SV: Re: [sdiy] Digital VCO

[Michael Bacich]

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)

> 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).  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.  If it does, I've never noticed it in the sound,   
It is true, however, that these systems use fairly high frequency  
master clocks for the counters.  Most of these synths seem to have  
pretty decent fine pitch control (for pitch bend, fine tuning, DCO  
detuning, etc.).  This type of system even allows for pretty  
convincing keyboard portamento over fairly wide pitch intervals  
(created by gradually changing the count value).

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.  This gives the system a more natural  
"analog" sound.  Since the two DCO's are not locked to the same  
master clock, the end result is kind of imperfect like a VCO.  The  
Oberheim Matrix 6 is a good example of all of this, as are some of  
the Roland DCO synths (JX-10, MKS-70, etc)

Mike B.


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