[sdiy] triangle to sine converter

Sat Oct 12 04:37:39 CEST 2002

Damn, I knew there was a catch!  Still, it'll do nicely in my MIDI2CV as a low-CPU way to generate pseudo-sine LFOs.  It'll be easy in a PIC to scale the amplitude of the triangle wave with frequency - as the frequency doubles, double the amplitude.

Seb

Tom May wrote:

> What you've got there is a curve made out of parabola pieces.  Running
> the triangle through an integrator will do this.  An integrator is
> just a low pass filter.  Your output amplitude will be inversely
> proportional to frequency, which won't make a very good tri-sine
> converter unless you tune the filter simultaneously with your
> oscillator.
>
> Tom.
>
> Using Mozilla 4.79, Seb Francis <seb at is-uk.com> writes:
>
> > <!doctype html public "-//w3c//dtd html 4.0 transitional//en">
> > <html>
> > Hi,
> > <p>I was working out how to generate a sine wave LFO in my MIDI2CV converter
> > without wasting loads of PIC CPU on floating point maths routines, and
> > I managed to work out the following simple algorithm (written in tcl -
> > great language for quickly trying out ideas):
> > <p>set dy 0
> > <br>set ddy 1
> > <p>set y 0
> > <br>set t 0
> > <br>while {$t <= 100} {
> > <br>    incr y $dy
> > <br>    incr dy $ddy
> > <br>    if {$dy >= 10 || $dy <= -10} {
> > <br>        set ddy [expr {0 - $ddy}]
> > <br>    }
> > <p>    set valueAtTimeT($t) $y
> > <br>    incr t
> > <br>}
> > <p>This produces what _looks_ like a perfect sine wave (see gif below),
> > and it got me thinking about analog triangle to sine converters. 
> > Basically the above algorithm increments "y" by the level of a triangle
> > wave ("dy" is following a triangle wave pattern, symmetrical about 0). 
> > That is to say the gradient of a sine wave is a triangle wave about 0. 
> > I'm not sure that this sounds totally right to me (although the result
> > looks like a sine) - I don't remember enough maths from school unfortunately.
> > <p>Now isn't the rate of voltage change (voltage gradient) across a capacitor
> > proportional to the current charging/discharging it?  So a triangle
> > wave through a resistor, into a transistor-pair current mirror, charging/discharging
> > a capacitor would produce a perfect sine wave.
> > <p>Does this work?  I can't remember ever seeing a triangle to sine
> > converter circuit like this.
> > <p>Just some ramblings of my brain ... ;-)
> > <p>Seb
> > <br> 
> > <p><img SRC="cid:part1.3DA610ED.432086F0 at is-uk.com" height=190 width=600></html>