[sdiy] XOR ringmod

[Scott Gravenhorst]

"Speth, John" <John.Speth at coherentinc.com> wrote:
>> >Are the XOR gate ring modulator designs a true ring mod 
>> effect? in other 
>> >words, if I put in 220Hz and 880Hz will I have sidebands at 
>> 660Hz and 
>> >1100Hz?
>> 
>> They're equivalent to ring modulating pulse waves. This means much
>> more complex sidebands than if the carrier is a sine wave. Those
>> sideband frequencies will be there along with frequencies due to all
>> the harmonics in the pulse wave.
>
>The downside to XOR ring mods is that even if one input is 
>silent, the output will follow the other input. This is not how 
>an analog ring mod would work. 

There is another way to look at logic states other than levels 
of "0" and "1".  It's deceiving to think of logic level 0 as the
same as a zero volt input to an analog ring modulator.  If the logic 
levels are considered "N" and "-N", then the XOR ring mod does work 
just like an analog one given the same input states of N and -N 
(using rectangular waveforms).  If need be, CMOS logic can actually 
work with such states as voltages if powered by a bipolar supply.

When I've used an XOR type ringmod, I've done so using the outputs of 
PWM circuits from 2 VCOs.  This is then passed to VCF/VCA which then 
eliminates the problem of a nonsilent output.

>It does have a unique sound though 
>and it's dirt cheap to implement. I built an analog ring mod with 
>a parallel XOR ring mod in the same panel and find it useful to 
>have a choice. 
>
>Also, you can sub in any flavor of AND or OR gates with 
>essentially the same effect with slightly different sound. The 
>bonus or downside here (depending on how you look at it) is that 
>depending on where one input "ends up" when it goes silent, you 
>may or may not get output when the other input is oscillating. 

Cool, perhaps a multi logic device module...

>John Speth
>

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