[opus] Stereo dropping to mono with libopus 1.2 RC

[Matěj Steklík]

OK, so at the link : https://uloz.to/!yyVrCY2Y8sn1/devil-s-elbow-opus-7z
(https://uloz.to/%21yyVrCY2Y8sn1/devil-s-elbow-opus-7z) (change the language
to English by clicking at the flag at the right upper side of the web page 
or just simply click at "Stáhnout pomalu" - it may take some time as the 
file is 55 MB+ and the download service is free) there is 7zip archive with 
5 music files. The tune is "Nick Warren - Devil's Elbow" and is freely 
available from the author's Soundcloud account. I use this song for encoder 
testing because it requires quite a high bitrate for artifacts to be 
inaudible (especially the "gas leakage" kind of sound at the very beginning 
(repeated several times during the full song lenght) of the track. The 
content of the 7z file is the original track in WAV, 2 files encoded with 
libopus 1.1.5 at 32 and 48 kb/s (only with --bitrate 32 or --bitrate 48 
option, other settings on default) and 2 files encoded with libopus 1.2 RC1,
again at 32 and 48 kb/s with default setting. For the 32 kb/s encoded files,
there is noticeable drop of stereo (or large part of frequencies) at the 
beginning of the track roughly between 0:03 and 0:08 which is actually 
audible for both libopus 1.1.5 and 1.2 RC1 (I haven't noticed it before), 
but in the case of 1.2 RC1 it's much stronger and probably a bit longer. I 
think this "effect" occurs several times again further in the track but 
those occurences are always very short (in tenths of a second). At 48 kb/s, 
this is not an issue (at least not very obvious) for libopus 1.1.5 but still
happens for libopus 1.2 RC1, although the stereo/frequency drop is shorter 
than in the case of 32 kb/s. At 64 kb/s (file not included in the 7z 
archive) I can't hear it for both 1.1.5 and 1.2 RC1. The other thing is that
while the general sound quality of 32 kb/s encoded files is clearly higher 
for libopus 1.2 RC1 than for 1.1.5, at 48 kb/s the sound seems to be a bit 
clearer and closer to the original when using 1.1.5 than 1.2. This could be 
my subjective feeling as the difference is really tiny (unlike the bitrate, 
which is noticeably higher for the same bitrate setting for libopus 1.2 RC1 
for every file).




Best regards,
Matěj
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