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

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

On 24/05/2012 04:43, David Savinkoff wrote:
> Hi Nicolas,
>
> Here is an anti-alias filter for the YM2149.
>
> This piecewise selective filter works by filtering the falling
> edge of a sampled pulse-wave differently from the rising edge.
>
> Piecewise selective filtering is effective because harmonics on
> one part of a wave partially define harmonics on other portions.
>
> Piecewise selective filtering can efficiently reduce aliasing
> with minimal harmonic removal.
>
> I disclose this information into the public domain so that it
> cannot be patented. May 23 2012 David Savinkoff.
>
> David

Hello

I'm slowly processing my unanswered mails ...

So, in your PWMaliasFilter, you give :

	if (x0 >= y0)
	/* YM Pull up   */
		y0 = x0;
	else
	/* R8 Pull down */
		y0 = (3*(x0 + x1) + (y0<<1)) >> 3;

But in the LowPassFilter we had :

        if (x0 >= y0)
        /* YM Pull up:   fc = 7586.1 Hz (44.1 KHz), fc = 8257.0 Hz (48 
KHz) */
                y0 = (3*(x0 + x1) + (y0<<1)) >> 3;
        else
        /* R8 Pull down: fc = 1992.0 Hz (44.1 KHz), fc = 2168.0 Hz (48 
KHz) */
                y0 = ((x0 + x1) + (6*y0)) >> 3;


Is it normal that the YM pull up behaviour in low pass filter is now 
used in the R8 pull down part for the PWMaliasFilter ?


Also, what do you mean about patent ? What part of this anti-alias 
filter would be patentable ? The coefficients used ?


Nicolas