Making music previews

This site uses a lot of music ‘extracts’ or previews to make sense of its contents. Naturally, they come from assorted commercial audio recordings -and therefore might trigger copyright claims. To avoid that, and to be able to put up a ‘fair use’ defence should one be made, it’s important that the previews are algorithmically produced: that is, they should be short previews of modest audio quality, no exceptions. The moment I start letting a preview go on a bit, in order to capture this or that lovely moment, I run the risk of breaching fair use guidelines. Similarly, if I up the audio quality to be lossless flac instead of 128kbps MP3, the record companies would be justified in going for my jugular!

In short: I need to produce all my music previews in an automated fashion that is consistent and predictable. In other words, it needs to be scripted! And here’s the script I use:

find . -name "*.flac" -print0 | while read -d $'\0' file; do
  filename=$(basename "${file/.flac}");
  directory=$(dirname "$file");
  ffmpeg -t 40 -i "$file" -af "afade='t='out:st='35:d='5" -codec:a libmp3lame -b:a 128k "$directory/$filename-preview.mp3" < /dev/null ;
  rm -f "$file";

It runs on Linux, obviously. It’s run in the parent directory of whatever set of folders contains the full-length FLAC tracks I’m hoping to convert to audio previews.

The critical line is the one starting “ffmpeg…”: that uses the ffmpeg utility to extract a 40-second audio sample from the full-length FLAC original, starting from the very beginning of the file. It converts it into an MP3 track at 128kbps, which is about the minimum MP3 sample rate you can use for music tracks -so the quality of the preview is guaranteed to be as low as it can reasonably be and in no way a decent substitute for the lossless original. The “afade=t=out…” switch in the middle of the ffmpeg command ensures that there is a 5 second gradual fade-out of the audio, starting from the 35th second. That just avoids the music preview stopping abruptly when its allotted 40 seconds is up. Finally, the script names the new MP3 file to be the same as the original, but with a “-preview.mp3” extension… and for good measure, it deletes the original FLAC. (So this isn’t a script you want to run against your master music library!)

The script thus guarantees musical samples which are of fixed length, low quality and with a nice fade-out -all conditions which, I hope, will keep the copyright guys happy!