[opus] Opus frame size

[Manpreet Singh]

The Opus RFC seems to recommend a frame size of 20ms for most applications.
For wideband speech, the sweet spot range is recommended to be 16-20kbps.
20ms frames => 50 frames per second. For a VoIP application, the header
overhead per frame (IP+UDP+RTP+SRTP) is 44bytes => 17.6kbps at 50 frames
per second.

So a 20ms frame size seems to cause a roughly 100% overhead of header data.
Therefore, higher frame sizes seem to make more sense to reduce this
overhead. Is there any significant downside in using a higher frame size
for VoIP applications?

Secondly, the sweet spot is mentioned as 16-20kbps for wideband speech "for
a 20ms frame". Assuming this is just the codec bitrate (without any
headers), does the frame size really influence the Opus bitrate?

Thanks,
Manpreet.
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