Re: [hatari-devel] Improved YM2149 sound

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On 26/05/2012 21:36, David Savinkoff wrote:
Hi Nicolas,

I'm going to air my thoughts so that you can analyse my reasonings.

As regards Paulo's tables, they were measured well, making it possible
for me to decide what was causing the non-linearities. I first thought it
may be compression due to transistor gain so I was looking at sigmoid
functions, especially tangent... but Paulo's tables disagreed. So I
thought of a voltage source that could pull up with a given Thevinen
resistance... Paulo's tables agreed. Next came circuit analysis and
experimentation... The rest is history. Paulos's tables give justification
to the math model. The math model allows me to determine the actual
resistances and ac voltages... and allows me to use the derived formulas
in further circuit analysis... The case is cracked wide open and can be
tested against itself. Paulo's tables told me enough to let me see the
rest of the picture... The math model goes beyond the tables now.

What the math model does:

- Paulo may have measured with per=0, but the math model gives a voltage
level for any discarded high (or low) frequency because the residual voltage
level is the dc component of that frequency.

- Modeling of all combinations of discarded high frequencies (Quartet)
and all combinations of non-discarded frequencies (Normal).

- Paulo's tables do not impose limitations on the math model because the
math model now describes Paulo's tables.

The code meets the objective of the math model as mentioned above.
The code does not differentiate between Normal mode and Quartet mode because
it is not necessary to do so... There really is no Quartet mode, just
filtered high frequencies... just like a real ST.

When "normal" (non sample) music is played and any voiceX goes to per=0
(or per is any filtered frequency), the new code treats the voltage exactly
as if it were filtered. This is what we want. Especially when it happens
transiently. The new code does way more than Quartet mode. Simplicity
leads to Generality!

A table to represent Normal (ac + dc components) and Quartet (dc component)
would have to be dimensioned:

((32 #ac+dc#)+(32 #dc#)) * ((32 #ac+dc#)+(32 #dc#)) * ((32 #ac+dc#)+(32 #dc#))

instead of the current:

(32 #ac+dc#) * (32 #ac+dc#) * (32 #ac+dc#)
Note that the current table treats Quartet mode the same as Normal mode by
using the same table-value for both.

I noticed that using the "-1" correction is a good substitute for a large
look-up table. It works good because Paulo's Quartet table is related to
the Normal table. Furthermore, the "-1" correction is already summed into
the table with the other values.

Picking 1 less from the Normal table as a Quartet value is a better
approximation than picking the same value (the current method in Hatari).

Picking 1 less also allows us to use the current table also!

I'll submit a comment patch tonight (pacific standard time).

Please let me know if my explanations are not clear, or if I'm
overlooking something. I'm now going to ponder the code changes
you mentioned.



I agree with your explanation, and especially with the fact that there shouldn't be 2 "methods" depending on the fact that the ST is playing samples or not, in quartet more or not.

Having a math model that fits all case looks good to me (I know some ST emulators are using different tables by trying to "detect" if the YM is currently used to play samples or not, I never really "liked" this idea).

But still, I think there should be a difference on whether you're modulating volume on a voice with (tone ON and per=0) and on a voice with (tone OFF and per=0), but in the case of your patch, only per=0 is taken into account.

Well, on the other hand, it's true that for per<=5 (or 80% of nyquist freq), output is set to 1, so we get the same behaviour as a "1" output when tone is OFF. Except that if voice is ON, we have a square wave on a real ST, instead of a constant "1" signal in Hatari's model.

If I understand, you apply a 0.5 coeff in that case to mimic the fact that the output power of the square wave is half the one of a constant wave. And that 0.5 coeff becomes "-1" at the volume level, since it's an exponential function. Is that it ?


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