[sdiy] Additive Synthesis questions

[Aaron Lanterman]

On Apr 11, 2010, at 12:30 AM, Ian Smith wrote:

>>> It's easiest to make square waves, just have to turn something on
>>> and off. Everything else is a lot harder to make with electronics.
>>> Sine waves are very natural and make up everything else, but as far
>>> as I know, the on/off nature of electronics makes it impossible to
>>> produce a sine a wave without wave shaping.
>> 
>> Arrgghhh!
>> 
>> -- Don
> 
> Is that a "Ian's right." or did I get it horribly wrong?

Uhm, sorry, probably "horribly wrong." ;)

A sine wave is the solution to a second-order differential equation, which is actually pretty easy to build in circuits. A state variable filter with sufficiently high Q will self-resonate; the first-order feedback loop in the SVF is what tames the filter into acting as a filter and not an oscillators. (Admittedly most filters will have a little tiny bit on nonlinearity in the feedback path to make this behave better). This approach isn't popular for VCOs since it's hard to get a precise control of the pitch. 

If you go up in the radio frequency range, there are dozens of ways to make sine waves.

There's a classic circuit that uses the weird properties of an incandescent bulb:

http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0007

http://en.wikipedia.org/wiki/Wien_bridge_oscillator

- Aaron