[opus] Opus 1.3 different default bitrate between opus encoder and opus multistream encoder

[Kevin O'Connor]

I use the Opus multistream encoder for both mono and stereo encodings and after updating from 1.1.3 to 1.3 I noticed the size of the produced Opus files had doubled for 1-channel encodings whereas switching to the standand Opus encoder gave me roughly the same sizes as before. In these tests, I'm encoding an 8kHz mono stream containing only speech with the following options set:

frame length = 20ms
application = voip
bandwidth = auto
max bandwidth = narrowband
bitrate = >unset<
force channels = auto
input precision = 14
vbr = true
fec = false
vad = false
complexity = 4
signal = auto

Using the standard Opus encoder, I get a bit rate around 10kbps and using the Multistream encoder, I get a bit rate of around 20kbps.

I can explicitly set the bit rate back to 10kbps to get around this difference, but thought it was worth reporting in case this isn't expected behavior.

Kevin O'Connor
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