i'm too afraid to ask, but
what exactly happens when you specify "--ns-sfb21 <negative value>" in lame commandline?

I believe it lowers the ath calculations for sfb 21 so that more high frequency content is encoded.

It was added to reduce ringing, whereby sfb 21 cuts in and out rapidly on some material.

There may be other reasons but you will need Dibrom, John V etc to tell you for sure.

what does a negative or positive value do ?

i mean,
i understand you can specify a positive ie. "--ns-sfb21 3" or
a negative value ie. "--ns-sfb21 -3".
One of these will reduce 16KHz> content more and the other will
try and encode more of that 16> content
If i'm not mistaken at this point

Yes, a value like ''--ns-sfb21 3'' will reduce the contents over 16Khz and the benefits can be less ringing and a reduced bitrate in VBR encoding (it was mentioned that ns-sfb21 <positive value> reduce the bitrate bloat).

thanksfor the reply,

so if a + value will try to reduce the bloat because of 16+ content.
does a negative value work the other way around ?
i read somewhere only "--ns-sfb21 4" actualy works (don't know
what he meant exactly)

Yes, it should do the opposite (increase the econding of contents above 16Khz, bitrate demanding and may increase artifacts significantly).

thanks for taking the time to answer my question ,

morelli