Don't encode on copy

artifacts/2026-02-14/m4b_conversion.md

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+# 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
+- [x] Initial specification.
+- [x] Backend implementation for stream copy.
+- [x] Verification.