Automatically IDing compressed audio

We've talked about identifying compressed audio by
looking for high frequencies in spectrograms. This new
RadioWorld article shows a way to analyze files in bulk,
programmatically, so you can sort your library by
likeliness of being uncompressed.

This approach does require some light coding. I haven't
had a chance to try this yet, but it should be a
fun experiment over the holiday break. If I get it
working, I'll share my findings.


RadioWorld: What's In Your Audio Library

Edited by eric_a

The easiest way is to look at a real-time spectrum analysis graph while the song is playing. MP3 has only one scale factor for frequencies above 16 kHz, so there will almost always be a visible transition at the 16 kHz mark, with the spectrum graph becoming very choppy and erratic above that point. Or some older, cruder MP3 encoders simply filtered out everything above 16 kHz, especially at lower bitrates.

MP2 (MPEG I Layer II) actually handles high frequencies more gracefully, and will show the spectrum graph dynamically altering its bandwidth in approximately 0.5 kHz increments. Many radio stations still have all or part of their music library in MP2 format, because it is deemed to be "broadcast-quality" (at least at higher bitrates), while MP3 is not.

M4A (AAC) encoding is much more difficult to identify based on a spectrum analysis, because it usually preserves the entire audio spectrum up to 20 kHz quite accurately. Unfortunately quite a few tracks on iTunes were transcoded from MP3 to AAC, because although Apple recommends that artists and publishers upload their music in uncompressed or lossless form, it does not require them to do so. (And even if they did require music to be submitted in a format like WAV or FLAC, people could just cheat and convert their MP3s to WAV first.) The telltale signs of MP3 encoding will still be visible even after it has been transcoded to AAC.


Edited by VWestlife

MediaInfo will tell you exactly what they are:

https://mediaarea.net/en/MediaInfo

And sometimes a corrupted MP3 file will show up as blank in an audio editor. MP3Diags will fix any problems with the file:

http://mp3diags.sourceforge.net/


Edited by VWestlife

Thanks for the analysis of this Mike. I've come up with better rips of most of these songs now, my question was more out of
curiosity as to why I could play, but not edit them. You're the first one who's been able to answer that.