Convert OGG and Opus to M4B

Apple Books won't open an .ogg or .opus file, and most phone audiobook players are lukewarm about them. Kataribe converts OGG Vorbis and Opus files to a single .m4b with chapters, cover and metadata, in your browser, with nothing uploaded.

LibriVox is the common source: a public-domain novel downloaded as a zip of Ogg Vorbis sections, one per chapter, with filenames like prideandprejudice_03_austen.ogg. Drop the lot in. Natural filename order keeps section 10 after section 9, each file becomes a chapter, and the long filenames can be cut down to "Chapter 3" in the chapter list before you build. Fill in title, author and reader, drop in a cover, and the result is a book Apple Books treats as a book.

Opus files — from archive.org, a podcast archive or a recording app — are handled the same way. Vorbis and Opus are both lossy, so this is a lossy-to-lossy re-encode; matching or exceeding the source bitrate is the sensible choice, and 64 kbps AAC suits a 64 kbps source. The desktop tools that do this are mostly Windows-only or unmaintained; this one runs wherever a modern browser does.

After you download the .m4b

Apple Books

  1. Mac: drag the .m4b into Books, or File → Add to Library. It lands under Audiobooks with the cover and chapters.
  2. iPhone / iPad: connect the device and sync audiobooks in Finder (or iTunes on Windows), or AirDrop the file and open it in Books.
  3. Books remembers your position and shows the chapter list from the file — that's the whole point of M4B over a folder of MP3s.

Audiobookshelf

  1. Copy the .m4b into your library folder as Author/Title/Book.m4b (or upload it from the web UI).
  2. Run a library scan; Audiobookshelf reads the embedded title, author, narrator, cover, and chapter markers.
  3. Chapters appear in the player on web, iOS, and Android — no chapter files or metadata edits needed.

Android players

  1. Copy the .m4b to the phone (USB, Drive, or Syncthing) into a folder your player watches.
  2. Smart AudioBook Player, Listen, Voice, and the Audiobookshelf app all read M4B chapters and cover art.
  3. One file per book means one entry in the library, with your position remembered across chapters.

Frequently asked questions

Does it handle Opus as well as Vorbis?
Yes. Both .ogg (Vorbis) and .opus files are read by the same FFmpeg engine, and you can mix them with MP3, M4A, WAV or FLAC in one book. Everything is encoded to AAC at the bitrate you pick.
Why won't Apple Books play OGG?
Apple's apps don't support the Ogg container. Converting to M4B gives you AAC in an MP4 container, which Apple Books, iPhone and every audiobook player read, with chapters that OGG files never had.
LibriVox offers some books as M4B already. Why convert?
If your recording has an M4B download, use it. Many older or multi-part recordings don't, or the file has no cover and a generic title. Rebuilding from the sections gives you editable chapter names and your own metadata.
Should I pick the same bitrate as the source?
The same or higher. Re-encoding can't recover detail the source didn't have, and going lower loses more. For a 64 kbps Ogg, choose 64 kbps AAC.
Are my audio files uploaded anywhere?
No. Kataribe runs FFmpeg compiled to WebAssembly inside your browser, so the files never leave your device. The engine (about 30 MB) downloads once and is cached; after that there is no account, no queue and no watermark.
How long does a full book take?
Encoding runs many times faster than real time on a modern laptop, but a full-length book still takes minutes, not seconds. Keep the tab open until the download appears. If every input is already AAC, the audio is copied rather than re-encoded, which is much quicker.
Is this really free?
Yes. Every input format, every bitrate, cover art and metadata, no sign-up, and nothing added to the audio.
Does it work with DRM-protected audiobooks?
No. DRM-protected files (Audible .aax and similar) won't open, and we don't help with that. Kataribe is for audio you already have in plain MP3, M4A, WAV, FLAC or OGG.