Here's a fun one. I'm involved in maintaining the audio system for an auditorium used by a non-profit. After a flood and remodel, including replacing some audio components (like microphones), it was observed that the microphone on the main podium always had a 60 Hz hum. The hum depended on where the microphone was facing. Sometimes it was there, sometimes it was not.
Being a non-profit facility, there are no fancy DSPs to notch out the hum or anything like that, so more creative solutions were investigated. It was determined through dumb experimentation that orienting an identical microphone 180 degrees to the one with the hum and setting the gain similarly would nearly eliminate the hum.
Eventually, the working theory became that a relatively new large pad transformer installed across the street was being picked up by the microphones.
Orienting one microphone 180 degrees from the other caused the hum to be picked up out of phase from the main, and thus could be mixed in to cancel out the main mic hum.
Ultimately the real solution was simply buying better microphones, but there was a period of some months while a microphone sat off stage, pointed backwards.
It's been quite a few years since I last worked in live show production, but on any show at a venue where we couldn't be sure of access to clean power, humbuckers (not the guitar pickups, a nickname for what I believe was just a dumb 60hz notch filter or ground loop isolator inlined on house power taps) were a standard pack out in the road kit.
I would have expected that this decades-old and well established component of power infrastructure would have been commoditized by now and integrated into any dedicated AV performance/production space such as an auditorium.
> what I believe was just a dumb 60hz notch filter
A simple notch filter won't fix 60 Hz hum. Or, at least, when I've tried to eliminate annoying 60 Hz hum in my own amateur recordings, it's never been very effective. The problem is that the 60 Hz hum isn't a sine wave. It's more like a square, so you've got a bunch of harmonics up the frequency spectrum to worry about too. You can try to also notch out 120, 180, 240, etc. but it starts to get weird sounding fast.
I can confirm this from experience. In addition if there are any professional sound engineers involved it's likely a coincidence due to lack of funding.
With the orchestra spread out on stage from left to right, and a bi-directional mic overhead with the capsule facing left-right, you get a channel which is largely the difference between what a listener at the same position would be hearing from each ear. Not exactly, but something like that. So not very listenable on its own.
On the same pole facing down at the entire band, you have the omnidirectional mic trying to capture the whole thing as good as possible, suitable for live broadcast from this other channel alone. They didn't have stereo radio yet anyway.
Afterward back in the studio, starting with only the two channels on reel-to-reel tape, the "difference" channel can be phase-inverted to an auxiliary tape, then you have three channels suitable for mixing.
And with analog techniques like this they can be mixed "down" to 5.1 surround sound.
From a single mic stand and only two live signals.
The history of balanced lines (common mode rejection, differential pair, etc.) is fascinating. Apparently the first twists (as in "twisted pair" to pick up external interference as similarly as possible on each conductor) were achieved not within a bundled cable, but between utility poles. Every two spans would constitute a full twist, with two single wires alternating from left to right on the cross member.
But as for the acoustic implementation, even that has a long history. The Dead borrowed the idea from fighter pilot headsets, the only difference being that pilots were contending with a noisy cockpit rather than feedback. Same general idea that the unwanted sound hits both mics somewhat equally while the voice hits both mics somewhat unequally.
Therein lies the difference between Cat5 and Cat5e: Cat5 pairs were all twisted at the same frequency, while Cat5e pairs must not be twisted at the same frequency.
(This is also how we were able to transition so quickly and cheaply from Cat5 to Cat5e: All that was needed to produce Cat5e on a line that had been producing Cat5 was just a relative speed change on the machines that twisted up the individual pairs.)
