3

I have a set of mp3 files, some of which have extended periods of silence or periodic intervals of silence. How can I programmatically detect this?

I am looking for a library in C++, or preferably C#, that will allow me to examine the sound content of these files for the silences.

EDIT: I should elaborate what I am trying to achieve. I am capturing streaming sports commentary using VLC and saving it to mp3. When a game is delayed, or cancelled, the streaming commentary is replaced by a repetitive message saying commentary is not available. By looking for these periodic silences (or total silence), I can detect if there is no commentary and stop the streaming recording

For this reason I am reluctant to decompress the mp3 because if would mean my test for these silences would be very slow. Unless I can decode the last 5 minutes of the file?

Thanks Andrew

1 Answers1

1

I'm not aware of a library that will detect silence directly in the MP3 encoded data, since its not a trivial task to detect silence without first decompressing. Luckily, its easy to find libraries that decode MP3 files and access them as PCM data, and its trivial to detect silence in PCM Data. Here is one such Library for C# I found, but I'm sure there are tons: http://www.robburke.net/mle/mp3sharp/

Once you decode the data, you will have a list of PCM samples. In the most basic form, the algorithm you need to detect silence is simply to analyze a small chunks (could be as little as .25s or as much as several seconds), and make sure that the absolute value of each sample in the chunk is below a threshold. The threshold value you use determines how 'quiet' the sound has to be to be considered silence, and the chunk size determines how long the volume needs to be below that threshold to be considered silence (If you go with very short chunks, you will get lots of false positives due to samples near zero-crossings, but .25s or higher should be ok. There are improvements to the basic approach such as using historesis (which is basically using two thresholds, one for the transition to silence, and one for the transition from silence), and filtering.

Unfortunately, I don't know a library for C++ or C# that implements level detection off hand, and nothing immediately springs up on google, but at least for the simple version its pretty easy to code.

Edit: Also, this library seems interesting: http://naudio.codeplex.com/

Also, while not a true duplicate question, the answers here will be useful for you:

Detecting audio silence in WAV files using C#

Community
  • 1
  • 1
bdk
  • 4,769
  • 29
  • 33
  • I elaborated my question above that makes this solution less favorable - but I thank you for the suggestion as I hadn't thought of it and it might be my only option - AS –  Apr 17 '11 at 02:57
  • These guys are able to split up MP3 files without decoding, but deep in their manual they say that in order to detect silence, they do have to decode. Still, at least you'd be able to use this method to split off a piece of the mp3 and then only decode that http://mp3splt.sourceforge.net/mp3splt_page/ – bdk Apr 17 '11 at 03:17
  • Is the commentary always from the same site and always the same message? If so, they may just be streaming a canned mp3 in a loop and you can cheat and just do a byte-for-byte compare of the streaming signal to the known commentary – bdk Apr 17 '11 at 03:19