Re: [hatari-devel] Sound lowpass filtering?

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 03/09/2023 à 04:51, Brad Smith a écrit :
> Thank you for the explanation. I think I now understand where the theory 
> comes from.
>
> So rather than being a voltage source, you treat the YM as a variable 
> resistor, which connected with C10 forms a lowpass filter whose cutoff 
> changes with that variation. This does explain why the effect would not 
> by in my LTSpice simulation attempt, since I was looking at the circuit, 
> and treating the YM as a simple voltage input source. I had just used 
> the 0.96V to 1.35V range listed in its datasheet.
>
> So OK, I can follow that reasoning based on this assumption about the 
> YM's output.
>
>  > Note this model doesn't change the pull-up filter coefficients for
>  > the many different pull-up resistances. Instead it simply accounts
>  > for 1000 ohm pull-down, and a single lower resistance pull-up.
>
> This feels like a large omission to me. Shouldn't we expect a different 
> resistance at each level of the YM output then? We have 3 channels each 
> with 32 levels. The approximation with only 2 steps seems like it should 
> be valid for 1 square wave at 1 specific level, but the difference 
> should be weaker at a lower amplitude, and stronger at a higher one. 
> With 3 channels playing music, the levels will be rising and falling 
> continually between many intermediate levels as the waveforms interact.
>
> What is the 2 step approximation's point of reference? Is it for 1 
> square at full volume?
>
> I'm not sure what the impact of this omission should be, but in the 
> future I'll try to simulate the difference to investigate. I think the 
> disparity in the results must be due to more than just this, but it 
> could be a significant factor.

Hi

on the ST there's also the specific case that the 3 YM output are wired 
together, creating some feedback from one channel to other channel.

This explains why there were several "volume tables" used in games/demos 
to convert 8 bit samples into 3 volume levels, using real voltage 
measure of the 4096 possible levels, while on the opposite on other 
computers (MSX and also CPC IIRC) the 3 YM outputs are not wired 
together making it much easier to build a conversion table.

Maybe this wiring on the ST is also affecting how final output is 
filtered / altered (at scart or rca) ?

In all cases, thanks for investigating on this :)

Nicolas