Humbug forensics

Humbug_forensics is a little python utility for removing and faking electrical frequency analysis foresnic data. Wait, What's this? Faking forensics?

When you run a mains powered recording device, or merely sit too close to a loaded power line with your ithingy, a small amount of A/C hum is picked up by the recorder. This hum comes from the alternating current of our electrical distribution network. And its not a perfect tone, the actual pitch varies somewhat based on the condition of the electrical grid.

In adversarial proceedings, like criminal cases, A/C hum is often used to establish the time a recording was made and whether the recording was edited. The theory is if a recorder was stopped and restarted, the hum on each side of the discontinuity would match a different time of day. This can be detected; its absense is used to "prove" that records of confessions were not doctored.

Except its easy to doctor. All you need to do is apply a narrow bandgap filter to remove the original hum and reinject new, fake, hum data into your audio signal. In fact, its so easy that I've created a program that does it for you.

humbugger --bandgap=60 --start="2013/8/1 12:00" --intensity=100 confession.wav parallely_constructed_confession.wav

Applied above, humbugger will remove 60Hz hum data from confession.wav and reapply hum data record from the US East Coast power grid at noon on August 1st, 2013.

Installation

humbug_forensics uses libsox, so you'll need to install that on your machine.

Ubuntu users:

sudo apt-get install libsox

Macos brew users:

brew install libsox

Once finished

pip install humbug-forensics

Cavets

humbugger uses data from http://dagrid.us - the hardware used to collect this data is downright sketchy and it doesn't have the best resolution or accuracy. Its entirely possiable this data doesn't match the hum data held by police departments, so a third party ENF foresnics expert may wind up matching your audio files to a completely different time than what you specified on the command line.

The point of this project isn't to reliably fake A/C hum data for fradulent purposes. The point here is to show that A/c hum data is easy to remove and manipulate and shouldn't be considered proof of anything.