[CELT-dev] Opus for audiobooks etc
jensend at iname.com
Thu Nov 17 15:22:00 PST 2011
OK, I hadn't realized that the higher-delay SILK modes were only saving
on frame and packet/transport overhead; I'd assumed that they'd be
getting some kind of prediction benefit.
> The low bandpass can be objectionable for the music parts in mixed content.
> Keep in mind that communication codecs are usually do a wideband at 16KHz,
> which sounds clearly and obviously worse for speech. (Although not
Right; I was imagining that some variety of SWB would be what one
usually wants for audiobooks.
> Here's Opus at 20 kbps beating AMR-WB, and at 32 kbps getting close to
> transparent (at 16 kHz samplerate, you may note):
Those were the results to which I was referring when I said "Some mono
tests seem to have given 32kbps Opus rather high marks." It's nice to
see that it beats AMR-WB -- IIRC Audible.com's low-bitrate codec is also
ACELP based and thus should be very comparable to AMR-WB. But AMR-WB+ is
a rather different beast, and since there's no royalty for mono AMR-WB+
decoding unless you're using it on a realtime channel, AMR-WB+ seems
like a credible competitor if you can stick to mono. I guess outside of
USAC it's the only answer I have to Greg's "disadvantage compared to what?"
Good to hear that those results may have been slightly anomalous and
that the stereo has seen major improvements since then.
> If your comparison points are Vorbis, MP3, Speex (or other
> pure-communication codec), or AAC it should be no contest.
I imagine so. It'd be nice to see how wide that gap is; has anyone
looked into doing an Opus vs. Vorbis, MP3, or HE-AAC test at lower
bitrates, esp for speech?
> For your application the improvements for encoder VBR and automatic speech
> detection that we're currently working on will probably be relevant. This
> use case would probably also benefit from additional look-ahead in the encoder,
> (and potentially two-pass rate control)
Great to hear. Is two-pass planned for after 1.0?
A couple of months ago I was in touch with Chris Lascelles, the guy
behind the Ear Bible (www.earbible.com), suggesting he keep an eye on
Opus because of its improvements over Speex. In his response he
mentioned that though they used Speex for their original product they
switched to Vorbis for a second version because they can use much
cheaper hardware that way. (I don't own one, so I can't check the
bitrates, but the original had 1GB of flash and a recorded bible is
anywhere between 55 and 85 hours, usu. in the 70s, so that's probably
~32kbps; I don't know whether the Vorbis version has more flash.) Does
anyone know of people who are looking at developing an Opus hardware
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