SV: Re: [sdiy] Digital VCO

[Michael Bacich]

On Apr 24, 2006, at 7:54 PM, Michael Bacich wrote:

> 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.

Just to be clear, I probably should have mentioned that the  
aforementioned analog CV is not provided by or controlled by the  
timer/counter chip, nor is it even directly related to the CV.  It is  
always provided by a DAC that gets its marching orders directly from  
the CPU, which looks at the requested pitch value, and outputs the CV  
that's closest to the one that would be ideal for that pitch value.   
These DACs usually have a seven or eight-bit amplitude resolution, so  
that's how much control they have over the ramp amplitude over the  
entire frequency range of the DCO.

The counters, however, have much greater precision in terms of  
frequency resolution (16 bits).  In practice, I doubt if the voices  
actually use the entire 16 bit range of available count values --  
that would ruin pitch resolution at lower frequencies.  These systems  
may also use additional simple binary dividers on the master clock  
signals, in order to get to lower octaves without sacrificing counter  
resolution in the working range of the voice.

Mike B.