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.

How to convert OGG or Opus files to an M4B audiobook

  1. Unzip and check the numbering pattern. LibriVox names put the section number in the middle: title_01_author.ogg, title_02_author.ogg. Natural sorting handles that because the prefix is identical for every file, and section 10 follows 9. Archive.org bulk downloads sometimes add a numeric suffix instead; those sort correctly too, but check the list before building.
  2. Replace the filename-derived chapter titles. With no tags to read, each chapter defaults to the filename with underscores turned to spaces. LibriVox's catalog page lists the section titles; typing them in takes a minute for a novel and is the difference between "Prideandprejudice 03 austen" and "Chapter 3: Mr. Bennet visits".
  3. Choose a bitrate that covers the source. Vorbis and Opus both squeeze more quality per kilobit than AAC-LC, so matching the number is the floor, not the target. For a LibriVox Vorbis section pick 64; for a podcast Opus file at 24 to 48 kbps pick 64 as well; drop to 48 only if the source is already rough or space is very tight.
  4. Author, reader, year. For public-domain recordings the author is the writer and the narrator is the volunteer reader, which Audiobookshelf shows as two people. The year field is yours to decide; the publication year is what most catalogs show, the recording year is what LibriVox lists.
  5. Cover, then build. LibriVox and archive.org pages carry cover art as JPEG; save it and drop it in, or make your own PNG. Every file is decoded and re-encoded, so a 60-section novel takes a few minutes; the tab has to stay open until the download appears.

OGG/Opus and why it has to be re-encoded

Copy mode exists in Kataribe for exactly one case: every input already contains AAC. Vorbis and Opus are different codecs, so their frames can't be placed in the audiobook container the way AAC frames can; the file has to be decoded to PCM and encoded again. Any tool that claims to convert OGG to M4B without re-encoding is either wrapping Opus in MP4, which Apple Books and most audiobook apps don't play, or not telling the truth.

Lossy to lossy costs something every time. The way to keep it small is to give the AAC encoder at least as many bits as the source used and preferably more, and to do it once: don't convert OGG to MP3 and then MP3 to M4B, go straight from the OGG.

Vorbis and Opus are different codecs in the same container

.ogg files hold Vorbis, .opus files hold Opus, and .oga can hold either. Kataribe accepts all three and lets FFmpeg identify the stream. Vorbis dates from 2000 and is what LibriVox and older archive.org rips use; Opus is the 2012 successor with much better low-bitrate speech, used by newer podcast archives and by recording apps that target small files.

Opus decodes at 48 kHz regardless of the original sample rate, so an Opus source hands the AAC encoder 48 kHz audio, while a Vorbis rip of a 44.1 kHz CD stays at 44.1. Neither matters for speech, but it explains why two books from different sources report different sample rates in a player's info panel.

Mixing OGG with other formats in one book

A LibriVox recording sometimes has one section only available as MP3, or a dramatic reading combines Opus narration with a WAV intro. Drop them together; the build encodes every file to AAC at the same bitrate, and the chapter list doesn't care what the source formats were.

The one thing to keep uniform is mono versus stereo, since each file keeps its own channel count through the encode and a switch mid-book is something some players stumble on.

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.
Does Kataribe read the Vorbis comment tags in OGG files?
No. Tags in Ogg Vorbis and Opus files are Vorbis comments, and the builder's tag reader handles ID3v2 only. Title, author and narrator need to be typed, and chapter titles start from the filenames.
What about .oga files?
Accepted. .oga is the Ogg audio extension some tools use for Vorbis or for FLAC-in-Ogg, and FFmpeg identifies the codec from the stream, not the extension. Either way the audio is encoded to AAC.
Opus is a better codec than AAC. Why convert down?
Because the players are the constraint, not the codec. Apple Books and most audiobook apps don't open Ogg or Opus, and the M4B convention is AAC. Choose a bitrate a notch above the Opus source to cover the efficiency gap; 64 kbps AAC for a 32 or 48 kbps Opus file.
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.