SV: Re: SV: Re: [sdiy] Digital VCO

[karl dalen]

--- Eric Brombaugh <ebrombaugh at earthlink.net> skrev:

> karl dalen wrote:
> > Wery simple, most of the so called DCO based units used 8253 counters
> > (each unit contains 3 16 bit counters) the output square are shaped into a 
> > short pulse that *pings* a normal VCO integrator and there you have your
> > ramp! (sawtooth). Then the rest is just a matter of waveshaping into
> whatever
> > wave you want. Roland did some special ASIC waveshapers+counter combination
> 
> > chip for the Juno series, the 106 might have it to.
> >   
> Interesting. So did they wrap a servo loop around from the comparator to 
> the charging current input to keep the amplitude stable?

In the special ASIC i dont know how they did it but in thee 8252 based DCO's
they simply keep a lookuptable that feeds a CV (usually a DAC generating a
voltage)
directly at the integrator thuru a resistor. Many manufacturers used 3 way
switching
between resistors to get a broader range from a 12bit DAC.(se note about
CEM3396). 

>ve done that 
> in the past for an adaptive oscilloscope sweep and I found that at low 
> frequencies the loop gets pretty hard to keep stable. Maybe if you put a 
> current DAC driven from the frequency setting to control the coarse 
> charging current / amplitude and then a fine control servo that levels 
> it out based on a comparator. Still sounds messy.

The relationship between frequency and amplitude CV is exponential so simply
put a expo converter between the DAC CV and the integrator.
 
> 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? Plus the fact that the frequency resolution decreases as 
> the count value decreases? Did they just use very fast clocks to drive 
> the counters?

Usually  12-10Mhz clk rates rest is lookuptabels.

Look for the CEM3396 , that chip is virtually used in every *later* DCO 
based synth, it descibes everything you want to know.

KD