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

It's definitely a problem.  Anyone know if Robert Hegemann still around/available to take a look at his code?

-----Original Message-----
From: Maik Merten via Lame-dev [mailto:[email protected]] 
Sent: Thursday, 18 July 2024 6:00 pm
To: [email protected]
Subject: Re: [Lame-dev] CBR/ABR quality regressions for q0, q1, q2 and q3

Hello Ross,

yes, with LAME's current release (and also LAME's SVN), it's easy to 
come up with samples where -q0 sounds worse than -q4 in CBR and ABR 
encoding. This is especially apparent with low bitrates, but can also be 
detected at the default bitrate of 128 kbps.

Personally, I haven't had a sample where -q4 sounds worse than -q0 (but 
plenty where -q0 sounds worse than -q4), which is why I'm confident this 
is an actual bug, but I certainly recommend giving it a try with your 
own ears.

To easily be able to distinguish the difference, one can experiment with 
ambitious low-bitrate encoding (assuming test.wav is CD-quality stereo 
music):

lame --resample 44100 --lowpass 16000 -b96 -q0 test.wav q0.mp3
lame --resample 44100 --lowpass 16000 -b96 -q4 test.wav q4.mp3

(these are settings below MP3's comfort level, but one wouldn't expect 
-q0 to do worse than -q4 nonetheless)

Depending on the input, -q0 can collapse into a smeary, metallic mess 
where -q4 holds up noticeable better. Usually, the lower the bitrate, 
the more obvious the differences become.

Some more discussion and audio samples have been posted over at 
https://hydrogenaud.io/index.php/topic,126120.0.html

Best regards,
Maik


Am 18.07.24 um 02:59 schrieb Ross Levis via Lame-dev:
> 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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