Splitting negative and positive halves of the waveform

[Taylor]

I've had some ideas lately about processing the 2 halves of a waveform separately.  Not sure anything interesting will come out of it, but I want to experiment. What's a good way to do this? Some sort of gain stage, followed by 2 counter-oriented diodes, each going to a separate out? Anything more clever? Anything I need to watch out for (some sort of DC problems maybe)?

[Nasse]

Old Elektor book has a circuit claiming to do this, "Electronic magnifying glass" or something, opamp and diodes

[brett]

Hi

followed by 2 counter-oriented diodes, each going to a separate out? Anything more clever

While you certainly can do that, I suggest that you check out web postings for "ideal rectifier".  This prevents the voltage loss across diodes.  Alternatively, you can work out some arrangement for forward-biasing the diodes (which is not usually as easy as using an ideal rectifier).  An ideal rectifier can be as simple as an op-amp, a couple of resistors, a couple of caps and a couple of diodes.

cheers

[JKowalski]

Sounds interesting, what exactly are you going to do?

You'll have to worry about the 0.7V drop of a diode in this case. If you simply use a diode to route the signal two different ways (top and bottom halves), each half wave will be lowered 0.7v towards the ground (or Vref) and because of this,you will lose the bottom 0.7V of your signal from each portion. You'll get massive crossover distortion. You need something closer to an ideal diode (diode without any voltage drop).

You can make a close ideal diode with just an op-amp and a diode, like here:

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

The "improved precision rectifier" is your best bet, it's also useful because as it also is a standard inverting op-amp gain stage. Gain controlled by -R2/R1.


You'll also have to make sure your signals don't slip significantly out of phase with each other, which would problems when you try to recombine them. Though it might be cool to do it on purpose?

Are you planning on making the signals go on seperate out jacks, or processing them in the pedal? Are you planning to recombine them at all? If so, seperate out jacks would require the signal to be capacitivly coupled, which will will alter the waveform in major ways and make mixing them together again impossible.


EDIT: Haha, brett posted while I was typing. Oh well.

[Taylor]

Crossover distortion and slipping the phase of the two are actually exactly what I have in mind.

Thanks, both, for the point in the right direction.

[JKowalski]

Not gonna give up your secret plans?  If you even have any, that is.


The first thing that comes to mind for me is a distortion - people often like the sound of assymetrical clipping, having an adjustable TS or something for both halves of a waveform would be a interesting project.

Do you mind if I steal your idea for my own tinkering? 

[Taylor]

Well, my plans are pretty vague at this point, hence the need for a test-bed of sorts. The basic idea is about de-syncing the halves. At low delay, this should result in crossover distortion, at higher delays (more than one cycle), I'm not even sure what will happen. Hopefully something fun, though. It might be one of those things that you dream up, thinking it's going to sound crazy, and then it's nothing special.

Regarding your twin-TS idea, you might check out a commercial pedal by Schumann called the Two Face - essentially what you're talking about, but with 2 Fuzz Faces.

http://www.schumannelectronics.com/two_face.html

[JKowalski]

Ah yes, I was thinking to myself "someone's got to have done this before!"

Still doesn't kill my need to tinker, though.

[JKowalski]

By the way - you might want to consider making your modifications to two, opposite phase (inv, noniv) copies of the original, unsplit signal, and then splitting them afterwards. Otherwise, it is easy to lose your nice and straight Vref bias to filtering,and whatnot.

[Nasse]

Elektor 301 schaltungen, circuit nr 214 perhaps

[brett]

Hi
Has anyone tried phase-shifting half of the signal, with the amount controlled by a LFO?
I imagine it would be a tremolo-distortion thing.  Also, what if one half was inverted before being recombined (for some warbling octave-up!)
cheers

[puretube]

Hi
Has anyone tried phase-shifting half of the signal, with the amount controlled by a LFO?
I imagine it would be a tremolo-distortion thing.  Also, what if one half was inverted before being recombined (for some warbling octave-up!)
cheers

that was what I wanted to imply here at first, but didn`t work out in my "only 1 chip" context...

[JKowalski]

Hi
Has anyone tried phase-shifting half of the signal, with the amount controlled by a LFO?
I imagine it would be a tremolo-distortion thing.  Also, what if one half was inverted before being recombined (for some warbling octave-up!)
cheers

That's pretty much what Taylor is planning on trying out.


Hey puretube, do you have a sound clip of your experimenting with that?

[puretube]

Hi
Has anyone tried phase-shifting half of the signal, with the amount controlled by a LFO?
I imagine it would be a tremolo-distortion thing.  Also, what if one half was inverted before being recombined (for some warbling octave-up!)
cheers

That's pretty much what Taylor is planning on trying out.


Hey puretube, do you have a sound clip of your experimenting with that?

sorry, no: the experiments weren`t recorded... only the final (non-shifting) octavator