Re: [hatari-devel] Allow a wider sample rate range for the ST YM2149 filter

[David Savinkoff <dsavnkff@xxxxxxxxx>]

----- Nicolas Pomarède wrote:
> Le 17/02/2015 21:33, David Savinkoff a écrit :
> > Hi,
> >
> > This patch allows the 'ST' mode YM2149 filter to be used
> > with sample rates of 40 KHz and greater, instead of the
> > current (experimental) 44.1 and 48 KHz.
> >
> > This allows one to use rates such as:
> > 50066 Hz ( = ym2149 clock / 2.5)
> > or
> > 41721.667 Hz ( = ym2149 clock / 3.0)
> > or even
> > 62582.5 Hz ( = ym2149 clock / 3.0)
> >
> > To make a master sound capture file which is then
> > high quality re-sampled later.
> >
> 
> Hi
> 
> patch looks simple :)
> 
> But do you mean that only some specific ratios should be allowed above 
> 40 kHz, else the filtering would not be as good ?

It is the sample rate conversion that causes aliasing distortion.
(simple ratios are best - eg. ym clock/2 or ym/3 or ym*2/5=50066)

The filter's sample rate 'only' affects the filter's cutoff frequency,
nothing else. This filter is intended to produce the 'ST' tonality,
as this filter is only used in ST mode.

> 
> Nicolas
> 

I decided to look into improving the sound; given the procedures 'evil'
mentioned on atari-forum (and Eero added to the documentation).

Some specific ratios may work better (at least theoretically). I figure
that 40KHz and greater is the best for flexibility. Also, limiting the range
is not necessary.