audiobookshelf/artifacts/2026-02-14/m4b_conversion.md

M4B Conversion Specification

Overview

Audiobookshelf provides a tool to merge audiobook audio tracks into a single M4B file. This specification documents the improvements made to this tool to support "Stream Copy" (no re-encode), preserving audio quality and significantly reducing processing time.

Feature Goals

• Support merging multiple audio files (MP3, M4A, AAC) into a single M4B container.
• Avoid re-encoding when the source audio is already compatible (e.g., AAC in M4A/MP4 container) or when the user explicitly chooses "Copy".
• Preserving all metadata including:
  • Title, Artist, Album, etc.
  • Chapters (from library item metadata).
  • Cover art.

Technical Details

Backend Implementation

The logic resides in server/utils/ffmpegHelpers.js and server/managers/AbMergeManager.js.

FFmpeg Strategy for "Copy"

When codec: 'copy' is requested:

1. Concatenation: If multiple files exist, they are concatenated using the concat demuxer in FFmpeg.
   • Command: ffmpeg -f concat -safe 0 -i files.txt -c copy -f mp4 output.m4b
2. Metadata and Cover Embedding: The concatenated file is then processed to add the ffmetadata and cover art.
   • Command: ffmpeg -i input.m4b -i metadata.txt -i cover.jpg -map 0:a -map 1 -map 2:v -c copy -disposition:v:0 attached_pic -f mp4 output_final.m4b

Frontend Implementation

The user interface is accessible via the Manage page of a book, under the M4B Encoder tool.

Options

• Codec: Options include AAC, OPUS, and Copy.
• Bitrate: Custom or presets (ignored for Copy).
• Channels: Custom or presets (ignored for Copy).

Current Status

• Initial specification.
• Backend implementation for stream copy.
• Verification.