[sdiy] Level Shifting question

[Richard Wentk]

At 10:19 29/10/2002 -0800, Ethan Zer0 wrote:
>I appreciate the replies I've had to my questions so
>far, however, due to my lack of terminology perhaps I
>have not accurately described the problem I am trying
>to solve.  I'll give it another whirl here:
>
>1. if I have a C.V. that is +/-1v  I would like to
>shift it to 0-2v.  What do I call this in modular
>synth terms?

That's a straight level shift, or DC offset. The general rule is to add 
half of the full range. So in your example the full range is 2V. You'd add 
1V DC to input CV and you're done.

But there's a problem here. If you know your CV is always going to be 
+/-1V, it's easy. Just use a simple DC mixer and add 1V. To make a DC 
mixer, just use a couple of opamps. You need to use high quality models 
with low offset voltages, but otherwise it's a very simple circuit.

But if your CV could be +/- 1V or +/- 10V or anything in between, you're 
stuck. This is especially true if you try to do something wild, reckless 
and dangerous like using another modulation source, maybe an ADSR or 
another LFO, to control the amplitude of your +/- CV.

The problem now is you don't know what half the full range is, because it's 
changing all the time. So... what to do?

There's a right way and a wrong way to deal with that one. The wrong way is 
to use peak detectors, envelope followers, and whole bunch of other stuff. 
It'll work, kind of sort of, some of the time, but realistically you'll get 
errors, and it's not a simple solution.

A better solution is to divide the output of the LFO/ADSR/whatever by two, 
and add that to your original CV. Now you have a 'half of the full range' 
signal that tracks the modulating source perfectly, and your original CV 
will stay happily positive. Again, you can use a DC mixer for this, 
assuming it has some kind of input scaling knob on at least one of the 
channels.

This will work the other way as well. If you have a positive voltage swing 
and want to convert it to +/- something, subtract half the output of the 
modulating source. Or use a fixed DC voltage if it's a steady amplitude. 
Decent DC mixers will have both adding and subtracting (also called 
non-inverting and inverting) inputs.

If they don't, it's not hard to build one that has both. If you research 
how to build inverting and non-inverting amplifiers out of opamps, you'll 
see how it's done.

>2. if I have a C.V. with + and - voltage, I would like
>to invert only the - voltage.  What do I call this in
>modular synth terms?

That's a rectifier, like people have said. You don't usually find 
rectifiers as stand-alone modules. But they're often used in the circuits 
inside modules.

That's maybe a shame, because rectifiers can be useful, and people should 
probably do more with them. E.g. if you rectify a +/- triangle, you get a 
triangle an octave higher. (With a DC offset... But you know what to do 
about that now. :) )

Richard