"Brian Gaff" wrote in message
After all take an unscreened dynamic mike into many places and you can
hear hum and whine being picked up by the capsule.
I remember encountering this when I was about to record the speeches at my
sister's wedding. I'd made up a long length of screened cable with 3.5 mm
socket/plug to extend the output cable of my electret microphone that was on
the table, so the camera could be a reasonable distance away. I'd tested all
this just before and everything was fine. At the end of the meal, when the
speeches were about to start, I turned on the camera and got a horrendous
buzzing which was only present if the camera was powered from the mains and
not if it was powered by battery. Luckily I had a fully-charged spare
battery which lasted for the speeches, and I hastily put the other battery
on charge for all the "afterwards" shots of the guests chatting, the dancing
etc. The perils of unbalanced rather than balanced microphones and cables...
Moral: Always have spare batteries even if you expect to work off mains. No
matter how many times you test beforehand, always expect a last-minute
I'm still not sure what had changed between my earlier testing during
setting up and the actual event an hour or so later. I don't think any extra
lights had been turned on (fluorescent tube interference).
That's slightly different from the dynamic microphone problem that you
describe, which is apparent if you try to record sound from the telly by
putting a dynamic mike near the speaker (real low-tech stuff) and the
scanning circuitry induces sharp 50 Hz buzz and noticeable 10125 or 15625 Hz
line whistle (for 405 or 625 line TV).