samples where aps or ape has a problem Options

[2Bdecided]

I'm sure this has been asked before, but I can't find it, and it's not in the FAQ (it should be - please!)

On which samples is --alt-preset extreme audibly better than --alt-preset standard.
On which samples is --alt-preset insane audibly better than --alt-preset extreme.
On which samples is --alt-preset insane not transparent?

Cheers,
David.

[JohnV]

You and JohnV have removed any serious quality worries, but there was also the issue of bloat. I've read that -Z may increase the bitrate. Maybe I don't have the right test samples, but this is barely happening here - it's a 1kbps increase.

Can someone who has a CD which produces a low bitrate using aps try encoding it again with aps -Z please?


DickD - yes! Will someone else listen to aps -Z please? I'm a good listener, but I'm in a noisy environment and I can't hear above 16kHz - so other people really need to check what I'm saying! It doesn't have to be badvilbel - there's plenty of other fun samples - or just grab a CD at random.

Yes, using aps/ape -Z increases bitrate but not always. Also one have to understand that there are only some positions on a track where noise-shaping 1 and noise-shaping 2 can sound different, the positions where one or both noise shaping types use scalefactor_scale.

Noise Shaping 1 (APS/APE -Z):
When Lame notices that all scalefactors are even numbered, it halves the values thus saving some bits in coding. This is quality wise "lossless" and is no different outcome than using no scalefac_scale at all (except for the bit saving).

So when Noise Shaping type 1 is used, only lossless bitrate saving happens in certain places. (You can see with EncSpot how many percent of the track is encoded using scalefac_scale).


Noise Shaping 2:
Noise Shaping 2 adds another trick for bitrate saving. When the encoder decides to use scalefac_scale (not necessarely in same places as with NS1), the scalefactor values are doubled. Since scalefactors are used to control the reduction of quantization stepsizes (which control how much noise per SFB is present), the dynamics is larger for the stepsize adjustment. It can go more closer to the "edge" of allowed noise, which of course saves bits.

With Noise Shaping 2, Lame should use scalefac_scale only when it's "safe". Obviously this is not always succeeding, and NS2 scalefac_scale can introduce even quite bad audible distortion sometimes, especially when using vbr-old nspsytune. The masking threshold/allowed noise is not perfect because of the imperfect psychoacoustics, and sometimes it fails when it has the chance to go more closer to the edge. I'd think with better psychoacoustics NS2 would work ok.

Now because of code level tweaking, APS/APE use less often Noise Shaping 2 than without code-level tweaking, but it's still not enough to be totally "safe".

IMO NS2, because it's not safe, is not the right type for VBR coding. But CBR acts differently and low bitrate CBR can gain quality from the bitrate savings given by NS2 scalefac_scaling.