paulears Posted December 19, 2014 Share Posted December 19, 2014 Not the usual dropout scenario, where level drops and the signal gets cut off, but it is happening on multiple mics, and looking at the display things are a bit strange. Have a look.http://www.limelight.org.uk/dropout.jpgTowards the left are the pretty typical RF traces in Red and Green - the RF levels going up and down as the person wearing the pack moves around. Them just before 58 on the horizontal timeline, both traces flatline - no change in RF level at all. During these periods, the audio is usually fine, but sometimes cuts and restores. Quality is normal. You can see the squelch cut in when the level drops right out. We are using channel 38, plus a few extra licensed channels in the band above. This strange effect has only just started happening, and is found on all the receivers to a degree. A nice reliable and consistent performing system now is a bit tricky. We've swapped the aerials and the traces swap as expected and the steady sections still happen. The blank sections are the same on both aerials but at different levels, very similar to the difference in the good sections. We wondered if the DA was faulty - but if so, any induced noise there would be at the same level on both receiver channels, not staggered. Maybe some kind of wide band interference? This would make the difference in signal level make sense, but what could impact the usual ch 39 plus the channels in 657-8MHz? This has made us all think - and we have no idea what is happening. The display with it's flat line just makes no sense? So - anyone else ever had the same? More importantly, a solution! Link to comment Share on other sites More sharing options...
peza2010 Posted December 19, 2014 Share Posted December 19, 2014 Whats the rough distance from Aeriel to packs??and are they paddles or standard aerials? Ive had a few strange problems with paddles fixed by removing the boosters from the paddle feeds. Link to comment Share on other sites More sharing options...
paulears Posted December 19, 2014 Author Share Posted December 19, 2014 Verticals, with reflector - and pretty well just a stage width at most. Link to comment Share on other sites More sharing options...
chelgrian Posted December 19, 2014 Share Posted December 19, 2014 So - anyone else ever had the same? More importantly, a solution! We had a very similar issue which was eventually traced to the mast head amplifiers we had installed clipping. What was very odd is that they had been installed for several years with no issues then suddenly started causing a problem. We removed the amps as we found they were not necessary and everything worked fine. All we can think is that it was marginal before and for some reason there was a slight voltage drop in the power to the masthead from the ASA and it started to clip. Link to comment Share on other sites More sharing options...
TimmyP1955 Posted December 25, 2014 Share Posted December 25, 2014 Have you run the Senny software to make sure that there are no intermodulation problems with the channels you are using? Link to comment Share on other sites More sharing options...
paulears Posted December 25, 2014 Author Share Posted December 25, 2014 yep - and using Sennheiser's suggested frequencies - it's now been clear for two days so hopefully whatever was causing it has moved on! Link to comment Share on other sites More sharing options...
paulears Posted December 27, 2014 Author Share Posted December 27, 2014 I'm not sure but I may have found the culprit. Walked through FOH, taking a short cut and my IEM dropped out, and I could hear a faint 'whine'. I tracked it down to a display of those fan things with LEDs in them, that they sell to the kids. Turning the three of them off, restored my comms feed. I wonder if a few are very RF nasty, and when the FOH people start up a nice display of them when the punters are around, some are noisy enough to produce the wide band noise floor increase we can detect. Not absolutely certain, and I need to fiddle a bit, but it may well be these that are responsible, and of course the kids in the audience have them, running rather close to the stage? Link to comment Share on other sites More sharing options...
pmiller056 Posted December 29, 2014 Share Posted December 29, 2014 Excellent bit of detective work! I presume any 'CE' marking on the toys is for 'China Export'. I'd guess that the completely unsuppresed twirly motors (for maximum cheapness) are a nice wideband sparkgap transmitter and the associated wiring is a multiple of 1/4 wavelength at the frequency that you are trying to use.... Peter Link to comment Share on other sites More sharing options...
paulears Posted December 29, 2014 Author Share Posted December 29, 2014 I haven't pulled one apart - perhaps a Big Clive job - but I assume there is some kind of chopped DC supply to the LEDs, via some kind of slip ring to make the whirling LEDs do strange patterns and stuff. This afternoon I was FOH again, and got loads of nasty buzzing, didn't think at the time, because I was somewhat stressed, but I'd bet this too was the damn things doing a slightly different thing. Link to comment Share on other sites More sharing options...
dbuckley Posted December 29, 2014 Share Posted December 29, 2014 If its those fan things I think they are, then they have a quite clever approach to make them very cheap; the power feed to the whirly bit is by an interrupted slip ring, so the little chip that runs the lights times the gaps to do the syncronisation. Link to comment Share on other sites More sharing options...
paulears Posted December 30, 2014 Author Share Posted December 30, 2014 That's almost certainly what these are, however, I haven't managed to 'borrow' one to open it up. Link to comment Share on other sites More sharing options...
Jon Wiles Posted January 5, 2015 Share Posted January 5, 2015 I had something similar back in '96 on a tour. There was a regular blat over some of the VHF channels we were using.Mentioned it to the house guys, who said that it started during panto and couldn't get rid. (we were there in the February. We did our fitup/rehearsals and opening night. Phoned the DTI (as it was then) and they arranged for someone to come in the next morning.They had a yagi and an analizer......cutting a long story short, it turned out to be a faulty fire exit sign in the Council building next door on the 3rd or 4th floor. Managed to persuade them to isolate it and all was then fine Link to comment Share on other sites More sharing options...
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