pure sine wave converter?

[bonkdav]

hello all! are there any circuits out there that convert a guitar into a pure sine wave? or that make it more pure? kind of an organic synthy super smooth sound ya know. i tried searching but i suck at it.

[R.G.]

As a practical matter, no.

There is a lot of work in the synth world and musicology on "pitch extractors" to pull the fundamental out of the straight signal. Some of these work OK-ish sometimes if you get them set just right. The successful ones are complex.

More pure is easier. Just lowpass filter it. The problem is that you have an instrument that puts out fundamentals from maybe 60Hz up to about 1312Hz in standard tuning. The harmonics are as big as or bigger than the fundamental and worse yet vary in amplitude with string, string age, pluck intensity, fret, pickup, and probably seconds left to play.

I spent a good long time trying to come up with something that would produce the original sine. The best I came up with so far was a filter bank with envelope generators and selection logic.

It goes like this:
The second harmonic is the nearest to the fundamental in frequency. It's 2x the frequency of the fundamental. So you need a filter that cuts off in well less than an octave; call that -20db per octave. That's a sharp filter, maybe 1/5 octave bandpass. So gang up five of these per octave for the four octaves of guitar fundamentals. On each one's output put an envelope generator, and on the output of those, put a comparator. The comparators then tell you "Hey, I got some signal in this frequency band!".

You take the outputs of the comparators and run those into a priority encoder  - a bit of logic that lets the output come out of the comparator that's for the lowest frequency and inhibits the output of all the others above it. Then put a VCA on the output of each filter and drive the VCA control from the priority encoder output. Only the filter band with the lowest frequency gets through, and you have your almost sine wave.

That's 20 filters, 20 envelope detectors, 20 comparators, one priority encoder and 20 VCAs.

Or you could program a DSP to do the work.

A sine wave is not an organic synthy super smooth sound. It's actually very bland, almost boring. Flutes put out almost a sine wave.

[bonkdav]

thanks a ton R.G.  That was exactly what i was looking for.  Your right it doesnt quite seem practical, plus i now hear in my head what it would sound like and not the sound i was hoping for.

[StephenGiles]

We keep on getting this, the EH Guitar Synth does the job admirably - see Mark Hammer's site.

[bonkdav]

thanks for pointing me to that schem you found, seems interesting but still a bit out of my league. ill keep these circuits in mind for when the time comes. thanks guys.

[StephenGiles]

For this job, you really need the complete works - there's no short cut I'm afraid.