AU audio format: Ruby parsing library

This page hosts a formal specification of AU audio format using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Ruby generated by Kaitai Struct depends on the Ruby runtime library. You have to install it before you can parse data.

The Ruby runtime library can be installed from RubyGems:

gem install kaitai-struct

Code

Parse a local file and get structure in memory:

data = Au.from_file("path/to/local/file.["au", "snd"]")

Or parse structure from a string of bytes:

bytes = "\x00\x01\x02..."
data = Au.new(Kaitai::Struct::Stream.new(bytes))

After that, one can get various attributes from the structure by invoking getter methods like:

data.magic # => get magic

Ruby source code to parse AU audio format

au.rb

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

require 'kaitai/struct/struct'

unless Gem::Version.new(Kaitai::Struct::VERSION) >= Gem::Version.new('0.9')
  raise "Incompatible Kaitai Struct Ruby API: 0.9 or later is required, but you have #{Kaitai::Struct::VERSION}"
end


##
# The NeXT/Sun audio file format.
# 
# Sample files:
# 
# * <https://github.com/python/cpython/tree/b8a7daf077da/Lib/test/sndhdrdata>
# * <ftp://ftp-ccrma.stanford.edu/pub/Lisp/sf.tar.gz>
# * <http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/AU/Samples.html>
# @see http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/AU/AU.html Source
# @see http://soundfile.sapp.org/doc/NextFormat/ Source
# @see http://soundfile.sapp.org/doc/NextFormat/soundstruct.h Source
# @see https://github.com/andreiw/polaris/blob/deb47cb/usr/src/head/audio/au.h#L87-L112 Source
# @see https://github.com/libsndfile/libsndfile/blob/86c9f9eb/src/au.c#L39-L74 Source
# @see https://github.com/chirlu/sox/blob/dd8b63bd/src/au.c#L34-L49 Source
# @see https://github.com/mpruett/audiofile/blob/b62c902/libaudiofile/NeXT.cpp#L65-L96 Source
class Au < Kaitai::Struct::Struct

  ENCODINGS = {
    1 => :encodings_mulaw_8,
    2 => :encodings_linear_8,
    3 => :encodings_linear_16,
    4 => :encodings_linear_24,
    5 => :encodings_linear_32,
    6 => :encodings_float,
    7 => :encodings_double,
    8 => :encodings_fragmented,
    9 => :encodings_nested,
    10 => :encodings_dsp_core,
    11 => :encodings_fixed_point_8,
    12 => :encodings_fixed_point_16,
    13 => :encodings_fixed_point_24,
    14 => :encodings_fixed_point_32,
    16 => :encodings_display,
    17 => :encodings_mulaw_squelch,
    18 => :encodings_emphasized,
    19 => :encodings_compressed,
    20 => :encodings_compressed_emphasized,
    21 => :encodings_dsp_commands,
    22 => :encodings_dsp_commands_samples,
    23 => :encodings_adpcm_g721,
    24 => :encodings_adpcm_g722,
    25 => :encodings_adpcm_g723_3,
    26 => :encodings_adpcm_g723_5,
    27 => :encodings_alaw_8,
    28 => :encodings_aes,
    29 => :encodings_delta_mulaw_8,
  }
  I__ENCODINGS = ENCODINGS.invert
  def initialize(_io, _parent = nil, _root = self)
    super(_io, _parent, _root)
    _read
  end

  def _read
    @magic = @_io.read_bytes(4)
    raise Kaitai::Struct::ValidationNotEqualError.new([46, 115, 110, 100].pack('C*'), magic, _io, "/seq/0") if not magic == [46, 115, 110, 100].pack('C*')
    @ofs_data = @_io.read_u4be
    @_raw_header = @_io.read_bytes(((ofs_data - 4) - 4))
    _io__raw_header = Kaitai::Struct::Stream.new(@_raw_header)
    @header = Header.new(_io__raw_header, self, @_root)
    self
  end
  class Header < Kaitai::Struct::Struct
    def initialize(_io, _parent = nil, _root = self)
      super(_io, _parent, _root)
      _read
    end

    def _read
      @data_size = @_io.read_u4be
      @encoding = Kaitai::Struct::Stream::resolve_enum(Au::ENCODINGS, @_io.read_u4be)
      @sample_rate = @_io.read_u4be
      @num_channels = @_io.read_u4be
      raise Kaitai::Struct::ValidationLessThanError.new(1, num_channels, _io, "/types/header/seq/3") if not num_channels >= 1
      @comment = (Kaitai::Struct::Stream::bytes_terminate(@_io.read_bytes_full, 0, false)).force_encoding("ASCII")
      self
    end

    ##
    # don't read this field, access `_root.len_data` instead
    # 
    # value `0xffff_ffff` means unspecified size
    attr_reader :data_size
    attr_reader :encoding
    attr_reader :sample_rate

    ##
    # number of interleaved channels
    attr_reader :num_channels

    ##
    # Most resources claim that this field must be at least 4 bytes long.
    # However, most programs don't enforce it, and [Audacity](
    # https://www.audacityteam.org/) even generates .au files with this field
    # being 0-byte long. According to <https://nixdoc.net/man-pages/IRIX/man4/dmedia/next.4.html>,
    # "NeXT files require that this chunk be at least 4 bytes (chars) long,
    # whereas this chunk may be zerolength in a Sun .au file."
    # 
    # By convention, size should be a multiple of 4 -
    # see <https://github.com/chirlu/sox/blob/dd8b63bd/src/au.c#L132-L133>.
    # Page <http://soundfile.sapp.org/doc/NextFormat/> also mentions that for some
    # sound playing programs, this field must have an even byte size. So a multiple
    # of 4 is probably best for compatibility.
    # 
    # Must be null-terminated. It is usually an ASCII text string, but this space
    # might be also used to store application-specific binary (i.e. non-ASCII) data.
    attr_reader :comment
  end
  def len_data
    return @len_data unless @len_data.nil?
    @len_data = (header.data_size == 4294967295 ? (_io.size - ofs_data) : header.data_size)
    @len_data
  end
  attr_reader :magic
  attr_reader :ofs_data
  attr_reader :header
  attr_reader :_raw_header
end