[CELT-dev] Opus for audiobooks etc

Geoff Shang geoff at QuiteLikely.com
Thu Nov 17 14:49:55 PST 2011


As a blind person, this subject interests me greatly.

On Thu, 17 Nov 2011, Daniel Jensen wrote:

> The only comment I've seen about use of Opus for audiobooks was jmvalin
> saying in response to someone on his blog that Opus's ability to do
> fullband would be a key advantage here. This seems kind of
> counterintuitive to me- can people even ABX human speech at a 32 or even
> 24kHz sample rate from speech at 48kHz, much less hear a large quality
> difference?

32 kHz may be hard to notice.  24kHz I'd think would not be, and 22kHz is 
definitely noticeable.   It's not to say that this isn't acceptable to 
many, but it's easy enough to hear and I personally would prefer something 
higher, particularly if I'm going to pay for it.

> A number of audiobooks I've listened to have used 22kHz mp3s
> without being clearly objectionable, and in my personal use I've had
> decent results using the -voice LAME setting (downsamples to 32kHz and
> encodes as 56kbps abr).

hmm.   I just tried reencoding some speech material I have here at 64kbps 
MP3 (44.1kHz mono) which is not ideal I know, at 56kbps CBR 32kHz mono 
using lame -q 1.  I'm fairly hard-pressed to tell the difference, though I 
might with better source material.  Still, it'd be hard to tell in 

> For audiobook use, I don't know that the SILK modes or anything else
> with that low of a bitrate will be good enough, and when you're storing
> hundreds of hours of speech 64kbps adds up fast. I'd guess the sweet
> spot for audiobooks would be between 20 and 32 kbps, and this seems to
> my unschooled understanding to be a region where Opus's low delay might
> put it at a serious disadvantage.

Interesting you say this.  Audible (which uses its proprietory AA/AAX 
formats) offers books in a few quality encodes.  The best one, which IMHO 
is noticeably better than the others, is their AAX format (also known as 
Enhanced Format).  This format is at 64 kbps (not sure which codec it 
uses).  This means that a longer book, such as a book from Dianna 
Gabaldon's Outlander series, comes in at over a gig.  And yet they happily 
provide this format.  With storage what it is today, I don't think that 64 
kbps is the size problem it once  was, and the improved sound is quite 

As for the sampling rate of the Audible files, 
https://discussions.apple.com/thread/2422463?start=0&tstart=0 gives the 
rate for AAX-format books as being 22.05 kHz, but I doubt this.  To me 
they sound better than this, maybe 32kHz.  It sounds like it's not quite 
44.1kHz but the difference isn't really noticeable unless you are in a 
position to compare the two.  Certainly the listening experience is very 
comfortable, even on hi-fi setups.

And I'm glad the Opus developers considered audiobooks as a possible 
application.  I have to admit that it never occured to me, but the 
benefits are clearly obvious.


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