Changes the pitch of an audio file; uses ffmpeg but simplifies building the command.

change-pitch-on-video.rb

#!/usr/bin/env ruby

require 'awesome_print'
require 'optparse'
require 'ostruct'
require 'shellwords'

# Uses approach specified at
# https://superuser.com/questions/292833/how-to-change-audio-frequency/1076762#1076762

class PitchChanger

  BASE_FREQUENCY = 44100.0

  attr_reader :input, :output, :factor, :noexec

  def parse
    OptionParser.new do |parser|

      parser.on('-i', '--input_file INPUT_FILE', "Input file") do |v|
        @input = v
      end

      parser.on('-o', '--output_file OUTPUT_FILE', "Output file") do |v|
        @output = v
      end

      parser.on('-f', '--conversion_factor FACTOR', "Conversion factor (decimal)") do |v|
        @factor = Float(v)
      end

      parser.on('-n', '--noexec', "Don't execute, just output command") do |v|
        @noexec = v
      end

    end.parse!
  end


  def validate_options
    errors = []
    unless noexec
      unless File.file?(input)
        errors << "Input file not specified (with -i) or does not exist."
      end
      unless output
        errors << "Output file not specified (with -o) or does not exist."
      end
    end

    unless factor.is_a?(Float)
      errors << "Conversion factor relative to 1.0 not specified (with -f)"
    end
    unless errors.empty?
      raise "Error(s):\n\n#{errors.join("\n")}\n\n"
    end
  end


  def build_command_line
    ffmpeg_option = "-af 'asetrate=#{BASE_FREQUENCY * factor},atempo=#{1 / factor}'"
    "ffmpeg -i #{input} #{ffmpeg_option} #{output}"
  end


  def call
    parse
    validate_options
    command = build_command_line
    puts command
    `#{command}` unless noexec
  end
end

PitchChanger.new.call