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Sound ghosties

Discussion in 'Sound, Music, and Intercom' started by CynicWhisper, Mar 31, 2008.

  1. CynicWhisper

    CynicWhisper Member

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    So I work in a church and randomly today, during the sermon, I wasn't even touching the sound board (truthfully I was reading Stephen King under the counter) and all of a sudden the volume jumps and feeds back. I've been working there two years and this has never happened before.

    The only difference in the system today was that I was running a new double tape deck (gag, tapes) and had left the tape in knob turned up even though there was no feed running through from the tape. Now I would understand if the feedback sound jump was instantaneous, but the system had been acting perfectly normally for a good ten minutes. I run an Ev m2 board (craptastic) and akg wireless body mics.

    Any ideas for the reason for this random jump?
     
  2. museav

    museav CBMod CB Mods Departed Member

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    Did it jump and stay or was it momentary? It could be caused by a number of things from intermittent wiring to some type of processing to simply a weak battery in one of the wireless mics.

    If the tape deck is wired such that it could cause a feedback loop then that could be a problem but it would tend to be there all the time, although any changes in the levels on the deck or mixer could make a difference. Or it could be something like the deck setup for serial playback between the two wells and at some point it switching to the well where you had the input up. But these are just guesses without knowing the deck or how it is wired and run.
     
  3. FMEng

    FMEng Well-Known Member Fight Leukemia

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    The cause of the level jump was a brief fade in the wireless mic signal RF. The reason for the level jump and feedback is that the wireless system's audio companding lost tracking when the signal faded.
    Companding is used in wireless systems to lower the noise floor of the audio. Inside the beltpack, the audio is compressed. At the receiver, the audio is expanded back to normal. This is a great thing if the compression and expansion track together. When they don't track, the result is a change in audio level.
    http://en.wikipedia.org/wiki/Compander
    In your case, something caused the RF to fade for a moment. Maybe the person with the mic moved to just the wrong spot where a null occurs. The battery could have been getting low, which lowers the transmitter strength. Maybe a car with a dirty CB transmitter drove by. Or the lady wearing the tin foil hat shifted in her seat.
    I once had un-explained drop outs with a particular pastor at my church, but nobody else. I finally discovered that he stuffed the wire antenna of the belt pack transmitter in his pocket, which was full of keys and coins. That detuned the antenna, which lowered the signal strength. Once I told him to let the antenna drape naturally, no more problems.
     
  4. CynicWhisper

    CynicWhisper Member

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    The battery was strong. The tape deck just appeared today, apparently we got it used off of someone and personally, I think it's a cheap pos, but I had to use it since I had to play a tape and record at the same time. It had rca in and out, that's it. I also don't know what would have happened had I not taken down the slider momentarily and brought it back up. But I didn't bring down the tape in knob until a few minutes later, and nothing happened again.
     
  5. avkid

    avkid Not a New User Fight Leukemia

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    That doesn't matter at all.
    It wasn't the deck.
     
  6. museav

    museav CBMod CB Mods Departed Member

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    I'm by no means a wireless mic expert but as I understand it, companding is used to increase the dynamic range possible. At the transmitter the audio signal is compressed both upward and downward, in effect squeezing it towards the middle. This reduces the signal's dynamic range making it easier to transmit via RF with fewer interference problems. At the receiver the audio is both upward and downward expanded equal to how it was compressed at the transmitter, restoring the dynamic range and also having the benefit, via the downward expansion, of lowering any extraneous noise picked up during transmission. As far as I know, in many wireless mic systems the compander's transmitter compression and receiver expansion ratios are either fixed or vary based on the audio levels.

    Companding can certainly have audible effects but a fixed ratio system would always have the same ratio, and thus would not seem capable of causing a jump in level. An audio level dependent, variable ratio system usually applies increasing compression and expansion with increasing signal levels, so if the receiver lost signal it would seem to then apply minimum expansion rather than maximum expansion. So I'm obviously missing something, can you explain further how one could lose compander 'tracking' for a wireless mic system and have that result in a sudden increase in output level?
     
  7. FMEng

    FMEng Well-Known Member Fight Leukemia

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    You are correct that if the receiver loses the signal completely and mutes, then the expander would essentially turn down the gain.

    In this case, what happend is that the RF dropout caused a noise burst, which fooled the expander into thinking it was being fed louder sound that it needed to open up for, increasing the audio gain substantially. The noise burst was probably very brief but long enough to confuse the level detector. The feedback lasted longer because the release time is slower.

    This is why it's import to pay attention to the modulation level of the transmitter, and keep it a high as practical. If the normal modulation level is good, then a noise burst won't have as much effect.
    I hope I'm making sense.
     

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