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

I'm not a LAME developer but I am a developer that uses LAME. I'm curious if you are both saying that using -q4 with a CBR encoding
would actually produce better results than -q0?

-----Original Message-----
From: Maik Merten via Lame-dev [mailto:[email protected]] 
Sent: Thursday, 18 July 2024 4:40 am
To: [email protected]
Subject: Re: [Lame-dev] CBR/ABR quality regressions for q0, q1, q2 and q3

Hello again,

the regression was introduced with

https://sourceforge.net/p/lame/svn/6147/

With this commit, CBR and ABR encoding was switched over the the newer 
VBR-psy-model.

Other than that, the encoding loops for CBR and ABR apparently were not 
changed. With the older psy-model (up to revision 6146), 
noise_shaping_amp being set to 1 or 2 does not appear to have a 
detrimental effect. This demonstrates IMO that noise_shaping_amp is not 
in principle incompatible with CBR/ABR, but presumably something went 
awry when switching over to the VBR-psy-model.

Disabling noise_shaping_amp for the affected modes still seems to be the 
easy workaround, but if it's possible to have noise_shaping_amp work as 
intended, that'd of course be very neat.


Best regards,
Maik


Am 12.07.24 um 18:57 schrieb Maik Merten:
> 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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