Re: Crystal's new chip

At 03:36 PM 7/7/97 -0500, you wrote:
>> I don't have a schematic in front of me, but I thought Star Castle used one
>> of those "4 pin" shift registers. I don't remember them having an adjustable
>> clock speed. I thought it was done internal to the chip.
>
> Yeah, but those noise generators aren't used for the background sound at all.

Huh? I'm not sure what this sentence means. (Really I'm not being facitous! :)

I was originally refering to your need to change the "frequency" of the
background noise, and like I said I don't have the schematic here, but I
know they either used the all_in_one noise generator chip, or they did it
themselves using shift_regs/xor_gates. They then piped the output into a
low pass filter section, then controlled the attack/decay with one of those
current control amplifiers. I wasn't sure where the adjustable clock, going
into the noise generator, was.

>> There is an adjustable frequency, on Star Castle, it changes the frequency
>> of the background drone. I think it's done with a simple resistor DAC going
>> into a VCO, though I know some games used a digital divider (Boxing Bugs).
>> Either way you *could* just sample the 16 (or so) different frequencies.
>
> It's a 3 bit DAC, so you'd actually only need to sample the resulting 8
different VCO outputs, but there is a continuous-time LPF on the output of
the DAC, to let the VCO ramp up and down smoothly. If you just sort of
"switch samples" you won't get that smooth transition between the two.....

Easily done with pitch bend...

>
>> The beauty of a synthesizer chip, is that it can play a sample at different
>> "note" values, not to mention the MIDI "Pitch Bend". I believe you could
>> get by with one sample of either noise or oscillator. Through note changes
>> and pitch bends you should have nearly unlimited control of the
>> "frequencies". All the PLL stuff is built into the synthesizer, along with
>> all the wavetable control. There's nothing left for the PIC but "Play this
>> sound, at this note, with this pitch bend, with this ADSR envolope". Very
>> cool, very simple.
>
> I agree for all the other sounds (i.e. the noise generated ones.) The
envelope control on the chip is VERY cool (which is another pain, that I am
currently dealing with, is the envelope control, which, on the original
boards is generated by the exponential ramp of the bias current to the
Current-Controlled Amplifiers.)
>
> The PLL would still have to be external, because if you are going to use a
wavetable for the background sound, and just vary the rate you step through
it, you, again, want to ramp smoothly between the two frequencies. You can
do this by changing the feedback divider of the PLL, and, the PLL will
re-acquire its lock based upon the kind of LPF that you give it. This still
seems a bit tricky to me.

No need, you're missing the beauty of the synthesizer chip. It has pitch
bend. I can't remember what the midi specs is, but the resolutions is real
high. Something link 4096 or 2048 steps between half notes! Whether it's
done with a PLL (most likely) or whether they just tweak an analog clock
inside the IC, I don't care, the point is you have control of the frequency
of the played waveform with very high resolution.

If you find someone with a midi synthesizer, hooked up to a midi keyboard,
and press a note and play with the pitch bend control you'll see what I
mean. You can't here any stepping of frequency between one note and the
next, even though it's being fully digitally controlled.

As far as stepping through the wavetable ourself, I don't think that would
be possible. The table needs to be accessed by the synth chip in a truely
random way, in order to play back the 32 simultaneous sounds. And it's kept
in a "proprietary compressed" format. You might have access to that info, I
obviously don't, and don't need it. I would use the synth chip it's self to
save the samples. I'm not concerned with how it saves them. I'll just take
what ever data it saves in RAM, and ROM it.
>
> Although the CS9236 may have its own onboard PLL, I'm sure it's used just
for its own internal clock generation. There's no output clock from the
9236 (usually called CLKOUT on all of our chips.)
>
> I just re-read this post, and it seems kinda negative. Actually, that's
not my intent. You're right, for most of these sounds, it's pretty easy to
use that chip. I'll bet what it is, is actually a lobotomized DSP, or
rather, a dedicated DSP, so we both have basically the same idea.

All that *and* wavetable synthesis. Looks like a very nice chip, they told
my friend that they would be available near the end of the month (or maybe
they already are -- I'm having dinner with him tonight, I'll find out), but
we're definitely going to grab some.

The external ROM looked very standard in it's interface. Just a basic
parallel data and address busses.

-Zonn
Received on Mon Jul 7 14:09:31 1997