[opus] Opus for speech: VBR vs CBR

[Daniel K]

Hello Basil,
Yes I actually did a comparison and the quality was lower with VBR. If I
use CBR and set the bitrate to 20kbps, the quality was better than
VBR+20kbps (actually uses a lower bitrate).

But I should mention that this is device specific. On some devices,
VBR+20kbps uses around 17-18kbps with active speech and the quality is
good. On others, it ends up using 11-13kbps and the audio isn't as good.
But forcing CBR at 20bkps sounds much better in the latter case.

That is why I ask what determines the choice of bitrate when using VBR and
if it can be influenced to use a value closer to the target bitrate.

Thanks!

On Thu, Jan 22, 2015 at 6:20 PM, Basil Mohamed Gohar <
basilgohar at librevideo.org> wrote:

> On 01/21/2015 07:51 PM, Daniel K wrote:
> > Hi guys,
> >
> > I'm using Opus for speech in wide-band mode (sampling rate 16000) and
> > 20ms frames with signal type set to SIGNAL_VOICE.
> >
> > I have a few questions here:
> >
> > 1.
> > When I choose VBR mode, the codec seems to choose the bitrate on its own.
> > However, that seems to be an issue on mobile devices. In some cases,
> > when I configure the bitrate to say 20kbps, I see that the outgoing
> > codec bitrate at runtime is sometimes far less (11-12kbps even). That
> > leads to a lower received audio quality.
> > When I choose CBR mode however, the outgoing bitrate is fixed at 20kbps
> > and produces better audio quality.
> >
> > Is there a way to bias the VBR mode to use a value closer to the
> > configured target bitrate or fix this low bitrate issue in any other way?
> >
> > 2.
> > When using CBR mode, there seems to be an issue when I use frame sizes
> > >20ms. On the receiving end, there appears to be a distortion (a
> > crackling noise) that does not happen when using VBR mode on the
> > encoding side (all else the same).
> > Has anyone experienced this and/or can suggest a fix/workaround for this
> > issue?
> >
> > 3.
> > Overall, is using VBR vs CBR mode noticeably better or worse in terms of
> > quality/bitrate? Any recommendations here?
> >
> > Thank you!
> Daniel,
>
> When you say, "lower perceived quality" at the lower rates, did you
> actually do a comparison, or are you assuming it must be lower quality
> because it's fewer bits?  If the quality is lower when the bitrate the rate
> controller chooses drops, then that's a bug, because the default, VBR mode
> should maintain a consistent quality even if the bitrate fluctuates.  In
> fact, the bitrate should vary because different segments of audio, voice or
> otherwise, do not require the same bits for the same quality.
> --
> Libre Video
> http://librevideo.org
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