integrity of a digital recording

April 24, 2012 2:19 PM

I have a digital recording of spoken voices. I need to know if the recorder was turned off and back on again during the conversation. Is there software for the Mac that will do that?
posted by Capri to Computers & Internet (7 answers total)

Best answer: You can't prove that it wasn't -- someone with digital editing software could easily clip out the clicks and whatnot, and line up the waveform perfectly (or crossfade the adjacent tracks a bit to accomplish the same thing) -- but you might be able to find out if it was.

You'd download Audacity or some other freeware digital audio workstation (or pay for one), open the file inside it, and look at the suspected interruption. You'll have to zoom waaaaaaaaaaay in, and look for a spot in the waveform where the line jumps instead of moving smoothly.

Best if you reach out to someone with expertise in the subject, as you might interpret something that isn't an edit as an edit. Also best if you remember that not finding the jump doesn't mean the audio wasn't stopped/started or re-edited.
posted by davejay at 2:25 PM on April 24, 2012 [3 favorites]

As well as looking for sudden clicks, one approach is to measure the average ambient noise across the entire signal. Then look at average noise in windows across your signal (say, every five seconds), to see how much difference there is between the noise in the window and the noise in the whole signal. Look for a large plateau-ing or valley-ing in the average level of noise going from one window to the next, which could signify a point where two segments of audio were joined. Even if the jump sounds smooth to your ears, and even if crossfaded, there could be enough of a difference in the average ambient noise levels to highlight where two disparate clips were joined together.
posted by Blazecock Pileon at 2:45 PM on April 24, 2012

If someone is looking to hide the editing, skilled audio editors can place ambient sound underneath different portions to mask it. So, unless you know the people working on the audio were clumsy/uneducated, there's no foolproof way to tell on this one.
posted by amoeba at 2:51 PM on April 24, 2012

Response by poster: I'm not concerned that this recording has been edited subsequent to when it was made, or that I've got to prove anything in court. I just don't want to assert that it was turned off and on again if it wasn't, and want to be reasonably sure about what actually happened -- not FBI level accurate.

I've used Audacity and found that where I think there's a break, I get a flat line with a tiny blip in the middle that sounds like a hiss. I think the recording was interrupted there because it sounds like the speaker (who is also the one at the controls) finishes a sentence, then there's a background speaker who sounds like he's cut off, and then silence, and then the main speaker appears to start in the middle of a word and the background speaker is making an S sound like he just finished saying a word that ends in S. But the quality of the recording is dreadful, so what I've just said is not obvious on first (or even 12th) listening. It's just what I think is happening. I was looking for a visual way to see what happened. Audacity doesn't show me anything super obvious, just a flat line. This isn't the only flat line in the recording, so I still don't know. (I'm also having some trouble figuring out how to use Audacity, due to my total inexperience with this sort of thing.)
posted by Capri at 2:55 PM on April 24, 2012 [1 favorite]

Best answer: A flat line says that there was *nothing* in the recording for that period. There is always room noise (in fact, on any shoot, the first thing you do is turn on all the lights and gear, and ask your actors and crew to stand dead still for for a minute so that you have some reference "silence" to dub in in case you need it for background for ADR later).

So, yes: If you have a block in your recording that goes completely flat, that's an indication that for that period someone unplugged or muted the mic, or did something similar to interrupt the recording. There should always be a little wiggling to that line.
posted by straw at 3:01 PM on April 24, 2012 [1 favorite]

All the more reason to loop in someone with more experience. Unfortunately, even with that expertise, the best you may be able to assert is "I suspect the recording was stopped and started, because X." And no software solution can automatically discover that suspicious waveform for you.
posted by davejay at 3:39 PM on April 24, 2012

A note on the flat line means silence thing -- is there any chance the recorder was one with automatic gain control, that mutes the microphone after a short period of silence, then picks it up again when someone starts talking again? If so, it would sound as you describe as well.
posted by davejay at 3:40 PM on April 24, 2012 [3 favorites]