[sdiy] Dual Frequency Shifter update

[Tim Daugard]

My contribution:

http://home.earthlink.net/~synthfred/h_alg121.htm


> If you log your signal down, filter it (you always need to toss it through
> a filter, "dome" or whatever) and exp it back you have a run into a pair of
> problems:

> 1) Signal is inherently bi-polar, log and exp is inherently operating on
> positive at one side of their respective function and bi-polar on the other
> side. Unfortunatly the bi-polar side is "inside" where as the positive is on
> the outside.

The log amp function I created is bipolar. The reponse may be an approximation - I haven't
checked for log accuracy. I created the module to be used as a continous compressor - like
the video amp of a spectrum analyzer.

The signal is AC coupled to the input of an opamp and compressed as it varies farther from
the bias point IN EITHER DIRECTION. When tested with vocals, it can compress the signal so
that anything from a whisper to a shout results in approximately the same level output.
Distortion is minamal as distortion, however the output is: sign X post gain X log(input)


> 2) We have moved the non-linear to the wrong side of the filter, so it is
> treating the now distorted signal by a linear filtering process and then apply
> the reverse function of the non-linearity will *NOT* get the right signal back.
> It's FUBAR by now. You guys of any should know this by heart and use it into
> deep dangerous and nasty areas ... you know....

Hey, I'm a bass player and work with a guitarist. I like the signal FUBAR at least to the
point where everyone else says STOP IT . . . You're the bass player.

> Exercise: Take a sine, toss it through an logarithm (see all the nice overtones
> at different amplitudes and phases),

okay

http://home.earthlink.net/~synthfred/images/f08_24.gif

(theoritical diagram, but verified with oscilloscope)

> phase shift them differently and put them

I haven't built a phase shifter yet, someone else will have to do this.

> back together again through the exponential function. Do we have an undistored
> sine? Oh, how did you do that and maintained the sign?

Unknown, but the sign is maintained. (or may be inverted, I would have to count
inversions)

> I think you are out for a fishingtour on dark waters chasing a red herring!

Actually, now I'm going to have to get back in the lab and build a phase shifter.

Lets see a logged quaditure LFO summed with logged signal and then anti-logged should do
something interesting. Now I gotta build a sine/cosine LFO. Anyone got schematics?

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm