cleanup tutorial DVD (which has helped immeasurably for getting rid of the buzz & hum on other projects and I'm sure will on this one). But a software solution is still pretty dang time consuming.
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Old September 24th, 2008, 01:41 AM   #8
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Okay.. I thought you had already recorded what you could and were faced with "fixing it". If not, then my post was more or less irrelevant, sorry.

Still ...a pretty unique product, I recommend you investigate it as a backup plan in the future. Way!! more than a clicks and buzz fix. It can memorize a noise and remove it. (like a motorized camera running for example... which is what led me to it.)
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Old September 24th, 2008, 08:04 AM   #9
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Could it have been a simple ground loop problem? Maybe a ground lift would have helped? It's something that happens quite often to me whenever I use a house system's feed.
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Old September 24th, 2008, 09:38 AM   #10
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Originally Posted by Chris Swanberg View Post
Okay.. I thought you had already recorded what you could and were faced with "fixing it". If not, then my post was more or less irrelevant, sorry.

Still ...a pretty unique product, I recommend you investigate it as a backup plan in the future. Way!! more than a clicks and buzz fix. It can memorize a noise and remove it. (like a motorized camera running for example... which is what led me to it.)
No, you are correct in that for this event I have recorded the signal and the signal contained the radio traffic which will need a post-production fix. What is odd, is that the traffic did not appear until late in the production. I may be able to clip it out with the Floorfish (or the plugin you mentioned) since the radio signal is very weak.

I also have ambient audio recorded from my AT897 shotgun, and ambient audio from the second cam (just an on board mic on the GS320, so nothing of any note). So I have other audio sources, and considering the noise level on the sound board feed, it might be better to just use the ambient recording from the shotgun.

But should this situation arise again, I want to be more prepared with a solution that gets it right before writing to tape.

Last edited by Jason Robinson; September 24th, 2008 at 09:39 AM. Reason: spelling
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Old September 24th, 2008, 10:12 AM   #11
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Originally Posted by Jason Robinson View Post
What is odd, is that the traffic did not appear until late in the production.
Since what you are picking up is an AM broadcast station, the late arrival of the traffic was likely due to sunset in the area that the signal originated from. AM stations have a problem in that their signals will travel great distances at sunset due to bouncing off the ionosphere. Therefore, they have three options... shut down at night, go to reduced power, or switch to a directional signal using a phased antenna array. All of this so that they don't interfere with other stations on the same frequency in other markets.

The basics for detecting, rectifying, and amplifying an AM radio signal exist in many electronics setups. The first time this happened to me was when I was in my early teens. It was around 2AM and I had been playing my electric guitar. It was very faint and I struggled to listen. It turned out to be a Canadian broadcast station. At first, I couldn't tell exactly where the sound in the room was coming from. Then I discovered sound was coming through my amp speakers. It scared the living crap out of me! I thought the 'other world' was trying to speak to me.

As was stated, balanced cabling to shield out unwanted signal will take care of this in most cases. In the example above, I had an unbalanced guitar cord, going to a distortion pedal, then on to the amp. Antenna, detector, and amplifier. Voila... you have an AM receiver.

-gb-
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Old September 24th, 2008, 10:45 AM   #12
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Reminds me of a time back in my TV news reporter days, I was doing a stand up, long mic cable on the street. When we got back in the edit booth the local beautiful music station (AM) gave me a nice sound bed. LOL. The station was a couple buildings away from me with the microwave to the transmitter on the roof. (shorter cable later and it wasn't an issue)
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Old September 24th, 2008, 11:07 AM   #13
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As was stated, balanced cabling to shield out unwanted signal will take care of this in most cases. -gb-

1) So will a 1/4" to XLR adapter and the use of my XLR cable get me to what I need to use the headphone jack (complete with its power problems but at least minus AM station pickup)?

2) To completely eliminate both problems (power buzz & hum and AM station) I understand I would need to know the sound board well enough to be able to pull an AUX out (at either line or mic) and then have an appropriate limiter to chop a line down to Mic for input to the Beachtek.

I would like to go with solution #2 because it eliminates the power buzz, but I doubt I'll be able to know my way around every house PA mixer and get permissions from the tech behind it to fiddle with their output plugs to get what I need.
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Old September 24th, 2008, 01:18 PM   #14
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Originally Posted by Jason Robinson View Post
1) So will a 1/4" to XLR adapter and the use of my XLR cable get me to what I need to use the headphone jack (complete with its power problems but at least minus AM station pickup)?
No, I don't think so. As Seth pointed out earlier, you'd still be dealing with un unbalanced signal that is prone to picking up all sorts of noise, the more the longer the cable is that you use. If all you have is an unbalanced output (headphones, RCA, etc.), you can however run a SHORT cable to a balancing transformer, such as this one
http://www.rane.com/pdf/bb22dat.pdf
and run a balanced signal from there into your Beachtek, and that cable can be as long you need it to be. That would protect you much better from picking up noise.

Quote:
2) To completely eliminate both problems (power buzz & hum and AM station) I understand I would need to know the sound board well enough to be able to pull an AUX out (at either line or mic) and then have an appropriate limiter to chop a line down to Mic for input to the Beachtek.

I would like to go with solution #2 because it eliminates the power buzz, but I doubt I'll be able to know my way around every house PA mixer and get permissions from the tech behind it to fiddle with their output plugs to get what I need.
Well, keep trying. The basic ways to operate analog mixers are pretty similar between different makes and models of mixers. Typically, you have one AUX knob per input channel and per AUX output. The knob controls how much of that channel's signal is added to that AUX output. So as a starting point, plug your XLR cable into the mixer's AUX1 output, then adjust the kbobs labeled "AUX1" such that you open it a little bit for all channels you which to record, and close it completely for the other channels. That should give you something on your XLR cable. Then adjust the AUX levels from there.

It may take a little time to set it all up such that you get the right levels of everything, but the basic signal flow should be pretty straightforward that way. And you'd get a clean, balanced signal right out of the mixer. Just practise doing it, it'll be worth it!

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Old September 24th, 2008, 02:53 PM   #15
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Originally Posted by Greg Boston View Post
Since what you are picking up is an AM broadcast station, the late arrival of the traffic was likely due to sunset in the area that the signal originated from. AM stations have a problem in that their signals will travel great distances at sunset due to bouncing off the ionosphere. Therefore, they have three options... shut down at night, go to reduced power, or switch to a directional signal using a phased antenna array. All of this so that they don't interfere with other stations on the same frequency in other markets.
>>>>Well, sort of. When the FCC started laying things out for AM radio stations, there were at least three classifications based on power and coverage area. The clear channel stations (no connection to the company that eventually came to be called 'Clear Channel', were (are) 50 kW stations and mostly in big markets. They had the "right of way" , but even they go from omni to directional at night, mostly to not interfere with Canadian stations.

>>>>The less powerful AM stations have less right of way so they have to change pattern or go off the air. Some of these are directional during the day already, but change direction at local sundown and back at sunrise unless they have "presunrise or "post sunset" authorization. That authorization was given to some stations who pleaded that a 4:45 PM sign off in December (for example) would prevent them from even having a PM drive program (and the ad revenue it generated.)

Quote:
Originally Posted by Greg Boston
As was stated, balanced cabling to shield out unwanted signal will take care of this in most cases. In the example above, I had an unbalanced guitar cord, going to a distortion pedal, then on to the amp. Antenna, detector, and amplifier. Voila... you have an AM receiver.

-gb-
This is true!

Regards,

Ty Ford

Last edited by Greg Boston; September 30th, 2008 at 05:11 PM. Reason: repaired quote
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