M4B Authoring Tool

Overview

Audiobookshelf includes a built-in tool to merge multi-file audiobooks into a single M4B file. This tool is accessible via the Manage tab of any book.

"Stream Copy" (Zero Re-encode)

The most significant improvement to the authoring tool is the Stream Copy (Codec: Copy) feature.

Benefits

Quality Preservation: Since the audio is not re-encoded, there is zero loss in audio quality during the merge.
Speed: Processing time is reduced from minutes (for re-encoding) to seconds (for disk I/O).
CPU Efficiency: Minimal processing power is required compared to transcoding.

How it Works

Concatenation: FFmpeg uses the concat demuxer to join the source files into a continuous stream without decoding them.
Container Swap: The stream is wrapped into an MP4/M4B container.
Metadata Injection: Chapters, title, artist, and cover art are injected in a secondary pass, again using -c copy.

Feature Matrix

Feature	Support
Codecs	AAC, OPUS, Copy (Stream Copy)
Chapters	Automatically generated from Library Item tracks
Cover Art	Embedded as a video stream (attached_pic)
Metadata	Title, Artist, Album, and Comment tags

Usage

Navigate to a book with multiple audio files.
Go to Manage -> M4B Encoder.
Select Codec: Copy.
Click Merge.
Once complete, a new M4B file will appear in the "Files" tab, and you will be prompted to swap the library item to the new file (optional).

Requirements

Source files must be compatible with the M4B container (typically AAC/M4A/MP3).
Note: While MP3 files can technically be copied into an M4B container, some players may have better compatibility if MP3s are transcoded to AAC.

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