Question

Original Question

I want to be able to generate a new (fully valid) MP3 file from an existing MP3 file to be used as a preview — try-before-you-buy style. The new file should only contain the first n seconds of the track.

Now, I know I could just ‘chop the stream’ at n seconds (calculating from the bitrate and header size) when delivering the file, but this is a bit dirty and a real PITA on a VBR track. I’d like to be able to generate a proper MP3 file.

Anyone any ideas?

Answers

Both mp3split and ffmpeg are both good solutions. I chose ffmpeg as it is commonly installed on linux servers and is also easily available for windows. Here’s some more good command line parameters for generating previews with ffmpeg

• -t <seconds> chop after specified number of seconds
• -y force file overwrite
• -ab <bitrate> set bitrate e.g. -ab 96k
• -ar <rate Hz> set sampling rate e.g. -ar 22050 for 22.05kHz
• -map_meta_data <outfile>:<infile> copy track metadata from infile to outfile

instead of setting -ab and -ar, you can copy the original track settings, as Tim Farley suggests, with:

• -acodec copy

Answer 1

I also recommend ffmpeg, but the command line suggested by John Boker has an unintended side effect: it re-encodes the file to the default bitrate (which is 64 kb/s in the version I have here at least). This might give your customers a false impression of the quality of your sound files, and it also takes longer to do.

Here’s a command line that will slice to 30 seconds without transcoding:

ffmpeg -t 30 -i inputfile.mp3 -acodec copy outputfile.mp3 

The -acodec switch tells ffmpeg to use the special ‘copy’ codec which does not transcode. It is lightning fast.

NOTE: the command was updated based on comment from Oben Sonne