[sdiy] 566 Functional Replacement
tom at electricdruid.net
Mon Mar 19 18:55:02 CET 2018
This *is* definitely a 8-bit PIC design. Here’s a sketch:
Run an NCO as fast as you can. Output a triangle wave. You minimise the problems caused by aliasing, since the harmonics roll off so fast, plus you can whack the sample rate way up since all you need to do to generate the waveform is invert the phase accumulator for the second half of the wave. Wave output goes through a simple RC filter off chip, then back through an on-chip comparator for your square output with no aliasing either. Bonus points if you can open up the other comparator input to allow PWM.
Synth & Stompbox DIY
> On 19 Mar 2018, at 16:26, Tim Ressel <timr at circuitabbey.com> wrote:
> When I sed 'VCO' I did not mean a full-featured volt per octave VCO. I literally meant an oscillator that has voltage control. Cv in, triangle and square out, thats it. thats what the 566 did. that is what I want. 8 pins, 5 passives, and you're in business. thats what the 566 brought to the party.
> On 3/19/2018 9:21 AM, Mattias Rickardsson wrote:
>> Some thoughts about the dream VCO discussions here:
>> What would be the main reasons for wanting a tiny VCO IC?
>> I can find it hypothetically attractive, sure, but given that it will never have many features AND a small size at the same time, I'd almost always need to add stuff outside in the end since you rarely want just the basic functionality. If I'd use a dedicated VCO IC I'd rather have some features that are sometimes not in use than having to add near-basic functionality with external components. The only true reason for a small chip that I can come to think of is cost. I guess it somehow scales with size in IC fabrication?
>> Making wishlists with VCO features, ending up on the 3340 specification from 38 years back... aren't we missing something? :-)=
>> Many modern analog oscillators (both in modulars and in hardwired synths) have other features than the ones we had in the '80s. This should be reflected a bit in the wishlists we make for VCOs today, even if most of these features could be added outside the main chip. Exponential and linear CV should be mandatory, triangle/sawtooth/square outputs as well - and I'm thinking that a triangle core would be the way to go, even if proper sync needs to be dealt with in a less obvious way. That said, we're still stuck in 1980. Sine shaper, thru-zero FM, what more is desired for the synthesists of today? :-)
>> Btw, Tim P, that negative expo CV allowing a single inverting CV summer is a smart choice. What about the double cap pins in your suggested pinout? Do they give a better solution than the cap-to-ground used in both CEM and SSM ICs?
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> --Tim Ressel
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