[sdiy] An Improved Sine Shaper Circuit
don at till.com
Fri Apr 17 17:53:14 CEST 2020
> On Apr 17, 2020, at 1:56 AM, René Schmitz <synth at schmitzbits.de> wrote:
> Interesting circuit, and a great article.
> I'm pretty sure I have seen a similar technique before, because I have used it. (cusp canceling)
I am very familiar with cusp cancellation. I've used it also. And it's mentioned in the article.
This is not cusp cancellation. While the circuit looks the same, I'm subtracting 1 or 2 orders of magnitude more of the original triangle signal than is necessary to cancel the triangle cusps.
I discovered this using some machine learning tools (Jupyter Notebook, Numpy) and some of my own software to optimize a classic sine shaper circuit... one with emitter resistors and with cusp cancellation. And it kept pointing me to subtract more and more of the triangle signal. I thought something had gone wrong, but what the heck, follow the data. And sure enough, the harmonic spectrum really did get better as the transistor's tanh curve performed a different function.
Then I realized I was no longer correcting for a little nipple that got through the sine shaper, I had discovered a new way to approximate the sine function. And that it was crazy accurate. I couldn't find a reference to this zig-zagging "tanh(x) - beta x" curve mentioned anywhere before.
(I'll add some more about this in the article soon.)
Donald Tillman, Palo Alto, California
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