Re: CBR/ABR quality regressions for q0, q1, q2 and q3

I noticed that -V9 -q0 does sound identical to -V9 -q4 (at least on
my Sony XM3s), also I cannot spot any difference in encoding time
between the two (which I would expect from different -q settings, as is
clearly the case when using -cbr).

Also AFAIR this has been the case for a very, very long time.

I don't know if this is to be expected and a trivial observation or if
it does indeed add anything to the conversation.

Just my 2 cents,

     Tobias


Am 12.07.24 um 18:57 schrieb Maik Merten via Lame-dev:
> Hello there,
>
> it has been observed that with CBR and ABR, perceived audio quality
> increases when using faster settings than default.
>
> A discussion regarding this over at HydrogenAudio:
> https://hydrogenaud.io/index.php/topic,126120.0.html
>
> This is very easily noticeable with very low bitrates, but is also
> detectable with the default bitrate of 128 kbps. The discussion thread
> on HydrogenAudio includes samples.
>
> I traced this down to cfg->noise_shaping_amp, which is set to 1 for
> -q3/-q2 and is set to 2 for -q1/-q0. Choosing a value of 0 (as for the
> faster quality settings) improves perceived audio quality.
>
> I opened up a bug report a while back over at
> https://sourceforge.net/p/lame/bugs/516/ with trivial patches and an
> attempt at a theory of why quality might be affected negatively with
> noise_shaping_amp being set to 1 or 2. This, however, is merely
> speculative, as it's not quite clear to me what settings 1 and 2 are
> actually trying to achieve - presumably, those are intended to increase
> quality (and perhaps they do in VBR?), but I don't quite understand the
> reasoning behind those approaches.
>
> While setting noise_shaping_amp to 0 is a trivial thing to do, it might
> also be (pure speculation, again) that the actual root cause is with the
> CBR and ABR encoding loops somehow not interacting properly with
> noise_shaping_amp 1/2 and that a "true fix" might involve tinkering with
> those encoding loops.
>
> How would one progress on this issue? With -q3 being the default
> setting, rather regrettably CBR and ABR encodings are impacted
> negatively by default.
>
> Another, but most likely minor thing: It appears that noise_shaping_stop
> is unused, yet set up by lame_init_qval. I assume this could be removed?
>
> Best regards,
> Maik
>
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