[hatari-devel] YM2149 + C10 lowpass filter mapping

[Nicolas Pomarède]

Le 16/12/2011 12:21, David Savinkoff a écrit :
> Hi Nicolas,
>
> I've pondered over these patches for a couple of days and and
> believe the C10 capacitor filter is ready for inclusion in Hatari.
>
> The audio.c patch is the same as before, and the sound.c
> patch has prettier comments.
>
> This filter is better than the previous (unused) filter, so at least
> sound.c should be patched (if the filter selecting method in
> audio.c is unacceptable).
>
> I've listened to the mad_plus demo, sh11inv demo, blubber demo
> and key clicks to hear the theoretical sound difference between
> the ST and the STe (I don't have an STe). I think the demos
> sound better with some of the dry harmonics filtered.
>
> A two-channel bank-switched IIR filter is now used to model the
> YM2149 + C10 components. This is faster and way better than before.
>
> The YM2149 is filtered at 7.7 KHz when it charges capacitor C10,
> and filtered at 2.1 KHz when R8 discharges C10. This is exactly
> what happens on a real ST.
>
> David

Thanks,

I will try to test it and apply it this week end.

Just one question : on STF, why do you turn the filter only for 44100 
and 48000 Hz, but not for higher or lower freq ? Is 50066 good enough, 
it doesn't require a filter ? Are freqs below 44.1 kHz too "noisy", so 
the filter won't improve result anyway ?

Just wondering, what is the "mathematical" reason for this ?

Nicàolas