SV: Re: SV: Re: [sdiy] Digital VCO and cold sound!

[karl dalen]

--- Michael Bacich <weareas1 at earthlink.net> skrev:

> 
> On Apr 24, 2006, at 6:08 PM, Eric Brombaugh wrote:
> 
> >  So did they wrap a servo loop around from the comparator to the  
> > charging current input to keep the amplitude stable?
> 
> In most cases, they usually also have an analog voltage that rises in  
> proportion to the frequency of the programmable divider.  They use  
> that varying voltage to charge the ramp, resulting in a ramp that  
> stays fairly uniform in amplitude.  It's not perfect, but pretty good  
> enough.  Typically, they also use the output of that ramp to feed a  
> comparator to make the pulse wave.  They give the comparator a CV- 
> variable triggering threshold that allows the DCO pulse wave to have  
> PWM.  Essentially, this gives the DCO an entirely analog output.   
> (the fact that it's locked to a clock, however, gives it part of its  
> characteristic "cold" DCO sound -- the frequency never drifts)

Now , we'r talking! :-) Cold sound!
No way, DCO sounds much warmer then a  Moog oscillator! :-)

> > Also, if they were using counters for the reset pulse, how did they  
> > compensate for the reciprocal relationship between the count value  
> > and the frequency?
> 
> I'm not sure I understand your question.  Just to clarify, these  
> chips are used as programmable frequency dividers, not as "counters",  
> per se.  The official function name for the 8253 chip is "triple  
> programmable timer" (there are three independent timers in each chip,  
> each one can be used as a separate DCO voice).  The count value is a  
> binary word that's latched into the count value input of the counter  
> chip.  The actual counter output that's used by the synth is simply a  
> square wave at the frequency of the DCO.  More often than not, the  
> synth actually uses the direct counter output as the Square wave for  
> the voice, besides using it ti derive the reset pulse for the ramp  
> generator.
> 
> They can also use the rising and falling edges of the 50% square wave  
> to toggle a switch to alternate wave direction in a ramp-to-triangle  
> waveshaper, if they want to also provide a triangle wave (not very  
> common, though).

Yes they could but  has that ever been used?
Is there any unit that perhaps used it'?

  Most of this waveshaping stuff is done inside of  
> the CEM3394 chip that's used in the Oberheim Matrix 6, Matrix 1000,  
> and some of the Akais that had DCO's.  All the chip needs is the  
> square wave input and the CV's to control and select the waveshape  
> and the proportional saw amplitude.  This greatly reduces the amount  
> of ancillary circuitry needed by the system.  The Roland DCO synths  
> required a lot of analog circuitry per DCO.
> 
> > Plus the fact that the frequency resolution decreases as the count  
> > value decreases? Did they just use very fast clocks to drive the  
> > counters?
> 
> I don't know if it's true that the frequency resolution decreases at  
> lower frequencies. 

Its opposite, resoulution increases at lower frequency.
Its easily to look if a synth uses counter based oscilators
listen to rough steps at high frequency.

> The best of them actually provide two separate, free-running (analog)  
> master clocks for the counter banks, and use the two clocks for the  
> two DCO's of each synth voice. 

Best, i would say that a majority of them used that method of 
2 VCOs clocking the counter bank. Basically a easy way to
generate  pitch bend and vibrato.

KD