[sdiy] Thoughts on VCO's

[Scott Gravenhorst]

Pardon my deviation...

Richard Wentk <richard at skydancer.com> wrote:
>
>On 2 Mar 2009, at 21:38, David Ingebretsen wrote:
>
>> I've got a few questions regarding VCOs. While a critical part of any
>> synthesizer, are there really a lot of differences?
>
>Yes. Filters are over-rated. Oscillators are under-rated.

I agree.

>> Here are a few personal preference questions:
>
>These really come down to clinical vs musical applications. The  
>problem with clinical VCOs is that while randomness *can* be added,  
>it usually isn't.
>
>Let's say you have three very accurate VCOs. You want to add some  
>random movement to them.
>
>To do a good simulation of analog splurge, you're going to need to  
>use three independent noise sources, with independent smoothing and  
>filtering.
>
>Most people either won't have those modules, or won't bother.

This is a good point.  My own experience has been with very imperfect VCOs which I
really love.  

Then I jumped the shark into the digital world which can be ever so sterile.  I did
exactly what Richard just said in one synth design - and it was amazingly nice,
starting with perfectly sterile "oscillators" and then applying various amounts of
noise to modulate pitch, filtered different ways gives rise to some incredibly
subltly analog sounds.  The noise sources are truly independant (4 noise sources,
maximal length LFSR types 61, 62, 63 and 64 bits long) and each has it's own filter.
 It makes a HUGE difference.  However - this is a digital system (FPGA) where
complexity doesn't take up square feet.  But it does seem that imperfection is
somehow interesting and that imperfections can be modeled quite convincingly with
digital methods.

>So - imperfect is definitely better. If you want clinical  
>oscillators, you can always build them digitally, 

[GRIN]

>and make them do a  lot more besides. They won't be cheap, but then neither are 
>some of the commercial VCOs.

Not sure about the "won't be cheap" bit.  I've got an Avnet board with a 400K gate
Spartan-3A FPGA that supports some pretty heavy duty stuff - 8 voice poly synth, 4
"vco" per voice, SVF per voice, "vca" per voice...  at the time I bought it they
were $39.  Price went up to $49, but still...  With an FPGA, it's what _you_ design,
now how it's designed.

>At the limit, the old Moog 901 is probably the poster child for  
>crappiness from a design POV - incredibly poor tracking and  
>temperature stability - but nothing else sounds quite like it.
>
>It's a nice simple, cheap circuit too.
>
>Richard

-- ScottG
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-- Scott Gravenhorst
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