[opus] Opus for speech: VBR vs CBR

[Daniel K]

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!
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