Re: [stella] TIA Audio Polynomials

Subject: Re: [stella] TIA Audio Polynomials
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Sat, 22 Nov 2003 22:33:32 +0100
> The tia manual says "5 bit poly --> 4 bit poly"
> note that 465 == 31*15
> There's probably some way to chain the two 4 and 5 bit polynomials
> together to get that same bit pattern.  Suggestions are welcome.

Maybe the output of the 5 bit poly gets fed into the 4 bit
poly and vice versa?

Or maybe the 5 bit poly is used to clock the 4 bit poly?
So everytime the 5 bit poly produces a LOW, a new value
is generated on the 4 bit poly.

> Mode 15:
> Again, suggestions on how to simplify this are welcome.

To go along with the suggestion above, maybe the 5 bit
poly gets used to clock a pure tone. So everytime a LOW
is produced by the 5 bit poly, the audio output for that
channel is inverted.

> This seems to do an acceptable job of sample rate conversion.
> Not I switched the base frequency from 31400 to 31456 based on what I was
> seeing in the frequency analysis of my sample data.

How did you get this frequency? I opened up my NTSC VCS,
and the clock crystal reads 3.579575 MHz. So assuming that
the TIA produces two samples per scanline, that would be
one sample every 114 TIA clocks. This should give a sample
rate of almost 31400 Hz. My PAL VCS is only running at
3.546894 Hz, so the sample frequency should be lower too.

> C=6
> divide by 31, with an average 52% duty cycle

Is that 16 HIGHs and 15 LOWs, or 15 HIGHs and 16

> C=7
> LFSR [5 3] (tia doc says 5 bit poly / 2; this is incorrect)
> C=9
> LFSR [5 3]

So you are saying that C=7 and C=9 are actually identical?

> C=11
> set last 4 bits to 1

Do you know what it is that has it's last 4 bits set to 1?
It seems that the output of C=11 is all HIGH just like C=0.

Ciao, Eckhard Stolberg

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