Forum: Ruby Rendering an mp3 as a waveform image in ruby??

73c04e9ef9ca435c5b19a2e765ae6d20?d=identicon&s=25 Max Williams (max-williams)
on 2008-07-24 17:35
Does anyone know of a way, in ruby, to render an mp3 into a waveform
graphic?  Like you'd see in audio editing tools like Audacity,
Soundforge etc.  Just a jpeg as the end result is fine.  Hoping there
might be a gem out there for this...

thanks
max
05be5d6610e2c3f1780aa0e39e902e93?d=identicon&s=25 Farrel Lifson (Guest)
on 2008-07-24 20:34
(Received via mailing list)
2008/7/24 Max Williams <toastkid.williams@gmail.com>:
> Does anyone know of a way, in ruby, to render an mp3 into a waveform
> graphic?  Like you'd see in audio editing tools like Audacity,
> Soundforge etc.  Just a jpeg as the end result is fine.  Hoping there
> might be a gem out there for this...

I don't think there's a Ruby gem to allow direct decoding of mp3
files, however it should be doable using mpg123 or some other program
callable from the shell which can decode mp3's.

The command  'mpg123 -O file.raw file.mp3'  produces raw 16bit stereo
PCM data (I think sampled at 22KHz) from the .mp3 file and stores it
in the .raw file. So for every 4 bytes you read the first two bytes
would be the signal strength of the one channel and the second two
bytes would be the signal strength of the second channel. You could
then use String#unpack (I think 'ss' will unpack to shorts, which were
signed 16bits last I checked) to decode these bytes and get the values
you need. You could then plot these values using a gem like RMagick. I
haven't executed this code but it could look like:

File.open('file.raw') do |file|
  while !file.eof?
    first_channel_data, second_channel_data = file.read(4).unpack('ss')
    # Plot data here
  end
end

Farrel
73c04e9ef9ca435c5b19a2e765ae6d20?d=identicon&s=25 Max Williams (max-williams)
on 2008-07-25 10:40
Ah, that's useful, thanks farrel.
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