How to upload mp3 with custom duration/length

Does anyone know the right approach to change the duration/length of
an uploaded mp3 file? User has the option to upload mp3 file as full,
30 secs or 90 secs. So far, I have gotten it to work for full.

Do I have to use a processor (with ffmpeg) for this? Or is there a
built-in option in paperclip for this to happen?

Tried googling around to no avail....

By the way, if user selects 90 secs (for example), then I need
Paperclip not to upload the original. The same goes for 30 secs. In
theory, its one file or the other.

Your suggestions / feedback are welcome

Note that I am using Paperclip for all my uploads ...

This is what I have. When i submit the form, the page just hangs. No
errors, no nothing.

model:
  has_attached_file :media,
                    :styles => { :short => 'k128' },
                    :url => '/assets/:id/:style.:extension',
                    :path => ':rails_root/public/
assets/:id/:style.:extension',
                    :processors => [:process_audio]

Checkout http://blog.ncodedevlabs.com/2010/01/14/extracting-audio-length-using-rails/

I got mine to work with the following code, incase future devs stumble
across a similar situation:

module Paperclip
  class ProcessAudio < Processor

    attr_accessor :geometry, :whiny

    def initialize(file, options = {}, attachment = nil)
      super
      @file = file

      unless options[:geometry].nil?
        if options[:geometry] == 'full'
          @geometry = ''
        elsif options[:geometry] == '30'
          @geometry = '-t 30'
        elsif options[:geometry] == '90'
          @geometry = '-t 90'
        end
      end

      @whiny = options[:whiny].nil? ? true : options[:whiny]
      @basename = File.basename(@file.path, File.extname(@file.path))
    end

    def make
      dst = Tempfile.new([ @basename, 'mp3' ].compact.join("."))
      dst.binmode

      cmd = "ffmpeg -i #{@file.path} #{@geometry} -acodec copy -y
#{File.expand_path(dst.path)}"

      begin
        success = Paperclip.run(cmd)
      rescue PaperclipCommandLineError
        raise PaperclipError, "There was an error processing the
preview for #{@basename}" if whiny
      end
      dst
    end
  end
end