I recall a time, not too long ago, when the only form of photographic evidence admissible in a court of law was a Polaroid shot. This was a self-developing technique and as such could not be altered. With the current form of photography and digital images, a 14 year old kid could alter it and even give me Justin Beiber’s hair.
I wonder if we are now at the point where audio files, whether they are .mp3, .wav, or any other format, would come under suspicion of being altered and therefore not of any great forensic use?
With today’s spectral analysis software programs, regardless of how sophisticated they are, I can make a monkey talk, or someone say something that they never said. Most software programs have some form of synthesis and this can be as simple (and as time consuming) as building up a waveform from its constituent pure tone building blocks. Doing so would not leave any forensic clues behind.
This is not like the “olden” days when I was a grad student. In the 1970s and early 1980s, in order for speech or music to be altered, one either needed to physically cut the recording tape and re-attach it, or change the playback speed. In both cases, the tampering was obvious and a cut tape can be easily detected. And altering the playback speed changed everything from formant location to harmonic spacing. In short, in the 1970s and early 1980s what was recorded on a tape was undoubtedly really what was recorded- any tampering was quite obvious.
We now live in exciting times, and although it is pretty obvious that Justin Beiber didn’t lend me his hair, other subtle changes are not so apparent.
I was an expert witness in a murder trial back in the mid-1990s and I was charged with determining whether the person on the tape was the person who was charged. He wasn’t, by the way, but I suspect that he was there at the murder scene. It was easy to do a series of spectral analyses showing that the third formant (F3) of the accused low and front vowels was different from that which was on the tape. (The defense lawyer of course, would not have put me on the stand if they were the same).
Even back in the mid-1990s, a smart prosecuting attorney should have asked me about an altered audio file that could have been changed in order to get my client off. MP3 files have been around since 1993 but I guess that technology had not yet filtered down to the general public. The audio files were on tape cassettes but if I was unscrupulous I could have created an MP3 file of the vinyl tape, altered it, and then re-taped it.
I could have changed the location of the formant structure of my client’s voice (equivalent to changing the length of his vocal tract), I could have left the formant structure intact, but changed the fundamental frequency (voice pitch), I could have done both, or even treated each frequency region differently- of course he would have sounded like a Martian, but the point is, this could easily have been done in 1994.
Maybe it’s time to only accept audio Polaroids again in courts of law? Too bad we don’t (yet) have the equivalent of a Polaroid!