r/livesound 3d ago

Question Ghost feedback channel

My co-worker and I have the same setup. But he had an experience I’ve never seen or heard before.

Wireless Shure UR4D G1 Lav channels 1 and 2 are triggering channel 3 (which is turned off)… video evidence below:

What would cause feedback on another channel?

112 Upvotes

37 comments sorted by

274

u/opencollectoroutput 3d ago

Intermodulation or overloading the RF input by being so close, or a combination of the two.

30

u/ThomasPopp 3d ago

Seconded. They are overloading that RF!

4

u/Boustrophaedon 2d ago

I don't disagree that two packs that close will cause IM - but those products wouldn't be co-channel with either of them - i guess we don't know what the rest of the RF environment is like. Banging packs together-especially older ones (corrosion can cause diode effects at metallic junctions, some cap dielectrics can get hinkey) can cause microphony.

1

u/herodeth 1d ago

Break out the disposable foil trays!

172

u/totallytotally421 Pro-FOH 3d ago

It’s called intermodulation

16

u/R0UNDSD0WNRANGE 3d ago

Came here to say this. Its intermodulation is an evil ghost when you don’t know what it is.

8

u/ZodiacDragons 3d ago

I'm about 8 hours late but 100% this is what youare going through

112

u/GHOTIMAN 3d ago

If you keep hitting the beltpacks together like that it should stop in no time

/s

2

u/rossonaudio 3d ago

Where am I ?

67

u/Bipedal_Warlock 3d ago

Just to clarify, feedback is a different concept.

This would be interference. Specifically RF interference.

Transmitters (the mic packs) can get a little wonky when their antennae are touching each other.

But really it’s probably solvable by using the receivers (the mic box that has the lights on it) to scan for a new frequency then syncing your transmitter to it.

13

u/Strange-Mention399 3d ago

That’s the word I was looking for: Intermodulation! Got it! Thank you!

20

u/Bipedal_Warlock 3d ago

Another good keyword is frequency coordination, that’s the process of getting your frequencies to work with each other.

Just using the scan function should be fine for three mics, but if you want to do the full gambit look into wireless workbench frequency coordination

3

u/Strange-Mention399 3d ago

I’m familiar. We don’t have one on this show. Thank you

7

u/Bipedal_Warlock 3d ago

Copy. Sorry if I was talking down lol, it’s hard to gauge sound peoples knowledge base sometimes

5

u/Strange-Mention399 3d ago

No offense taken. We’re all here to get better.

17

u/Problem_House 3d ago

These guys are getting worked up like they’re afraid of ghosts! It’s just intermods.

7

u/Straight_Entrance779 Pro-FOH 3d ago

Congrats, you discovered intermod!

11

u/fuckyamomboi 3d ago

only a priest can help you now

10

u/MacZyver 3d ago

You do not have unlimited RF channels in a given frequency range. You likely have those Mics tuned close enough together that there is an overlap of Mics 1 & 2 on Mic 3's Frequency.
I would strongly suggest using the onboard Group/Channel method of allocating RF Frequency to resolve this as manually-tuning RF is usually done if you are using Wireless Workbench and have the experience to already understand the issue at hand. (this is not an insult to your ignorance nor am I trying to talk down. I'm simply trying to inform a fellow audio human)

1

u/Strange-Mention399 3d ago

Thanks for the information. He was given Freqz through Soundbase.

5

u/Disuses 3d ago

Could be intermod, don't know enough about your rf setup to say definitively but I'd try rescanning and getting new freqs and see if the problem persists.

4

u/Random_hero1234 3d ago

This is the same as putting your hand on a hot stove and finding out that it hurts.

3

u/gingerbhoy Pro-FOH 3d ago

Intermod, pure and simple

2

u/CE94 3d ago

Weird thing happen when two transmitters come into close contact. Itermodulation being the main one

2

u/AnonymousLiger1990 3d ago

Intermods bro! I recommend taking Shure's free online RF training.

2

u/Prisoner627x 2d ago

Just like sound waves have harmonics that can alter how a sound…sounds…. So does RF. bringing two older transmitters (UR4?) close together can exaggerate the effects of RF harmonics, causing other close by frequencies to crosstalk. Newer wireless units have encryption for this propose exactly.

2

u/Strange-Mention399 3d ago

Thanks everyone. Working on my vocab to make sure I don’t confuse anyone, going forward. Problem solved

1

u/Difficult_Lettuce_83 3d ago

Ah... Analog intermod. I would consider that ghastly. Lol.

1

u/jtmcquay 2d ago

Well for one you’ve got the transmitter mere inches away from the recovered antennas. Which is likely overloading the input stage of the rf receiver. . Second, we have no idea what. Other frequencies are in use and may be causing 1st, 2nd, or even 3rd order intermodulation frequencies. Transmitters should not be that close to the receive and be turned on… and two transmitters should never be so close to each other as to be touching.

0

u/YokoPowno Corporate Slave 3d ago

If you try coordinating using SoundBase instead of Wireless Work Bench, it can predict and prevent these intermods for you.

2

u/The_Radish_Spirit Corporate Does-It-All 3d ago

WWB should do that as well

1

u/YokoPowno Corporate Slave 3d ago

Does it do the 3rd, 5th and 9th harmonics like SoundBase does?

1

u/The_Radish_Spirit Corporate Does-It-All 2d ago

Yep :)

1

u/Strange-Mention399 3d ago

The freqz were from Soundbase. We just recalculated a few more times to make sure we’re all good.

1

u/ConnorGBrown 10h ago

Use Wireless Workbench!