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Forums Audio Engineering A moment of weirdness – multiple lavs

  • A moment of weirdness – multiple lavs

     Tom Galli updated 1 year, 1 month ago 3 Members · 4 Posts
  • Tom Galli

    January 7, 2020 at 12:56 am

    Aloha all, and happy new year!

    I’m going to describe this in detail, but the crux is, a setup that had been working flawlessly for 2 days suddenly experienced a bizarre, and production-halting, hiccup. After a few moments of frantic troubleshooting, I bypassed the issue, which self-corrected before I could return to look into it minutes later.

    The project is a small indie film. The recorder is a Tascam DR-680. I was running 6 wireless mics. CH 1-3 were Sennheiser lavs. CH 4&5 were Sony URX/UTX P03 lavs. CH 6 was an older Sony P02 with the plug-in transmitter attached to a shotgun on a boom.

    It was a 2-day shoot. The actress on CH 4 had lines throughout both days, so I kept her on the same mic and same channel for both shoots. Saturday, there were no problems. Late in the day Sunday, in between two takes of the same shot, suddenly I started getting a very loud, terribly over-modulated noise from CH 4. The director was calling for places; I yelled “hold for sound!”

    As I yelled, the noise momentarily cut out. Talking to myself, I said “What the…” It cut out momentarily again. Every time I spoke, the channel would cut out. I started to realize that what I was hearing was a ridiculously overblown audio feed of myself! I could hear myself moving, and when I took the headphones off of my head, I could hear feedback beginning to ring.

    I unplugged the receiver from the recorder, and moved it over to CH 5, which wasn’t in use for that particular scene. The noise followed it, moving to CH 5, so I was reasonably confident it was in that mic/receiver as opposed to the recorder. Not a problem, I had 2 mics available, as neither CH 2 nor CH 5 was in use at the moment. I waved the talent over, swapped her mic for #2, and we resumed shooting.

    When we finished the scene, I went back to troubleshoot, but the issue was gone. Which was good; I needed all 6 mics for some subsequent scenes, and everything functioned flawlessly.

    My first thought was that, somehow, the recorder had hiccuped, and the built-in mic was overwriting CH 4. One hole in that theory is that there is no built-in mic on the DR-680. Big hole. Another hole is that the sound moved when I swapped plugs around.

    During the time of the incident, the 2 unused lav mics were sitting on the table with me. Mic 5 was the same make/model as Mic 4, so maybe some freak environmental change caused the signal from mic 5 to bleed into mic 4? That would account for me hearing myself, but doesn’t explain why it happened only for those few minutes during 2 solid days of operation.

    The only oddity I found, in looking things over afterwards, is that phantom power was turned on to CH 5&6. I know I had turned phantom off for all channels when I set up at the start of the day, but maybe I hit that switch during handling, or maybe someone had been looking at my toys during a break. Anyway, I turned it off, but the situation had self-resolved before I made that discovery, so I don’t think it was the culprit.

    Anyone have thoughts on what would cause a hiccup such as I described?

    Tom G

    The difference between theory and reality is that, in theory, there is no difference between theory and reality.

  • Ty Ford

    January 10, 2020 at 5:02 pm

    Hi Tom,

    Thanks for the detailed report. No hard explanations here. Yes, phantom on wireless receivers is generally not a good idea. Having transmitters sitting next to receivers and other transmitters can cause problems. And then there’s what other RF might have been happening.

    I think the part where the problem followed your receiver is telling.

    Perhaps something nearby interfering with it.


    Ty Ford
    Cow Audio Forum Leader

    Want better production audio?: Ty Ford\’s Audio Bootcamp Field Guide
    Ty Ford Blog: Ty Ford\’s Blog

  • David Peterson

    January 15, 2020 at 1:52 am

    Interesting scenario! Seemed you had a good go in troubleshooting and thinking through it, I would have likely done similar ish in your shoes.

    Definitely seems kinda like a gained up mic situated close to yourself is being feed in, and the guess it is your recorder’s internal mic is a solid assumption! Except the DR680 lacks this feature, and from memory I think you’re right as I started out with a Tascam DR680mk1 myself (paired with a Sound Devices 552 in front of it. Still got both of them somewhere here in my office).

    Something else I would try, is see if any of your spare unused transmitters are accidentally on the same frequency as the receiver you’re using? As that seems very likely to me, even though it requires a few unlikely things to happen: the freq change, and for the spare transmitter to be sitting still turned on while in your bag (as obviously simply turning it off would immediately fix this). Does sound like you’re on some casual amateur indie film set too where people might be mucking with your equipment (a BIG NO NO NO!), which increases the odds this might have happened…. (although, later on when you used all six wireless I’m surprised this problem didn’t pop up again? Unless you did another rescan, which would have moved off from being on the conflicting frequency it was before)

  • Tom Galli

    January 15, 2020 at 7:45 am

    Aloha David,

    I didn’t do any troubleshooting other than what I mentioned; there was no time! The theory of having two units on the same frequency crossed my mind, but the units had been in constant use together for a day and a half without issue, and were used together again later. I did check the batteries on the units that seemed to be misbehaving, but they were nice and strong.

    I might have turned something off and on again, but I don’t recall doing so. Again, we were holding a scene while I tried to get things up and running, so “make it work” was priority one, and “figure out what happened” had to wait for a break. By the time the break came, the issue had self-corrected.

    As far as people mucking about with the gear, I suppose it’s possible that the talent got bored waiting for her cue and decided to press buttons on the transmitter, but everything else involved was in arm’s reach: the recorder, the receivers, the unused transmitters. Seems unlikely, but if that did happen, then after I swapped mics on her, it should have remained mucked up until I discovered the problem.

    I’ve got 2 full weekends of shoots coming in Feb with the same rig, so I’ll be setting it up before then and trying to duplicate the hiccup. I don’t like not knowing!

    The difference between theory and reality is that, in theory, there is no difference between theory and reality.

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