Audiobook authoring on macOS

The audiobooks and radio dramas I’ve collected over the years mostly come as a bunch of MP3 files, often accompanied by some JPG with cover art and sometimes a TXT file with a teaser text and some background information like the names of the voice actors.

However, nowadays, there’s a much better format: M4B. This is basically an M4A container with all the metadata required to make an audiobook – e.g. cover art, author information, chapter marks (incl. optional chapter titles and art) and whatever you can put into an MP4 metadata tag. And the audio is encoded using either AAC (aka. AAC-LC) or the (more optimised for voice) HE-AAC and thus more space efficient than MP3.

Software

To convert my existing files, I’m using 3 different macOS apps:

Audiobook Builder (ABB) – This is the main star of this line up. ABB allows you to specify essential metadata, throw a bunch of MP3 files at it, give all existing chapters proper names and then select the desired quality and compile everything into a nice M4B file. Often, that’s all what’s needed.

mChapters – lets you adjust existing chapters, remove them, add new ones, etc.

Subler – Normally more oriented towards movie files, Subler lets you edit all metadata and define any tag you can think of. Great to add metadata ABB doesn’t handle.

Process

It all starts with ABB. Create a new book, give it a name, author and cover art. Then, optionally, click the “More info” button and add release date and a teaser text (description). Switch to the Chapters tab and add all MP3 files.

If all MP3s/chapters have the same cover art (open the Show Details section), you can select them all, then “Join” (using the button in the toolbar) all chapters, right-click the cover art and Delete it. Then “Split” the chapters again.

Once you’re happy with the chapters, go to the Finish tab to configure the output. It’ll show your last used settings. Click the Build Options button to configure the details.

Output Formats

I once spent an evening scouring forums and websites for comparisons of different bitrates and different codecs and came up with this little guide for myself:

• HE-AAC is mostly for speech, good at lower bitrates, worse than AAC at higher bitrates
• AAC 128 is better than HE-AAC 96
• HE-AAC at 48 Kbps is perfectly fine for spoken content; comparable to AAC-LC at 64 Kbps, and MP3 at 96 Kbps
• AAC at 96 Kbps is comparable to MP3 at 128 Kbps
• AAC at 160 Kbps is mostly transparent (i.e. most people don’t hear a difference from the original source)
• AAC at bitrates between 192 and 224 Kbps is comparable to MP3 at 320 Kbps
• AAC at 256 Kbps is better quality than MP3 at 320 Kbps
• MP3 at 192 Kbps VBR-ABR is nearly indistinguishable from Original (=mostly transparent)
• MP3 at 256 Kbps VBR-ABR is basically indistinguishable from Original
• Resampling from 44.1 kHz down to 32 kHz produces worse quality due to interpolation and is not worth the storage savings

(All for Stereo. If you reduce to Mono, you can also reduce the bitrate.)

I’ve settled on two settings:

Voice only – If it’s just people reading from a book, I – personally – find HE-AAC at (Custom settings!) 40 Kbps, Mono and 44.1 kHz adequate. This will give a nice small file size while the voices will still be perfectly understandable. Especially if you’re planning on listening to this during a car or train ride.

Radio dramas/Music – Whenever there’s more than just voices involved, I tend to use plain AAC at the “High” setting, which is 128 Kbps, Stereo at 44.1 kHz. Although, I’ve also used HE-AAC at “High” (80 Kbps) at times. This is down to personal preference. If you intend on listening with headphones, you might want to experiment a bit with this.

Once happy, click the Build Audiobook button to create your .m4b file.

Postprocessing

Chapters

I have some books that are just one single MP3 file without any chapter markers. At best, I have a list of the chapter titles (without timings) for them. For those files, I can open the M4B in mChapters and create chapter markers. It shows the sound waveform so you can easily identify longer pauses and put a marker there. It’s also great to adjust the exact point in time where one chapter end and a new one starts. You can also set the language of the audio and metadata tracks.

Metadata

If you need to modify specific metadata tags, you can use Subler to do it. (While it also supports editing chapter information, it doesn’t provide a nice sound waveform view or playback feature to do it.)

Audiobooks repurpose tags from music files, so in this raw form, it might look weird. This is the usual mapping that ABB and most audiobook apps use:

* For series of books, Apple Books uses the title and book number in the ©nam field – e.g. My Little Story Part 1 – and just the name of the series, e.g. My Little Story without any book/part number in the ©alb field.
† Apple Books uses only the ©ART field whereas other apps might use aART for the main author and ©ART for further/secondary authors. You can separate multiple authors using the ampersand/Et symbol (&).
‡ Separate multiple narrators using commas.

Apple Books integration

There’s a way to make Apple Books recognise a custom audiobook as if it was bought from Apple. This will make the app ignore all provided metadata and instead take everything incl. the cover art from iTunes. It’ll also put series of books in some kind of “container” and show other books from the series.

To do this, you have to add the book’s iTunes ID into the plID field. To get the ID, find the book in Apple Books and click the (…) button at the top right. There, select “Copy Link”.

Now, paste the link somewhere, e.g. Safari’s address field or into a new TextEdit document. The link will look like this:

https://books.apple.com/gb/book/my-awesome-book-title/id123456789

In this case, the ID is 123456789. Using Subler, add a field of type “playlist ID”, enter this number as its value and save your changes. After adding it to Apple Books, it will be detected as if you’ve bought it from iTunes and presented in a nice fashion as shown above.