Re: [hatari-devel] YM2149 Anti-Alias Filter

[David Savinkoff <dsavnkff@xxxxxxxxx>]

On Sunday 17 June 2012 14:53, Nicolas Pomarède wrote:
> On 10/06/2012 23:27, David Savinkoff wrote:
> > Yes. By-the-way, the low-pass anti-alias cutoff frequency is not
> > critical as long as it doesn't filter the music, thus, a convenient
> > and existing filter is used. Note that this filter (and the previously
> > disabled filter) are an alternative to BLEPs.
> > https://ccrma.stanford.edu/~juhan/vas.html
> >
> > This anti-aliasing makes an improvement to the sound. Listen to
> > P7ACTION (STE) and other tunes in 505's BLUBBER.PRG for
> > clarity.
> >
> > The pull-up and pull-down resistance are of significance when making
> > DSP filters; however, these resistances are usually the same. To
> > model the YM2149, one must do circuit analysis and go beyond what
> > is written in DSP textbooks. This exercise got me to 'at least' make
> > some personal discoveries which others can now ponder over.
>
> Hello
>
> thanks for the explanations.
>
> While applying the patch, I noticed in Audio_SetOutputAudioFreq in audio.c
> :
>
>          if ((ConfigureParams.System.nMachineType == MACHINE_ST) &&
>                  (nAudioFrequency == 44100 || nAudioFrequency == 48000))
>          {
>                  /* Apply YM2149 C10 filter. */
>                  UseLowPassFilter = true;
>          }
>          else
>          {
>                  UseLowPassFilter = false;
>          }
>
> Should this be changed ?

No. Unless there is something I'm not aware of.

> Should we use piecewise filter all the time ? 

No. UseLowPassFilter=True is for ST at 44100 Hz and 48000 Hz
sample rates only; UseLowPassFilter=False is for all else
(Anti-alias filter for all else).  Both filters are piecewise, both
filters also anti-alias, but the ST filter affects the tonal quality
as the 0.1 microfarad capacitor does in the ST.

David
>
> Nicolas