[hatari-devel] Stereo DC filtering which accounts for DMA sound

[Nicolas Pomarède]

On 06/11/2011 04:57, David Savinkoff wrote:
> Hi Nicolas,
>
> "The Seven Gates of Jambala" on the Madness Demo has a noticeable
> difference in ambience. The stereo bass jumps and bounces from
> ear to ear because of shifting phase with the current DC filter.
> The stereo bass sounds refined, tight and proper with the new
> IIR filter (which performs identically to a first order Analog
> filter - eg. resistor + capacitor in the ST).
> Use headphones to examine the ambience closely.
>

Hello

I got time to test your patch, but I didn't hear such difference as you 
said with "7 gates of jambala" in STE mode or SH11INV in STF mode.

At least, that's good news, because there's no quality regression, but I 
don't hear differences using headphone.

For instance, I don't understand what you mean by "jump from ear to ear" 
? Do you mean sound goes from left channel to right channel ?

If so, that would be a bug in the previous dmaSnd.c file, but not an 
effect of the filter. There're 2 functions DmaSnd_LowPassFilterLeft and 
DmaSnd_LowPassFilterRight, and as Madness uses stereo output, I don't 
see how sound could go from one ear to the other ?

Anyway, if this patch is "mathematically" more correct than the previous 
one, I can apply it, it's just that I don't hear the differences (maybe 
others do ? You can try by getting hatari 1.5 sources and applying the 3 
patches sent on 4/11/2011 by David).

By the way, you say this emulates an RC filter ; but what are the values 
for R and C in the case of your patch ?

Thanks

Nicolas