I was out recording my car last week both for the purposes of scouting a good strip of road and practicing my auto coverage technique. When I got back to the studio I discovered that one of my tracks came back covered in a strange strange fluttering distortion, and I can't figure out what would have caused it.
Here's a pic of the setup:
(Side note: after coming back into the studio I wasn't entirely happy with my mic placement - I felt there was too much overlap in the perspectives. In the future I'll try to have shotguns for the side mics and will position them a few feet away from the main XY or MS setup.)
What you're looking at is a VP-88 plugged into a Zoom H4 sitting in between a pair of SM-81s plugged into a 744t. The audio from the VP-88 came out great sounding, but the SM81s ended up being unusable due to the strange fluttering on them.
Here's a (low volume) clip that is unaltered from the record:
I threw this clip into izotope to have a looksee, and saw this:
As you can see, the distortion is pretty evenly spread throughout the entire frequency spectrum, even running up past the ultrasonic range that my 96k recordings were made at.
when I zoomed in I saw this:
The left and right channels seem to have no relation to one another with regards to pattern, and its a clearly modulating signal on narrow but not static intervals.
My own thoughts are that its not likely to be the microphones or cables themselves because of how uniformly it is affecting both channels. It could potentially be some kind of radio interference in the air, but the fact that the VP-88 recorded clean with no hint of that kind of thing causes problems with that theory. It could also potentially be the 744t itself (I haven't taken it back out yet) but I don't really think it would be anything like the pre or A/D converter making this kind of thing.
I've never really heard distortion like this before, so I'm curious about what may have caused it. I really like this strip of road for recording purposes, but if this is caused by something being transmitted in the air I'll have to take it off my list.