beat_detection.rb

# Simple sound energy algorithm #1
# http://www.gamedev.net/reference/programming/features/beatdetection/

require 'rubygems'
require 'riff/reader'

file = ARGV.shift

class Wav
  def initialize(riff)
    @riff = riff
    raise if @riff.root_chunk.signature != 'WAVE'
  end

  def num_channels
    @nc ||= read_bytes(fmt_chunk.offset + 10, 2, 's').first
  end

  def sample_rate
    @sr ||= read_bytes(fmt_chunk.offset + 12, 4, 'I').first
  end

  def bit_rate
    @br ||= read_bytes(fmt_chunk.offset + 22, 2, 's').first
  end

  def data_speed
    @ds ||= read_bytes(fmt_chunk.offset + 16, 4, 'I').first
  end

  def block_size
    @bs ||= read_bytes(fmt_chunk.offset + 20, 2, 's').first
  end

  def each_sample
    file = @riff.file
    offset = data_chunk.offset + 8
    length = data_chunk.length
    pack_template = bit_rate == 16 ? 's*' : 'C*'

    file.pos = offset

    while file.pos < offset + length
      yield read_bytes(file.pos, block_size, pack_template)
    end
  end

  private

  def fmt_chunk
    find_chunk('fmt ')
  end

  def data_chunk
    find_chunk('data')
  end

  def find_chunk(fourcc)
    @riff.root_chunk.each do |chunk|
      if chunk.fourcc == fourcc
        return chunk
      end
    end
  end

  def read_bytes(offset, bytes, pack_template)
    @riff.file.pos = offset
    @riff.file.read(bytes).unpack(pack_template)
  end
end

wav = Wav.new(Riff::Reader.open(file, 'r'))

INSTANT_ENERGY_SIZE = 1024
HISTORY_BUFFER_SIZE = 44032
SENSIBILITY = 1.3

buffer = []
history_buffer = []
i = 0
wav.each_sample do |sample|
  if wav.num_channels == 2
    buffer << sample[0] ** 2 + sample[1] ** 2
  else
    buffer << sample[0] ** 2
  end

  if buffer.size == INSTANT_ENERGY_SIZE
    if history_buffer.size == HISTORY_BUFFER_SIZE
      instant_energy = buffer.inject(0) do |total, samp|
        total + samp
      end

      local_average_energy = history_buffer.inject(0) do |total, samp|
        total + samp
      end * INSTANT_ENERGY_SIZE / wav.sample_rate * SENSIBILITY

      puts '%f sec(%i): %s' % [i / wav.data_speed.to_f, i, instant_energy > local_average_energy ? 'beat!!' : '']
    end

    history_buffer[0..0] = buffer
    history_buffer.slice!(HISTORY_BUFFER_SIZE, INSTANT_ENERGY_SIZE)

    buffer = []
  end

  i += wav.block_size
end