## Re: [stella] TIA polynomials revisited

 Subject: Re: [stella] TIA polynomials revisited From: "Eckhard Stolberg" Date: Fri, 5 Dec 2003 21:36:41 +0100
```> I was wrong about the duty cycle; it is 13/31 or 13:18
> Numerous apologies.
>
> I had mode 3 and 15 slightly off.

> static const int poly5[] = { // 5 bit LFSR [5 3]
>         5, 3, 2, 1, 3, 1, 1, 1, 1, 4, 1, 2, 1, 1, 2, 2, -1 };

> static const int poly68[] = { // used by mode 15
>         5, 6, 4, 5, 10, 5, 3, 7, 4, 10, 6, 3, 6, 4, 9, 6, -1 };

This is the same poly as in your previous version. If it
was off, what's the correct poly?

You said that mode 15 is the same as mode 7 except that
it is driving the div6 device instead of the div 2 device.
I think this is achieved by using the CPU clock as the
input into the 5-bit poly instead of the pixel clock.

Also you said that the math for mode 7 works out to be
the same as mode 9. So the output of the poly in mode 15
should be the same as the output of the 5 bit poly at
a third of the speed. So the following should be correct,
right?

static const int poly93[] = { // used by mode 15
15, 9, 6, 3, 9, 3, 3, 3, 3, 12, 3, 6, 3, 3, -1 };

> static const int divisors[] = { // frequency dividers
>         1, 1, 15, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 1 };

For mode 2 you have a divisor of 15. Since that isn't used
anywhere else in the TIA audio circuit, I'm wondering if
mode 2 actually has the 4-bit poly clocked by the output
of the div31 device with it's 13:18 duty cycle. Since this
device only changes it's state twice during it's 31 bits,
this would be similar to a div15.

Ciao, Eckhard Stolberg

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