Jim Nitchals wrote:
Thanks for the great response! Incidentally, I think
this'll make a great article for C=Hacking mag :)
> > Anyway, assuming it's right, I can't
> > use these optimal frequencies. Another point of
> > clarification, is this pulse width of 227 microseconds
> > the whole cycle? From 0 to + to 0 to - and back to +,
> > if you get what I mean... basically, does "pulse
> > widths" = "cycle length"?
> >
> These tests were performed using sine waves, not square waves,
> i.e. a full cycle from 0 to max, down to -max, back to 0.
Since the SID in the C64 doesn't have a sine wave - should
I use a triangle wave? It'd be close than a pulse wave...
> > I've arbitrarily chosen new widths of:
> > 0 bit : 286 ms = 3500 Hz
> > 1 bit : 428 ms = 2337 Hz
> > Should these be fine?
I've actually dropped these far lower, with still no success - 700 ms
and 2000 ms now, I believe - I don't care about the speed of it for
now - I just want it to work! :)
> Should be wonderful, but you can definitely use lower frequencies and
> improve reliability against jitter, if you have imprecise timing caused
> by other sources. Have you accounted for screen DMA and interrupts?
> Vblank interrupts will definitely ruin the timing; screen DMA may do
> harm to frequencies as high as you're pushing.
I'm actually having the hardware generate the waveform right now -
just tell the SID chip what waveform and frequency, and it does it -
then I set an interrupt that _should_ occur when cycle is about
complete - then a new frequency is popped in, according to the next
bit to be transmitted, and it's done over and over again. It sounds
exactly like a Supercharger tape to me - but the Supercharger doesn't
agree - it just tells me to rewind the tape immediately. BTW, all
interrupts and screen DMA have been killed while playing the .bin.
If there are any C64 fans around who want to see the source code, and
possibly help - let me know! I'd really like to get this going,
so me and some others can start some 2600 coding :)
> Here are some figures to work with (22.68 microseconds per clock at
> 44.100 KHz):
> Zero One
> 7 14
> 8 12-19
> 9 13-23
> 10 14-24 lowest amplitude needed is using widths 10, 15
> 11 14-25
> 12 16-27
> 13 17-28
> 20 23-54
> 30 33-87
> 40 43-107
Cool, but I don't understand :) I'm sure I should, and can understand,
but I don't. Duh - what do the numbers below the zero and one mean?
Are these the length of the wave in clocks?
Thanks!
Robin.
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