[opus] Stereo dropping to mono with libopus 1.2 RC

[Jean-Marc Valin]

On 19/06/17 12:05 PM, Matěj Steklík wrote:
> so it would be great if the final libopus 1.2 had officialy a
> parameter to force music "mode" of encoding.

Support is already there in libopus 1.2 (and has been since 1.0), so
it's just a matter of adding the option to opusenc. That's on the TODO list.

> I'd like to ask two
> additional questions: 1) is the --set-ctl 4024=3002 parameter a present
> alternative to that planned "force music encoding" option or is it an
> other workaround?; 

the --set-ctl is more of a hack, which is why the plain is to have a
real option.

> 2) if I wanted to set opusenc to not use SILK at all,
> is there a minimal bitrate which I have to set (such as 48 or 64 kb/s)
> or is it that when during VBR encoding the actual bitrate at one point
> drops below some value, the SILK or hybrid mode is used even if the
> target file bitrate is in CELT territory (48 kb/s+)? Thanks for answer.

There's a bitrate above which SILK will never be used, but it depends on
many things, including the frame size, complexity setting, and the
stereo width. I have to say that for stereo music, it's rather rare (but
not totally impossible) that SILK ends up being used above 48 kb/s.

Cheers,

	Jean-Marc


> Best regards, Matěj 
> ---------- Původní e-mail ----------
> Od: Jean-Marc Valin <jmvalin at jmvalin.ca>
> Komu: Matěj Steklík <matej.steklik at post.cz>, opus at xiph.org
> Datum: 19. 6. 2017 5:39:15
> Předmět: Re: [opus] Stereo dropping to mono with libopus 1.2 RC
> 
> 
>     Hi Matěj,
> 
>     I had a look at your files and it seems like the issue is that the
>     speech/music detector thinks that some segments are speech rather than
>     music and at low bitrates SILK tends to significantly narrow the stereo
>     image. We're planning on adding an option to opusenc to force it to
>     treat the input as speech/music. Just to confirm that's the issue, can
>     you try encoding with the following option:
>     --set-ctl 4024=3002
>     and see if that fixes the issue?
> 
>     Cheers,
> 
>     Jean-Marc
> 
>     On 18/06/17 03:45 PM, Matěj Steklík wrote:
>     > 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
>     >
>     >
>     > _______________________________________________
>     > opus mailing list
>     > opus at xiph.org
>     > http://lists.xiph.org/mailman/listinfo/opus
>     >
>