Class: Mp3file::MP3Header

Inherits:
Object
  • Object
show all
Defined in:
lib/mp3file/mp3_header.rb

Defined Under Namespace

Classes: MP3HeaderFormat

Constant Summary collapse

MPEG_VERSIONS = 
[ 'MPEG 2.5', nil, 'MPEG 2', 'MPEG 1' ]
LAYERS = 
[ nil, 'Layer III', 'Layer II', 'Layer I' ]
BITRATES = 
[
  # MPEG 2.5
  [ nil,
    [  8, 16, 24, 32, 40, 48,  56,  64,  80,  96, 112, 128, 144, 160 ],    # Layer III
    [  8, 16, 24, 32, 40, 48,  56,  64,  80,  96, 112, 128, 144, 160 ],    # Layer II
    [ 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256 ], ], # Layer I
  # reserved
  nil,
  # MPEG 2
  [ nil,
    [  8, 16, 24, 32, 40, 48,  56,  64,  80,  96, 112, 128, 144, 160 ],    # Layer III
    [  8, 16, 24, 32, 40, 48,  56,  64,  80,  96, 112, 128, 144, 160 ],    # Layer II
    [ 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256 ], ], # Layer I
  # MPEG 1
  [ nil,
    [ 32, 40, 48,  56,  64,  80,  96, 112, 128, 160, 192, 224, 256, 320 ],   # Layer III
    [ 32, 48, 56,  64,  80,  96, 112, 128, 160, 192, 224, 256, 320, 384 ],   # Layer II
    [ 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448 ], ] # Layer I
]
SAMPLERATES = 
[
  [ 11025, 12000,  8000 ], # MPEG 2.5
  nil,
  [ 22050, 24000, 16000 ], # MPEG 2
  [ 44100, 48000, 32000 ], # MPEG 1
]
MODES = 
[ 'Stereo', 'Joint Stereo', 'Dual Channel', 'Mono' ]
SAMPLE_COUNTS = 
[
  [ nil,  576, 1152, 384 ], # MPEG 2.5, III / II / I
  nil,
  [ nil,  576, 1152, 384 ], # MPEG 2, III / II / I
  [ nil, 1152, 1152, 384 ], # MPEG 1, III / II / I
]
SIDE_BYTES = 
[
  [ 17, 17, 17,  9 ], # MPEG 2.5, Stereo, J-Stereo, Dual Channel, Mono
  nil,
  [ 17, 17, 17,  9 ], # MPEG 2, Stereo, J-Stereo, Dual Channel, Mono
  [ 32, 32, 32, 17 ], # MPEG 1, Stereo, J-Stereo, Dual Channel, Mono
]
MODE_EXTENSIONS_LAYER_I_II = 
[ 'bands 4 to 31', 'bands 8 to 31', 'bands 12 to 31', 'bands 16 to 31' ]
MODE_EXTENSIONS_LAYER_III = 
[ nil, 'Intensity Stereo', 'M/S Stereo', [ 'Intensity Stereo', 'M/S Stereo' ] ]
EMPHASES = 
[ 'none', '50/15 ms', nil, 'CCIT J.17' ]

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(io) ⇒ MP3Header

Returns a new instance of MP3Header.



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# File 'lib/mp3file/mp3_header.rb', line 74

def initialize(io)
  begin
    head = MP3HeaderFormat.read(io)
  rescue BinData::ValidityError => ve
    raise InvalidMP3HeaderError, ve.message
  end

  @raw = head
  @version = MPEG_VERSIONS[head.version]
  @layer = LAYERS[head.layer]
  @has_crc = head.crc == 0

  if @version.nil? || @layer.nil?
    raise InvalidMP3HeaderError, "Bad MPEG version or layer."
  end

  kbitrate = BITRATES[head.version][head.layer][head.bitrate - 1]

  if kbitrate.nil?
    raise InvalidMP3HeaderError, "Bad bitrate."
  end

  @bitrate = kbitrate * 1000
  @samplerate = SAMPLERATES[head.version][head.samplerate]
  @has_padding = head.padding == 1
  @mode = MODES[head.mode]

  @mode_extension = nil
  if @mode == 'Joint Stereo'
    if [ 'Layer I', 'Layer II' ].include?(@layer)
      @mode_extension = MODE_EXTENSIONS_LAYER_I_II[head.mode_extension]
    elsif @layer == 'Layer III'
      @mode_extension = MODE_EXTENSIONS_LAYER_III[head.mode_extension]
    end
  end

  @copyright = head.copyright == 1
  @original = head.original == 1
  @emphasis = EMPHASES[head.emphasis]
  @samples = SAMPLE_COUNTS[head.version][head.layer]

  slot_size = layer == 'Layer I' ? 4 : 1
  pad_slots = has_padding ? 1 : 0
  @frame_size = (((samples.to_f * bitrate.to_f) / (8 * slot_size.to_f * samplerate.to_f)) + pad_slots).to_i * slot_size
  @side_bytes = SIDE_BYTES[head.version][head.mode]
end

Instance Attribute Details

#bitrate ⇒ Object (readonly)

Returns the value of attribute bitrate.



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# File 'lib/mp3file/mp3_header.rb', line 5

def bitrate
  @bitrate
end

#copyright ⇒ Object (readonly)

Returns the value of attribute copyright.



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# File 'lib/mp3file/mp3_header.rb', line 5

def copyright
  @copyright
end

#emphasis ⇒ Object (readonly)

Returns the value of attribute emphasis.



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# File 'lib/mp3file/mp3_header.rb', line 5

def emphasis
  @emphasis
end

#frame_size ⇒ Object (readonly)

Returns the value of attribute frame_size.



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# File 'lib/mp3file/mp3_header.rb', line 5

def frame_size
  @frame_size
end

#has_crc ⇒ Object (readonly)

Returns the value of attribute has_crc.



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# File 'lib/mp3file/mp3_header.rb', line 5

def has_crc
  @has_crc
end

#has_padding ⇒ Object (readonly)

Returns the value of attribute has_padding.



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# File 'lib/mp3file/mp3_header.rb', line 5

def has_padding
  @has_padding
end

#layer ⇒ Object (readonly)

Returns the value of attribute layer.



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# File 'lib/mp3file/mp3_header.rb', line 5

def layer
  @layer
end

#mode ⇒ Object (readonly)

Returns the value of attribute mode.



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# File 'lib/mp3file/mp3_header.rb', line 5

def mode
  @mode
end

#mode_extension ⇒ Object (readonly)

Returns the value of attribute mode_extension.



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# File 'lib/mp3file/mp3_header.rb', line 5

def mode_extension
  @mode_extension
end

#original ⇒ Object (readonly)

Returns the value of attribute original.



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# File 'lib/mp3file/mp3_header.rb', line 5

def original
  @original
end

#raw ⇒ Object (readonly)

Returns the value of attribute raw.



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# File 'lib/mp3file/mp3_header.rb', line 5

def raw
  @raw
end

#samplerate ⇒ Object (readonly)

Returns the value of attribute samplerate.



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# File 'lib/mp3file/mp3_header.rb', line 5

def samplerate
  @samplerate
end

#samples ⇒ Object (readonly)

Returns the value of attribute samples.



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# File 'lib/mp3file/mp3_header.rb', line 5

def samples
  @samples
end

#side_bytes ⇒ Object (readonly)

Returns the value of attribute side_bytes.



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# File 'lib/mp3file/mp3_header.rb', line 5

def side_bytes
  @side_bytes
end

#version ⇒ Object (readonly)

Returns the value of attribute version.



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# File 'lib/mp3file/mp3_header.rb', line 5

def version
  @version
end

Instance Method Details

#duration ⇒ Object 



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# File 'lib/mp3file/mp3_header.rb', line 121

def duration
  @samples.to_f / @samplerate
end

#same_header?(other) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/mp3file/mp3_header.rb', line 133

def same_header?(other)
  # Borrowed from ffmpeg.
  same_header_mask = 0xffe00000 | (3 << 17) | (3 << 10) | (3 << 19)
  (to_i & same_header_mask) == (other.to_i & same_header_mask)
end

#to_i ⇒ Object 



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# File 'lib/mp3file/mp3_header.rb', line 125

def to_i
  @raw.to_binary_s
    .reverse
    .each_byte
    .each_with_index
    .inject(0) { |m, (b, i)| m += b << (i*8) }
end