Need help identifying strange flutter distortion.

Question

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:

[soundcloud]rcoronado/strange-flutter-distortion[/soundcloud]

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.

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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.

Any thoughts?

Answer 1

Barring equipment failure, it looks like you managed to place your mics in the path of a microwave dish. They are still used to transmit data long distances (like from a radio station studio to its towers). Good news is they usually only transmit for short periods throughout the day, so you might be able to avoid it on the same stretch. They also travel in a narrow band, so moving your physical location a bit could help.

Did you not hear there interference when you were monitoring the sound?

I would also recommend using starquad cables for extra protection from EMI in the field.

Answer 2

Well, first off, keep the recording, because it's a really cool effect. Who knows when you'll find a use for it.

I assume you've tested the mics going into a different recorder? Have you also tested the cables? Did you test the VP88 going into the 744? You've probably done all of these by now, but I had to ask.

It's kind of hard to trouble shoot without that kind of info. Honestly, my first impression is that it does sound like some sort of digital artifact/interference. I could be dead wrong on that, obviously. If it is, it could be an issue with the recorder (testing other mics and cables in a different location will confirm/debunk that), or it could be some sort of transmission you're picking up. It hasn't been all that long since the digital changeover, so who knows what changes are still taking place in the various frequency bandwidths.

Keep us posted on this one, I'm really curious about it.

Answer 3

Huh. That is pretty darned weird. I am leaning towards it being a signal you're picking up instead of something else. Seems like EMI rather than RFI, maybe.

Were you close to any place that would have older wireless signals, be they wireless internet networks or low-spectrum wireless phones? Any cell towers nearby? Any wireless devices you had on your person or in your gear, like phones? Where was your car during this recording, and was it idling?

Absolutely wire up a test in EXACTLY the same way with EXACTLY the same cables. Then remove one mic at a time, then the cable, and see if the sound stops. I could potentially see this being an on-board problem, but I can't image what component would cause it.

Answer 4

Power? Did you have low/dead batteries or absence of phantom power. Underpowered gear can do some pretty odd things.

On a side note the SM-81 is a pretty sensitive mic. I associate it with recording acoustic guitars and other delicate sounds in studio conditions. Have you used this setup before?

Answer 5

I will admit first that I have not had a chance to listen to the recordings yet but I will do that once I am at my studio again tomorrow.

How far away are the nearest train tracks? I see the 81's have no shock mount so a train even miles away can produce low frequency vibrations through the ground that would be picked up on those mics without shock mounts.

So, I may completely amend this note tomorrow once I have listened to the tracks but I'm just throwing that out there.