Just thinking about it, couldn't you get an isolated recording of the hum, and then mix the inverse of it over the appropriate audio sections? That should leave you with a humless track.
- step 1: Set up a recorder in a relatively isolated environment, ensuring that the hum is being recorded.
- step 2: Record a 30 minute conversation with the target, ensuring that you have enough to splice together something incriminating.
- step 3: invert the hum recorded from step 1 and mix it into the track from step 2. This produces 30 minutes of humless audio with the target.
- step 4: edit the 30 minute recording to produce incriminating audio clip of about 30 seconds.
- step 5: overlay legitimate audio from 30 seconds of real hum onto faked recording.
It seems like it could work.
- step 6: Get caught for something much simpler that you overlooked, and ruin your life.
Please don't actually use this maliciously. I suppose it could be a decent defense in court if you could prove that it is possible to fake the hum.
If you've ever tried to do this you'll know that while it works in theory, getting it working in practise is much harder than you'd think. In the past I've tried to create acapellas using a song and its instrumental (both digital recordings taken from the same source) using this technique and even with the highest possible quality digital audio the results are mixed at best.
It wouldn't entirely eliminate the hum, but a balanced feed (note that XLR can be unbalanced as well) will reduce the EM interference and cross talk that adds to the background hum.
Another contributing factor is when power supplies to the equipment are different or "dirty". Often this can be resolved with something as simple as a multi-adapter with the earth pin disconnected (note, this shouldn't be attempted unless your hardware is already insulated. but then if you're trying to commit a crime, then an electric shock is likely preferable to life sentences in jail).
If the hum varies over time, I don't think you'll be able to perfectly invert it with a separate sample very easily. You'd probably have to simultaneously record it elsewhere (but close by) and you might still get artifacts--though those might end up missed in the overlaid track.
Probably doable, but a lot harder than your method.
Made me think, what about audio compression? Psychoacoustic algorithms might want to alter the signal, or cut off the some of the frequencies entirely. This could make the record neither valid nor invalid according to this method, since the hum would be too altered to be a valid information source.
I infer from the article that it's not a perfectly consistent background noise across the entire grid, but rather that it's consistent enough to get a match as long as the recording is long enough. As such, even if you did have access to their recordings, or made recording of your own, you still wouldn't be able to just invert it like that.
>which is, as the article states, a challenge even for recording professionals
Actually, it calls it an "annoyance", which is a good characterization of it. And that's for professionals trying to make very high quality recordings where they accurately capture the output of instruments within that frequency range while eliminating the mains interference in the same range.
If you're recording someone's voice on a portable recorder that's in your pocket or stashed somewhere in a room, you're not likely to capture much in terms of voice in that range in the first place. Even if you did, it would still be plausible that you didn't. So if you tampered with a recording and then replaced everything under 70 Hz with ambient noise recorded separately (including mains noise) in the same location, it would be pretty difficult to prove tampering.
A few minutes after posting that, I realized that a band-stop filter would work fine for spoken word. However, that might add a different set of tell-tales to the recording.
