Detecting Edited Audio
Interesting development in forensic analysis:
Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording.
Philip Harrison, from JP French Associates, another forensic audio laboratory that has been logging the hum for several years, says: “Even if [the hum] is picked up at a very low level that you cannot hear, we can extract this information.”
[…]
It is a technique known as Electric Network Frequency (ENF) analysis, and it is helping forensic scientists to separate genuine, unedited recordings from those that have been tampered with.
Dr Harrison said: “We can extract [the hum] and compare it with the database – if it is a continuous recording, it will all match up nicely.
“If we’ve got some breaks in the recording, if it’s been stopped and started, the profiles won’t match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we’ll have two different profiles within that one recording.”
Guy • December 12, 2012 1:28 PM
Most video cameras and audio-recorders have some kind of 50/60hz hum filter to prevent problems. That would seemingly destroy this source of information, but probably there are harmonics of those frequencies everywhere, and any synchronous motors that can be heard in the background will also produce a harmonic.
The really elegant aspect of this is that there are millions of latent recordings of the power signal that span as far back as there are recordings of high enough quality. That means that you could potentially take news footage from the 70’s or even 80’s and correlate the hum with other videos from that time.
How long until YouTube builds a map of these signatures and automatically time/date stamps and roughly locates the video?