Audiobook

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Audiobook media can be Audio CDs or downloaded files, several formats exist. For certain hardware media players a specific format will be necessary. This article documents how to create or convert an audiobook of various format types.

iPod-compatible format

The purpose of this section is to detail a process to create an iPod-friendly audiobook from digital media using Linux native tools.

An audiobook designed for iPods is largely the same as a MPEG-4 audio file — it contains both an audio stream and information about that stream (metadata). The two differences are: one, it is wrapped in a specifically-named MPEG-4 container (.m4b); and two, it has a chapter index added to the metadata. The chapter index uses the Quicktime standard.

The basic process is this: the audio source is decoded/uncompressed (it is usually preferable to have a decoded audio-only file, as this will simplify the concatenation/splitting of files later and prevent mixing of metadata tags); after this, the audio files are encoded to the AAC format — with a MPEG-4 container; last, the chapter index and audio information tags are added.

Multiple tools are available to help create audiobook files.

Tools

faac — open source AAC encoder
neroaacenc-bin^AUR — proprietary AAC encoder
gpac — multimedia framework for manipulating MPEG-4 media
lame — MP3 encoder and decoder

Extracting or decoding

The audio source file(s) will either need to be extracted, if a disc media, or to be decoded, if an encoded file.

If the audiobook is a disc media, various digital audio extraction utilities are available. A common way to extract audio from a CD to WAV-formatted files is:

cdparanoia -B

If the audiobook is a audio file, various programs can decode it. For example, for MP3s:

lame --decode part-01.mp3

(If no splitting of concatenation will be required, files can be directly encoded to AAC with a MPEG-4 container.)

Concatenation or splitting

Audiobooks parts are typically saved in about one hour, ten minute segments (usually the time extent of a audio CD). They can be concatenated or split as desired.

Join WAV files:

sndfile-concat track-01.wav track-02.wav… disc-1.wav  # or more easily
sndfile-concat track*.wav disc-1.wav

Join MPEG-4 compatible audio files (AAC, AVI, MP3…):

MP4Box -cat track-01.aac -cat track-02.aac… disc-1.aac

Split a wav file (wavsplit^AUR can do this (untested)).

Split a MP3 files into 10 minute intervals:

mp3splt -f -t 10.0 part-01.mp3 -o @n

Split a MP3 by chapters/tracks (this method scans for silence and assumes them to be chapter/track marks, splitting them on those marks — these settings are generalized and will likely required further adjusting to work):

mp3splt -f -s -p -min=3 part-01.mp3

Split MPEG 4 compatible audio files (metadata repair will likely be necessary) into 10 minute intervals.

MP4Box -split 600 target.mp4

Encoding

Multiple programs exist that can encode to AAC.

To encode with the open source encoder FAAC:

faac -q 80 -o part-01.aac part-01.wav
faac -q 80 -o part-01.m4b part-01.wav -w  # with a MPEG-4 container(wrapper)

To encode with the proprietary encoder Nero:

neroAacEnc -q 0.7 -of part-01.aac -if part-01.wav

Note: The FAAC quantizer quality setting of 55 (~64kbps) is recommended when encoding from an audiobook CD. This setting is for an audio source that is already an encoded file. The FAAC setting of 80 should reasonably accommodate sound quality while preserving file size close to the original file. Whenever converting from one lossy format to another, there will always be some amount of information lost.

Warning: Though ffmpeg can convert MP3 to an AAC encoding in one step (ffmpeg -i track1.mp3 -acodec aac -strict -2 track1.aac), support for AAC is experimental.

Chapter index merging

To merge a chapter index into a audio file it must be decided what type of index to use: one, recurring intervals with periodic chapter times; or two, particular intervals with definitive chapter times and names.

Add a MPEG-4 container to the audio file:

MP4Box -add disc-1.aac disc-1.m4b

Recurring intervals

To merge a chapter index with chapters at every 10 minutes:

mp4chaps --every 600 disc-1.m4b

Particular intervals

The method requires writing a chapter index as a text file and merging it into the audio file.

Note: mp4chaps will complain about not seeing a video stream when attempting to merge the chapter index with the --import option (a bug should probably be filed). An alternate method is to use MP4Box. Though MP4Box is typically seen used for video files it has the ability to add chapters while not introducing a video stream. Because of this the resulting media will be able to play on iPods and iTunes as expected.

The chapter index will need to be written as a text file before being merged into audio file. There are two standards for chapter indexes: one, the Quicktime standard; and two, the Nero standard. Either can be initially merged.

The Quicktime standard looks like this (thousandths of a second can also be added .000):

00:00:00 Track 01
00:08:40 Track 02
...

The Nero standard looks like this:

CHAPTER1=00:00:00
CHAPTER1NAME=Track 01
CHAPTER2=00:08:40
CHAPTER2NAME=Track 02
...

To merge the chapter index file:

MP4Box -chap disc-1.chapters.txt disc-1.m4b

To merge the chapter index file in one step along with the audio file and a container:

MP4Box -chap disc-1.chapters.txt -add disc-1.aac disc-1.m4b

The chapter index will need to be converted to the Quicktime standard (though MP4Box will recognize both chapter index standards when importing, it imports using the Nero standard). Convert to the Quicktime standard with:

mp4chaps --convert --chapter-qt disc-1.m4b

The chapter index can now be tested, for example, with VLC.

Tagging

A number of tagging programs can be used. Basic command-line programs provided by libmp4v2 have numerous abilities:

$ mp4tags -album "An Good Audiobook" -artist "John Doe" -disk 2 -disks 10 -genre Nonfiction -albumartist "Ms. Foo Bar" -song "Disc 2" -year 2004 disc1.m4b

Add coverart:

$ mp4art --add a-good-audiobook.jpg disc-1.m4b

Optimize the container:

$ mp4file --optimize *.m4b

Example of batching

A number of audio files can be processed using a loop:

for m in *.mp3; do
  lame --decode "$m"
  faac -q 80 -o "${m%.*}".aac "$m"
done

To write tags for sequential disc numbering:

for b in *.m4b; do
  [ "$n" ] && n=$((10#$n + 1))
  [ -z "$n" ] && n=1
  mp4tags -album "An Good Audiobook"… -disk $(printf "%02u" $n)… "$b"
done

Audible format

It can be a laborious process to convert and Audible book format — article.

A much easier way is to use this script to retrieve your activation bytes from a downloaded Audible audiobook — link.

You can then use mpv to listen to the file:

$ mpv --demuxer-lavf-o=activation_bytes=<your activation bytes> <filename>.aax

Or you can write the file to unencrypted .mp4 with ffmpeg (documentation):

$ ffmpeg -activation_bytes <your activation bytes> -i <filename>.aax -vn -c:a copy <output>.mp4

The resulting file will have chapter headers intact.