Sampling Short Sequences From Long MP3 Recordings?

mehl writes "I am a professor for social psychology at the University of Arizona and I am looking for help with finding / developing a special program. In my research, I ask participants to carry around a digital voice recorder while they go about their normal lives. The voice recorder then tracks the ambient sounds in their environments and produces an 'acoustic log' of a person's day. We then use these ambient sound recordings as source data for various person perception studies. For privacy reasons, we are required to sample brief snippets of ambient sounds instead of recording an entire day continuously ('Big Brother is listening to you...'). So far, we have achieved this by modifying the hardware of a digital voice recorder (triggering it with an external microchip). With the high turn-over in player models, however, this strategy has turned out to be short-sighted (every half a year we have to build a new chip). I am thinking about switching strategy, recording continuously in the first place (no problem with the current generation of flash memory) and then sampling (random) snippets after the fact from the continous recordings. Does anybody know of an existing program that can randomly (or pseudo-randomly; e.g., 30 sec every 10 min) and automatically sample short sequences from a day-long (18 hours) mp3 recording? What would it entail to develop such a program (for Windows)?."

Comments

Use mp3split

[mrAgreeable]

Sounds like a job for a really simple shell script driving mp3split. Sounds a lot easier than a custom chip!

This is really simple

[Anonymous Coward]

Jump to a a random offset, look for the sync-word, copy a number of frames, repeat. The MP3 format is made of frames, there is no per-file header and since the format is designed to be used in streaming applications and to be robust against errors, you can jump right to the middle and grab a couple of frames without worrying about the rest. Many webpages have the frame spec. Here is one.

Not difficult

[Effugas]

MP3 is a bitstream, so you can basically use the language of your choice to seek to arbitrary offsets, slice wherever you like for as long as you like, and whatever frames are broken will simply not get decoded. You may of course want to actually have on-frame-boundry edits (they generally sound better and play more reliably, especially on ipod which doesn't have great stream reassembly code). cutmp3 can work:

http://www.puchalla-online.de/cutmp3.html

There's lots of pure windows code to do this too:

http://www.programurl.com/software/cutter.htm

But if you want to code this yourself, there's some excellent Perl libraries for managing MP3:

http://search.cpan.org/~nuffin/MPEG-Audio-Frame- 0. 08/Frame.pm

(and most directly speaking to what you're working on)

http://search.cpan.org/~ilyaz/MP3-Splitter-0.02/ Sp litter.pm

It's not too bad to use Perl either, especially with the Perl Packager. Given only one host with the full Cygwin Perl install, you can create compiled executables that encapsulate everything you need down to a single file. It rocks!

http://search.cpan.org/~autrijus/PAR-0.85/script /p p

I imagine though that you'd eventually want to only analyze random chunks that contain speech, or at least speech like frequency distributions. This is trickier, and I don't know if there's Perl code to do it. Maybe you could investigate Praat's internal scripting language?

http://www.fon.hum.uva.nl/praat/

Praat is pretty mind-bogglingly cool -- it's worth checking out no matter what.

--Dan

P.S. Yes, I've been working on some mildly related stuff. How could you tell? :-)

Re:Java? Python? PERL?

[Fastolfe]

This made it on the front page? Lame.

Agreed..

In any case I couldn't imagine that it'd take more than half a day or so to do this in Java or Python.

Or five minutes in Perl with MP3::Splitter:

perl -MMP3::Splitter -e 'mp3_split($_,{},[ rand(64800), 30 ], ...) for @ARGV' filename.mp3