Re: Playing samples on both new and old SIDs

From: Steve Judd (sjudd_at_ffd2.com)
Date: 2000-07-31 02:02:44

Hola Richard, Robin,

On Sat, 29 Jul 2000, Richard Atkinson wrote:

> On Fri, 28 Jul 2000, Robin Harbron wrote:
> 
> > Both the PRG and Mapping claim that pulse width=(register value/40.95)%,
> > so that setting the pulse width to 4095 would get a duty cycle of 100%.
> > It wouldn't be the first time they were wrong, of course...
> 
> That is wrong; I've checked this with an oscilloscope. Either set the PW
> registers to 0 and rely on the envelope generator at full volume for DC
> offset, or use the zero frequency method as already described.

Yeah.  Presumably the pwidth value is just added to a running counter,
with the high bits determining high/low.

Robin, one thing to keep in mind with pulse waves is that they are
"symmetric" about zero -- that is, a 20% duty cycle is identical (to the
ear) to an 80% duty cycle.  Except for a minus sign, they have the same
Fourier transform (1 - h(x) versus h(x)).  You can verify this with your
ear, using a pulse width of, say, 256, and one of 4096-256.  I think that
pwidth=4095 is the same as pwidth=1.  (Maybe you already knew this;
professor Judd will now step down).

I was just fiddling around with the tunesmith instrument editor, and I was
suprised to see that with a zero frequency, a pwidth setting of 0 on voice
three puts the oscillator output immediately at $FF; a setting
of 4095 puts the output to zero.  Are the outputs inverted wrt the
register value?  If not, then why does pwidth=4095 and freq=0 work for
digis?

Finally, I just observe that any waveform can be used to set the output
value -- for example, using a sawtooth and a frequency of 1, I can set the
freq to zero and halt the oscillator output at arbitrary values.

Speaking of which... has anyone ever sampled some of the combination SID
waveforms using a low frequency, like 1?  Is it the same as at higher
frequencies?  It always seemed to me that a little Fourier analysis ought
to be able to clear up the mystery of the combination waveforms.

-Steve

-
This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.

Archive generated by hypermail 2.1.1.