[opus] Opus encoding rate for very quiet noisefloor

[Timothy B. Terriberry]

Kevin Connor wrote:
> However, I wanted to ask the list if this is expected/known behaviour.  I wonder if it is possible to engineer a scenario (using existing apis)  that would give one the benefits of _AUDIO mode (low delay, great fidelity, avoid voice-EQ) without this particular sensitivity that gives high bitrate for nearly inaudible noise floor.  It's a lot to pay for near subliminal comfort noise.  I'm prepared to hear that the answer is: choose VOIP mode ;)   Thank you!

libopus doesn't know what your volume knob is set to, so it can't tell 
if that's "near silence" or important signal that should be preserved. 
You can help it out using OPUS_SET_LSB_DEPTH [1]. If you're injecting 
noise at a level of +/- 2 in 16-bit input, try setting it to 14.

[1] 
https://www.opus-codec.org/docs/opus_api-1.1.2/group__opus__encoderctls.html#gaa23940eb477ff617edc14b8d66e104c0