Difference between revisions of "Audible Audio"

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Proprietary container format from audible.com.  There is no published specification.  It may contain one of five different encodings which are numbered 1 thru 5.  1-3 are rumored to be [[ACELP.net]] at varying bitrates.  #4 is [[MP3]].  #5 is some unknown Sony format.
Proprietary container format from audible.com.  There is no published specification.  It may contain one of five different encodings which are numbered 1 thru 5.  1-3 are rumored to be [[ACELP.net]] at varying bitrates.  #4 is [[MP3]].  #5 is some unknown Sony format.


* [http://www.audible.com/formats Audible.com FAQ]
* [http://www.digitalpreservation.gov/formats/fdd/fdd000103.shtml Description of the format from the US Library of Congress]
* [http://www.digitalpreservation.gov/formats/fdd/fdd000103.shtml Description of the format from the US Library of Congress]
* [http://en.wikipedia.org/wiki/Audible.com Wikipedia entry]
* [http://en.wikipedia.org/wiki/Audible.com Wikipedia entry]
To find files, use Google "filetype:aa audible".


=== File format ===
=== File format ===

Revision as of 16:16, 19 August 2009

Proprietary container format from audible.com. There is no published specification. It may contain one of five different encodings which are numbered 1 thru 5. 1-3 are rumored to be ACELP.net at varying bitrates. #4 is MP3. #5 is some unknown Sony format.

To find files, use Google "filetype:aa audible".

File format

The file is built up of 5 parts:

  • general header
  • text based metadata
  • some codec related header
  • offset table
  • data

General header

First 4 bytes is the file size including these four bytes.

Text based metadata

 number_of_entires (32)
 skip (8)
 
 key_length (32)
 value_length (32)
 key (key_length)
 value (value_length+1)

Offset table

 number_of_entries (32)
 type (32) [0 - meta?, 1 - meta?, 2 - audio packets, 3 - meta?, 4 - meta?]
 offset (32) [must be relative, as the first entry in the list is always 0]

Parsing the file

This following Ruby code works for parsing the known parts:

 video = File.new(ARGV[0])
 puts "File size: #{video.read(4).unpack('N')[0].to_i}"
 video.read(0xb4) # skip header
 sizes = video.read(4).unpack('N')
 puts "Number of entires: #{sizes[0].to_i}"
 video.read(1) # skip
 (1..sizes[0].to_i).each { |x|
   sizes = video.read(8).unpack('NN')
   key = video.read(sizes[0].to_i)
   value = video.read(sizes[0].to_i+1)
   puts "#{x} key(#{sizes[0].to_i})=#{key} value(#{sizes[0].to_i)=#{value}"
 }
 video.read(0x6f) # skip
 sizes = video.read(4).unpack('N')
 puts "Number of packet table entries: #{sizes[0].to_i}"
 (1..sizes[0].to_i).each {
   sizes = video.read(8).unpack('NN')
   puts "type=#{sizes[0].to_i} offset=#{sizes[1].to_i}"
 }

Sample output (stripped):

 File size: 5762745
 Number of entries: 19
 ...
 13 key(5)=codec value(7)=acelp85
 16 key(10)=HeaderSeed value(10)=1158166611
 18 key(9)=HeaderKey value(43)=3759801365 1641076194 2988088058 4282540117
 19 key(15)=EncryptedBlocks value(5)=39333
 ...
 Number of packet table entries: 5461
 type=0 offset=0
 type=0 offset=1045
 ...