[Vorbis] Questions on Opus encoding

[epicanis+xiph at bigroom.org]

On Tuesday, August 12, 2014 06:19:09 PM Ulrich Windl wrote:
> Hello!
> 
> I hope this is the right audience for Opus codec questions:
> From what I've read about Opus I got the impression that the codec will
> switch between speech and music encoding rather quickly. I wanted to try
> that for a CD track that has a vocal (talk) introduction followed by music
> (just two instruments). Using foobar2000 with some custom setup for Opus, I
> tried VBR encoding with a 256kbps setting (opus-tools-0.1.9-win32). My
> expectation was that the bitrate would be significantly lower than 256kbps
> while speech is encoded, while it would go up for the music part. However
> during playback I could not see a significant difference in foobar's
> (version 1.3.2 before you ask) display. Is this the way things should be?
> The other thing that confused me is that foobar displays 48kHz for the
> sampling rate in file properties (a bug?). Opusinfo displays "Original
> sample rate: 44100Hz".
[...]
As I understand it, one should think of the "bitrate" not as an actual 
"bitrate" but more like a "quality" setting that just happens to go from 0 to 
510 and coincidentally happens to roughly track the the resulting bitrate...

As far as the "speech" encoding, that (again, as I understand it) really
only comes into play at lower bitrates where the tradeoff in detail vs bitrate 
seems worth it (at higher bitrates, there are plenty of "bits" available to 
get the details of speech sounds with the high-quality general-purpose 
encoding method, so the special speech-optimized mode never kicks in.)