Class: FormatParser::AdtsHeaderInfo

Inherits:
Object
  • Object
show all
Defined in:
lib/parsers/aac_parser/adts_header_info.rb

Overview

This is a representation of the relevant information found in an Audio Data Transport Stream (ADTS) file header.

Constant Summary collapse

AAC_ADTS_HEADER_BITS_CHUNK_SIZES =

An ADTS header has the following format, when represented in bits: AAAAAAAA AAAABCCD EEFFFFGH HHIJKLMM MMMMMMMM MMMOOOOO OOOOOOPP (QQQQQQQQ QQQQQQQQ) The chunks represented by these letters have specific meanings, as described here: wiki.multimedia.cx/index.php/ADTS

[
  ['A', 12], ['B', 1], ['C', 2], ['D', 1],
  ['E', 2], ['F', 4], ['G', 1], ['H', 3],
  ['I', 1], ['J', 1], ['K', 1], ['L', 1],
  ['M', 13], ['O', 11], ['P', 2], ['Q', 16]
]
MPEG4_AUDIO_OBJECT_TYPE_RANGE = 
0..45
MPEG4_AUDIO_SAMPLING_FREQUENCY_RANGE = 
0..14
MPEG4_AUDIO_SAMPLING_FREQUENCY_HASH = 
{
  0 => 96000, 1 => 88200, 2 => 64000,
  3 => 48000, 4 => 44100, 5 => 32000,
  6 => 24000, 7 => 22050, 8 => 16000,
  9 => 12000, 10 => 11025, 11 => 8000,
  12 => 7350, 13 => 'Reserved', 14 => 'Reserved'
}
AAC_PROFILE_DESCRIPTION_HASH = 
{
  0 => 'AAC_MAIN',
  1 => 'AAC_LC (Low Complexity)',
  2 => 'AAC_SSR (Scaleable Sampling Rate)',
  3 => 'AAC_LTP (Long Term Prediction)'
}
MPEG_VERSION_HASH = 
{ 0 => 'MPEG-4', 1 => 'MPEG-2'}

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Instance Attribute Details

#aac_frames_per_adts_frame ⇒ Object

Returns the value of attribute aac_frames_per_adts_frame.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def aac_frames_per_adts_frame
  @aac_frames_per_adts_frame
end

#buffer_fullness ⇒ Object

Returns the value of attribute buffer_fullness.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def buffer_fullness
  @buffer_fullness
end

#frame_length ⇒ Object

Returns the value of attribute frame_length.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def frame_length
  @frame_length
end

#home_usage ⇒ Object

Returns the value of attribute home_usage.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def home_usage
  @home_usage
end

#layer ⇒ Object

Returns the value of attribute layer.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def layer
  @layer
end

#mpeg4_channel_config ⇒ Object

Returns the value of attribute mpeg4_channel_config.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def mpeg4_channel_config
  @mpeg4_channel_config
end

#mpeg4_sampling_frequency_index ⇒ Object

Returns the value of attribute mpeg4_sampling_frequency_index.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def mpeg4_sampling_frequency_index
  @mpeg4_sampling_frequency_index
end

#mpeg_version ⇒ Object

Returns the value of attribute mpeg_version.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def mpeg_version
  @mpeg_version
end

#originality ⇒ Object

Returns the value of attribute originality.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def originality
  @originality
end

#profile ⇒ Object

Returns the value of attribute profile.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def profile
  @profile
end

#protection_absence ⇒ Object

Returns the value of attribute protection_absence.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 3

def protection_absence
  @protection_absence
end

Class Method Details

.parse_adts_header(header_bits) ⇒ Object

If the given bit array is a valid ADTS header, this method will parse it and return an instance of AdtsHeaderInfo. Will return nil if the header does not match the ADTS specifications.



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 74

def self.parse_adts_header(header_bits)
  result = FormatParser::AdtsHeaderInfo.new

  AAC_ADTS_HEADER_BITS_CHUNK_SIZES.each do |letter_size|
    letter = letter_size[0]
    chunk_size = letter_size[1]
    chunk = header_bits.shift(chunk_size)
    decimal_number = chunk.join.to_i(2)

    # Skipping data represented by the letters G, K, L, Q, as we are not interested in those values.
    case letter
    when 'A'
      # Syncword, all bits must be set to 1
      return nil unless chunk.all? { |bit| bit == '1' }
    when 'B'
      # MPEG Version, set to 0 for MPEG-4 and 1 for MPEG-2
      result.mpeg_version = decimal_number
    when 'C'
      # Layer, always set to 0
      return nil unless decimal_number == 0
    when 'D'
      # Protection absence, set to 1 if there is no CRC and 0 if there is CRC
      result.protection_absence = decimal_number == 1
    when 'E'
      # AAC Profile
      return nil unless MPEG4_AUDIO_OBJECT_TYPE_RANGE.include?(decimal_number + 1)
      result.profile = decimal_number
    when 'F'
      # MPEG-4 Sampling Frequency Index (15 is forbidden)
      return nil unless MPEG4_AUDIO_SAMPLING_FREQUENCY_RANGE.include?(decimal_number)
      result.mpeg4_sampling_frequency_index = decimal_number
    when 'H'
      # MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an in-band PCE (Program Config Element))
      result.mpeg4_channel_config = decimal_number
    when 'I'
      # Originality, set to 1 to signal originality of the audio and 0 otherwise
      result.originality = decimal_number == 1
    when 'J'
      # Home, set to 1 to signal home usage of the audio and 0 otherwise
      result.home_usage = decimal_number == 1
    when 'M'
      # Frame length, length of the ADTS frame including headers and CRC check (protectionabsent == 1? 7: 9)
      # We expect this to be higher than the header length, but we won't impose any other restrictions
      header_length = result.protection_absence ? 7 : 9
      return nil unless decimal_number > header_length
      result.frame_length = decimal_number
    when 'O'
      # Buffer fullness, states the bit-reservoir per frame.
      # It is merely an informative field with no clear use case defined in the specification.
      result.buffer_fullness = decimal_number
    when 'P'
      # Number of AAC frames (RDBs (Raw Data Blocks)) in ADTS frame minus 1. For maximum compatibility always use one AAC frame per ADTS frame.
      result.aac_frames_per_adts_frame = decimal_number + 1
    end
  end

  result
end

Instance Method Details

#fixed_bitrate? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 61

def fixed_bitrate?
  # A buffer fullness value of 0x7FF (decimal: 2047) denotes a variable bitrate, for which buffer fullness isn't applicable
  @buffer_fullness != 2047
end

#frame_rate ⇒ Object

The frame rate - i.e. frames per second



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 67

def frame_rate
  # An AAC sample uncompresses to 1024 PCM samples
  mpeg4_sampling_frequency.to_f / 1024
end

#mpeg4_sampling_frequency ⇒ Object 



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 35

def mpeg4_sampling_frequency
  return MPEG4_AUDIO_SAMPLING_FREQUENCY_HASH[@mpeg4_sampling_frequency_index] if !@mpeg4_sampling_frequency_index.nil? && MPEG4_AUDIO_SAMPLING_FREQUENCY_HASH.key?(@mpeg4_sampling_frequency_index)
  nil
end

#mpeg_version_description ⇒ Object 



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 45

def mpeg_version_description
  return MPEG_VERSION_HASH[@mpeg_version] if !@mpeg_version.nil? && MPEG_VERSION_HASH.key?(@mpeg_version)
  nil
end

#number_of_audio_channels ⇒ Object 



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 50

def number_of_audio_channels
  case @mpeg4_channel_config
  when 1..6
    @mpeg4_channel_config
  when 7
    8
  else
    nil
  end
end

#profile_description ⇒ Object 



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# File 'lib/parsers/aac_parser/adts_header_info.rb', line 40

def profile_description
  return AAC_PROFILE_DESCRIPTION_HASH[@profile] if !@profile.nil? && AAC_PROFILE_DESCRIPTION_HASH.key?(@profile)
  nil
end