[vorbis] Beta4 artifact/bug in the bass area

[Moritz Grimm]

> I belive, that solving this low frequency artifact, will also help reducing
> the clipping in low bitrate modes.
> The lower bitrate you use, the more clipping ocures, maybe some frequencies
> are amplified to much somehow.
> Something similar could also happen at frequences of 130Hz, 300Hz, 500Hz,
> 1200Hz and some others.
> Can somebody verify this?

Well, I can't verify this, but the weird thing is that the artifact only
appears in this relatively short period of the tune and both the
bassdrum and the ionosphere noise have been there before. So it's very
unlikely that this is about the frequency alone. Afair in this part of
the tune the ionosphere sample is pitched down one octave (played at
half speed), so the overlapping sounds of the pitched down noise plus
the deep bass of the 808 drum (that consist of not much more than one
very dominant sine wave) seem to confuse the encoder. It appears that it
can't distinguish those sounds properly so he puts them together and
invents stuff...?

Moritz

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