Audiobook transcoding notes

Baltakateiʼs notes for transcoding Audiobooks.

Commands

Create a copy of the audiobook mp3 files

$ dir="/tmp/zasni/5/"
$ mkdir -p "$dir"
$ cd "$dir"
$ cp ~/1869..War_and_Peace/*.mp3 "$dir"/
$ ls -1 "$dir"
How To - Track 001.mp3
How To - Track 002.mp3
How To - Track 003.mp3
(...)
How To - Track 030.mp3
How To - Track 031.mp3
How To - Track 032.mp3

Create a Bash script named make_list.sh to generate a file list parsable by ffmpeg

#!/bin/bash
fout="mylist.txt";
while read -r line; do
  printf "file '%s'\n" "${line#./}" >> "$fout";
done < <(find . -type f -name "*.mp3" | sort);

Run the bash script make_list.sh

$ /bin/bash make_list.sh

Concatenate and save the mp3 files as a lossless, but large, WAV file:

$ ffmpeg -f concat -safe 0 -i mylist.txt output.wav

If ffmpeg reports the output WAV file is too large, add the -rf64 auto option to use a non-standard WAV format that supports larger files.:

$ ffmpeg -f concat -safe 0 -i mylist.txt -c:a pcm_s24le -rf64 auto output.wav

Convert the WAV file to a 32 kbps opus file:

$ ffmpeg -i output.wav -c:a libopus -b:a 32k output.opus

Note: the output.opus file is what I ended up saving since the ogg container mentioned later suffered playback issues due to how the album artwork was saved as a single video frame.

Alternatively, concatenate the multiple mp3 files into WAV format via a stdout/stdin pipe (no need to store a large file) to a second simultaneous ffmpeg operation that converts the WAV data into OPUS format which is written to disk.

$ ffmpeg -f concat -safe 0 -i mylist.txt -c:a pcm_s24le -rf64 auto -f wav - | \
    ffmpeg -i - -c:a libopus -b:a 32k output.opus

Extract album artwork from one of the original audiobook mp3 files.

$ ffmpeg -i How\ To\ -\ Track\ 001.mp3 -an -vcodec copy album_artwork.png

Add album artwork and output to an ogg container.

$ ffmpeg -i output.opus -i album_artwork.png -map 0:a -map 1:v -c:a copy -q:v 10 output.ogg

History

• 2023: BK-2020-03: Baltakatei wrote a Bash script to automated the conversion of a directory of mp3 files into an opus audio track of a single mkv file. The script is saved in BK-2020-03/user/mp3s_to_mkv.sh and is run as: mp3s_to_mkv.sh [DIR IN] [DIR OUT] [BITRATE]. Baltakatei found a bitrate of 48k (48kbps) for the audio track is sufficient for audiobooks.

See also

• Wikipedia:Audiobook transcoding notes
• libro.fm: DRM-free audiobooks

External links

• bk4, reboil.com