On Sat, 22 Nov 2003, Eckhard Stolberg wrote:
> 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.
I measured it.
I sampled C=4, F=7. That should be divide by 2, divide by 8.
Frequency analysis (audacity, view/plot spectrum/spectrum/512/hanning)
of the sample shows a peak at 1966 Hz.
1966 Hz * 2 * 8 == 31456 Hz
Any number of things might have affected this. Temperature is a factor,
both in the console and the audio card I used to sample. Clocks
on sound cards generally aren't really all that accurate. I've
seen and measured sound cards off by anywhere from 0.05% to as
high as 3%. Percent difference here is 0.09%, so it's well within what
I might expect from a modern sound card.
> > C=6
> > divide by 31, with an average 52% duty cycle
>
> Is that 16 HIGHs and 15 LOWs, or 15 HIGHs and 16
> LOWs?
16 high 15 low. Duty cycle is usually given as the percent time spent
on or high.
> > 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?
The TIA doc says they should be different, but I cannot hear a difference in
my sample, nor can I see a difference in the sample values. As far as I can
tell they are the same.
> > 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.
C=0 and C=11 difficult to do anything with, really. My short program
ran each mode for one second. That may as well be a flat signal with a
constant DC bias. Since the sound card filters out any DC components,
they both just look like a brief pop followed by silence in my sample.
I imagine Berzerk voice uses one of these, but I'm not sure which.
--
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
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