“Looping” an audio file with Sox, Lame and mkfifo

Today I needed a very long (3 hour) MP3 audio file to use for an experiment; a test file with some music on it. My first thought was to start a MP3 audio recorder, turn on the radio, and leave for 3 hours.

But impatience is among the three great virtues of a programmer, so I turned to Google instead, seeking command line tools for audio manipulation. It turns out that sox and lame will do the job. I installed the tools – here is the Debian / Ubuntu invocation:

apt-get install sox lame

then grabbed an MP3 file of a piece of music (Peter T. Noonan, album “Cafe at Arles”, track 9 “Life’s Old Road”, if you are curious) and repeated it a few times:

sox music.mp3 foo.wav repeat 3

lame foo.wav longmusic.mp3

This worked well, the first time… but consumed a lot of disk space. To get to 3 hours I would need enough disk space for a 3 hour uncompressed WAV file. Unfortunately Sox does not support MP3 output, and I didn’t want to compress to a format it does support, then uncompress and recompress again. So I used a Unix/Linux FIFO pipe instead of a file, with Sox running in the background to fill the pipe with data for Lame:

mkfifo foo.mp3

sox music.mp3 foo.wav repeat 10 &

lame foo.wav longmusic.mp3

a little while later, longmusic.mp3 is a very long MP3 file… but not long enough, because sox fails when the “virtual” WAV file it is writing reaches 2 Gb in size, just as it fails with a real WAV file at that size. That was about 1 hour and 41 minutes – not long enough; so I ended up looking elsewhere:

The Ugly Hack

It turns out that Lame will tolerate an MP3 which consists of several appended MP3 files as its input. It complains but keeps processing when encountering the extra headers in the middle of such a file. So this solution with cat, a pipe, and Lame, worked:

cat 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 1.mp3 | lame – longmusic.mp3

A little while later, I had a 3 hour long, valid MP3 file.