[sdiy] An Improved Sine Shaper Circuit

Donald Tillman don at till.com
Thu Apr 23 18:32:29 CEST 2020



> On Apr 21, 2020, at 11:22 PM, René Schmitz <synth at schmitzbits.de> wrote:
> 
> On 21.04.2020 21:45, Donald Tillman wrote:
> 
>> So, I'll claim that if a small amount of the original triangle wave is subtracted from a wave that's roughly sinusoidal, then it's actual cusp cancellation.
> 
> In your circuit the cusps are also cancelled, after all you still aim for a flat top of the approximated sine at +-pi/2. In that sense one could also call this cusp cancelling.
> 
> The way I see it, it is a different set of parameters for the same circuit, the math is fundamentally the same.
> 
> So where would you draw the line?

I see two dimensions for drawing the line.  :-)

One is that where just a little triangle is added to cancel the remaining cusps of an otherwise good sine approximation, that applies to any implementation; whether it's a diff amp pair tanh() function, or a diff amp pair with feedback that's no longer a tanh() function and thus the math is different, or a Buchla FET shaper, or a piecewise linear approximation, or a multistage contraption, or whatever.

The other is that you can optimize a diff amp pair for just a little cusp cancellation, or you can optimize it for tanh(x) - βx, but there doesn't appear to be an advantage to any intermediate point along the line.

  -- Don
--
Donald Tillman, Palo Alto, California
http://www.till.com





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