Rectify an LFO?

18watt · Post by **18watt** » Fri Apr 18, 2008 11:30 pm

Maybe I'm thinking about this the wrong way (I'm new to synths), but: when a guitarist applies vibrato to a note, the pitch only goes upwards. An LFO produces a wave that alternates... Is there a way, with current Moog products, to full-wave rectify an LFO?

misterhemi · Post by **misterhemi** » Sat Apr 19, 2008 1:27 am

I don't know which signals are present at the rear of a Voyager but if the LFO's signal is there it could be run to a bridge rectifier or something to that effect.

Maybe take a look at the owners manual for the Voyager or one of the interfaces (For example the CP-251) and see if the LFO's signal is present.

misterhemi · Post by **misterhemi** » Sat Apr 19, 2008 1:38 am

Ok, the LFO signal is present at the accessory port on the rear of the Voyager therefore it would also be present at the CP.

I found this on the AMS web site:

ACCESSORY PORTS

INPUTS
This is a standard male DB-25 connector that provides the following control voltage and gate inputs:

Control Voltages:

VOLUME
PAN
FILTER CUTOFF
OSCILLATOR WAVEFORM
OSCILLATOR PITCH
MOD2 (EXTERNAL MOD SOURCE)
MOD1 (CONTROLS PEDAL/ON MOD BUS AMOUNT)
SAMPLE AND HOLD IN
ENVELOPES RATE
LFO RATE
T.S. X
T.S. Y
T.S. A
Gates:

LFO SYNC
ENVELOPES GATE
SAMPLE AND HOLD GATE
RELEASE
T.S. GATE
OUTPUTS
This is a standard female DB-25 connector that allows access to the control voltage and gate output signals from the Voyager. A VX-351 CV Expander can be connected to this port to output these signals on its 1/4" jacks. Control signals that are routed to this port are:

KEYBOARD GATE
KEYBOARD CV
KEYBOARD PRESSURE CV
KEYBOARD VELOCITY CV
X, Y, AND Z CV OUTPUTS FROM THE T.S. INPUTS
MOD 1 AND MOD2
MOD AND PITCH WHEELS
MOD BUS OUTPUTS
FILTER AND VOLUME ENVELOPES
LFO TRIANGLE AND SQUARE WAVE
SAMPLE AND HOLD STEPPED AND SMOOTHED OUTPUT
NOISE

18watt · Post by **18watt** » Sat Apr 19, 2008 7:56 am

You're right, there is an LFO output from the Voyager (and also the 251, and a couple of the Moogerfoogers). Maybe I'll make a teeny diode bridge in a project box. I was just wondering if there was some existing circuit in my current setup I was missing.

Thanks!

analoghaze · Post by **analoghaze** » Sat Apr 19, 2008 11:13 am

Hey!

There is this:

http://www.synthesizers.com/q130.html

I own the Q119 sequencer and absolutely love it. Someday I will get more modules from Dot com. (Since I already shelled out for the power supply that powers 8 modules and I am only using one jack on it.... seems like a "waste")

That will do what you want it to, but if you have th eknowledge to put together a bridge rectifier circuit, I'd do that. I believe you can do it with 4 diodes..... maybe IN4001s, or IN4007.

Good luck.

18watt · Post by **18watt** » Sat Apr 19, 2008 12:09 pm

Hello there! See, I knew it wouldn't be long before I started tinkering!

I can fix something up with less than ten bux at Radio Shack, then. No problem. I'll use diodes, because they'll look snazzy all point-to-pointed.

BTW, I'm still having a blast w/the Old School. This morning's adventure was 1->2 sync, which made perfect sense after watching Mr. Rudess' youtube on the subject. I'm fairly certain I hear a bunch of osc. sync in Jan Hammer's stuff, too.

Gosh, I think I need an oscilloscope...

latigid on · Post by **latigid on** » Sat Apr 19, 2008 12:12 pm

Thankfully there isn't very much to a bridge rectifier

I'm not too sure why you would want to apply this to an LFO, though. Half-rectification would simply double the frequency but halve the amplitude (and would be "offset"). Full rectification would generate a DC signal.

Half-wave rectification would start to get interesting with non-triangle waves...

What was the intended use? Could you use a Pitch Wheel?

18watt · Post by **18watt** » Sat Apr 19, 2008 12:21 pm

It's possible that I don't really understand what I'm trying to do.

The goal was to take the pitch of a note and repeatedly raise and lower it by a small amount.

When a guitarist applies vibrato, it's not really a nice sine wave thing - the strings stretch, but don't slacken (using typical finger technique), so you'd get a series of humps rather than positive/negative curves, I think...

At least, I think so.

Yes, you could use a pitch wheel to do the same thing, but I was curious about using the mod wheel & lfo to attack the task.

hieronymous · Post by **hieronymous** » Sat Apr 19, 2008 7:52 pm

This is related to the guitar part of what you're talking about: sometimes vibrato is simply applied to a note, with the effect that you are talking about - the note really only goes sharp and returns to pitch, it can't go down. But often, a note is bent up a full step, and then vibrato is applied, in which case the pitch can go higher and lower, or just lower and back to the original note. So there are a lot of possibilities! Paul Kossoff of Free had a really crazy wide vibrato that even impressed Eric Clapton! And of course Clapton's vibrato is really nice. Hendrix applied a lot of techniques - traditional finger vibrato, and of course the whammy bar. Adrian Belew would often grab the headstock and physically bend the neck!

LDT · Post by **LDT** » Mon Apr 21, 2008 3:26 am

On the Voyager: Apply the lfo to osc. pitch and you have modulation that is positive only. So no workarounds needed.

18watt · Post by **18watt** » Tue Apr 22, 2008 10:41 am

It's still a sine though, isn't it?

It's the bumpy guy on the bottom of this picture that I'm after.

latigid on · Post by **latigid on** » Tue Apr 22, 2008 4:44 pm

Hmmm, perhaps this would work if you didn't use a smoothing capacitor. Give it a go!

misterhemi · Post by **misterhemi** » Tue Apr 22, 2008 7:57 pm

latigid on wrote:Hmmm, perhaps this would work if you didn't use a smoothing capacitor. Give it a go!

Yeah, it would have to be without a smoothing capacitor as that would smooth out the ripples and then he would be just left with a steady DC voltage, it wouldn't even follow the amplitude variations of the LFO because the capacitor would retain a charge.

LDT · Post by **LDT** » Wed Apr 23, 2008 5:57 am

Ok, now I see what you are after.
Although not the same thing, it strikes me that you could try with a triangle through the lag processor of the CP-251. Set RISE to zero and adjust FALL to taste. This should do an imitation of the pictured curve, but of course this would not work at all if the input wave is changing in freq.

(Other brands than .com also has rectifiers, but they are often called wave folders.)