Audible audiobooks -- What format of compression do they use?

[br0adband]

The question is pretty self-explanatory.

I've been trying to find some reasonably decent information about what format Audbile uses for their audiobooks but I haven't found anything worthwhile so far.

Does anyone have any ideas or places for more information? I think they're using low bitrate AAC but I'm not sure and I'm not about to cough up hard earned cash just to find out.

Thanks,
br0adband

[cordevax]

there's an audio codec out there called speex (i think) which is designed purely for spoken word.
i don't know if audible uses that, but if you're looking for a codec, this might be one to take a look at.
more details at hydrogenaudio.org

[br0adband]

Yep, I know about Speex. I'm all over HydrogenAudio almost daily but Audible uses something else and Speex is pretty much for computers only (not aimed at portable mp3 devices). Again, I'm led to believes it's an AAC-related format, probably just low bitrates.

br0adband

[Caribbean Soul]

Excellent question. FWIW, here's a quote I read on another board from someone who used them last year.
All the formats are "audible.com" format (download a s a .aa file), which is why you can't use them in any old MP3 player.
The person I quoted above was not happy with their medium fidelity downloads, but was okay with their high fidelity downloads. Again, this info is from last year with using an iPod and a cassette adapter in his car during his commute, and the downloads coming from their audible.com site.

I agree that I'd like more info on what they are doing.

[br0adband]

.aa huh...

I would still suspect it's just another variant of AAC to maintain the DRM stuff. If I remember correctly one of the original commercial uses of AAC audio compression was (and still is) LiquidAudio. I bet there is some correlation there between what LiquidAudio was and what Audible now is.

Hmmm...

/me hits Google :D

br0adband

[akellen]

According to their website (www.voiceage.com), VoiceAge provides a proprietary codec called ACELP.net that is used by Audible.com.