Subject: Re: [stella] TIA polynomials revisited From: Adam Wozniak <adam@xxxxxxxxxxxxxxxx> Date: Fri, 5 Dec 2003 21:36:57 -0800 (PST) |
On Fri, 5 Dec 2003, Eckhard Stolberg wrote: > > 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? poly5[] is definately correct. I don't have all my work right in front of me, so I'll have to double check poly68 changes later. Did the poly465[] array change between the two versions you're looking at? > 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. No. There is no simple multiplier that will convert between poly5[] and poly68[]. The "divide by 6" and "divide by 2" are clocked by the 5 bit polynomial, NOT the other way around. f(g(x)) != g(f(x)). Mode 15 is NOT "mode 7 only slower". > Also you said that the math for mode 7 works out to be > the same as mode 9. Yes. > 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? No, see above. > > 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. Possible. I'd have to go back and look at the original data again to be sure. Maybe this weekend. -- adam@xxxxxxxxxxxxxxxx http://cuddlepuddle.org/~adam/pgp.html Will code for food. http://cuddlepuddle.org/~adam/resume.html "The dinosaurs are not around today because they did not have a space program." -- Arthur C. Clarke ---------------------------------------------------------------------------------------------- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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