AmiMoJo's Journal: Compressing audiobooks with Opus

Opus is a free audio codec (RFC, BSD licenced library) for compressing speech. I've been experimenting with compressing audobooks so I can fit more on my phone. I should have splurged on the 128GB model.

For high quality audiobooks, i.e. downloaded from Google Play / Audible or ripped from CD, the results with Opus are excellent. Google uses M4A encoding at 32kHz but I found that by re-sampling to 16kHz and re-encoding with Opus at just 24kbits/sec VBR with the default settings I ended up with a file about 1/3rd the size and very nearly as good audio quality. The improvement over MP3 is even greater, down to around 1/5th the original size.

I have some older books that were ripped from tape and they needed some additional pre-processing to compress well. Noise reduction helps and you can do a fair bit with Audacity's filtering tools since voice tends not to be very dynamic compared to music.

I use fre:ac for encoding as it can go direct from m4a and do the down-sampling in one pass. You need the alpha version to get Opus codec support.

All this could have been avoided if the Pixel 4 hadn't sucked but maybe if I see a cheap Pixel 3 I can upgrade to 128 GB.