Re: Crystal's new chip

> 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.
 
> 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.....
  
> 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.

        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.

Joe
 
Received on Mon Jul 7 13:38:30 1997