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Why the Opus Codec Is Perfect for Audiobook Lovers

Posted in: Technology

Bookshelf with sign for Audiobooks above.

When a lot of people think about audiobooks, they will often think about streaming services like audible but at £7.99 per month, a subscription to audible is relatively expensive and there has been cases where users have had audiobooks removed from their library.

There are a variety of methods (both legal and illegal) of obtaining audiobooks with auction websites like eBay providing a large number of CD audio books for prices that are often cheaper than any audible package.

Finding the Perfect Audio Codec

Personally, I like to listen to audiobooks when I am going to bed as I also want to store a lot of audiobooks onto my micro-SD card, to do so I need to encode audiobook into an efficient audio codec. Finding an audio codec for encoding vocal lead content is slightly more interesting than just finding any old slightly efficient codec as our ears are sensitive to vocals at certain frequencies.

This is where Opus comes into its’ own as the codec incorporates both a speech (SILK) and music mode (CELT), which allows the codec (depending on bitrate and settings) to be efficient at a wider range of bitrates, with less artifacts.

Encoding the Audiobook

There are a number of GUI encoding programs that are able encode Opus audio, with differing features, but my suggestion would be foobar2000 for its relative simplicity and fantastic encoding features.

Probably the most important thing to consider when encoding an audiobook is what bitrate you are going to use as even though a lower bitrate means that you use less space on your storage medium, it will to at least to some degree also reduce the sound quality of the audio your are transcoding. So, to make your choice of bitrate easier to make I have included a simple table below;

12Kbps (VBR)	12Kbps Opus audio sounds a bit like a good quality phone call, so not fantastic but should be OK if you listening to your audiobooks though cheaper ear-phones.
16Kbps (VBR)	16Kbps is where things get interesting as speech audio starts to sound pretty good at this bitrate with only occasional artefacts. The problem with 16Kbps for me is that my ears are sensitive to high frequencies, especially sibilance making it unusable for me, due to the problems that bit rate has with artefacts at high frequencies.
32Kbps (VBR)	32Kbps is the bitrate that I personally use when encoding audiobooks for listening while going to bed. At this bitrate speech sounds really good, on top of using less than 15MB per hour of storage.
48Kbps (VBR)	48Kbps is not necessary for most audiobooks’ but there may be a few overlaying music which could take advantage of a slightly raised bitrate.

Published: 5th June 2019