CV theory and effects

The Moog Forum Moogerfooger Forum
1

ok we were all talking a while back about sounds sources and cv sources and we kind of covered the following:

  • things that can be used as both like noise
  • things that can only be used as CV like envelopes
  • and sound sources that can be made into CV sources like guitar with the use of a pitch-voltage converter

after reading the threads in question i left with the notion that CV and sounds are all just voltage going through the same wires (correct me if i’m wrong, i want to learn)

i just want to know if you can use the mf 104 to create delay on CV sources?

it’s designed to work with audio but, as the moogerfoogers don’t cut any bass freqencies out (like normal pedals) i thought it might carry over the CV signals

with the envelope on the mf 101 running through the delay and then to the ring mod you could make the loudness of one note effect the ring mod amount of the next note


the reason i ask is also related to the post eric made about a new fooger design
if the foogers can process CV then the VCA of eric’s design could be used to shape the strength of cv signals much like the shaping section of the mod buss on the voyager, only more patchable.

2

actually if you can’t use the delay/other effects to processs CV, could you (theoretically) build a device that shifts the freq of the CV into the audio realm 20-20000hz (for processing) then back again into the sub audio realm for CV purrposes

3

I’m no expert, but, in my experience this is true to a large extent. Most, if not all, audio sources can also be used as control sources.

This is most obviously true in the case of oscillators which are commonly used as audio and control sources. If the oscillator’s frequency is low enough it will moduate a filter and sound like tremelo. If the oscillator is at an audiable frequency it will still modulate the filter but it sounds more like a ring modulator. If the resonance on the filter is all the way up and the filter is self oscillating, and you modulate it with an audiable oscillator, it more less is a ring modulator.

But you can use other audio signals besides oscillators for modulation, they just don’t work as well usually. I think maybe it depends on the amplitude of the signal, but I’ve used a guitar signal, for instance, to modulate a filter.

I don’t think that Control Voltages and Modulators(like LFOs) are neccesarily the same thing though, even though they’re used the same way.


I’ve never tried to put a control voltage through a delay/echo, but, somehow, I don’t think it would work. I want to try it now though.


Sounds good, you’ll have to try it.
But, there are other ways to do this sort of thing if the Delay doesn’t work. Like the lag processor on a CP251. I don’t have one, but, if I understand it, it’s the same as a Portamento on an Oscillator: It delays the control voltage for the amount of time you set it for. On an oscillator it sounds like, or is, glide. But when you use it on an LFO it will delay the onset of the modulation (to whatever you’re modulating).
I might be off on this a little (or alot). But what I’m getting at is, you can process CVs so that there is a delayed effect. With LFOs you can get repetitions, so between the two, I’m sure you could get the effect you described (and probably everything else imaginable).

I don’t have a Voyager either, but I think you’re talking about using an expression pedal to vary the amount of modulation used in the mod router. If so, this function can also be replicated (in a more patchable way) on the CP251. You use an expression pedal to control an attentuator. The attentuator will vary the strength of whatever control signal that is patched to it.
A VCA can also be used to attentuate or add gain to control signals. I’m not sure what gain does to control signals though.


But this is the coolest thing about Moogerfoogers, and why I hope there will be new ones. I’ve been using analog synths for over 15 years, but it wasn’t until I bought my first MF that I really started to understand their potential or operation. I even had a MicroMoog for years, which was one of the most patchable synths made, and never even thought to try out any of the jacks on the back. I never had a manual for it and had no idea if I’d be harming it in some way to start connecting things. What a shame.
I’ve been thinking of starting to build a modular synth lately, but it’s such an enormous monetary investment. With the Moogers, you can get alot of the functionality of a modular without that huge commitment. I remember thinking when the Moogers came out that it was a little silly that they were in a guitar pedal format, but now I realize the wisdom of this form. You can use them individually, or collectively in little units that don’t take up the space of a wall.

4

I tried sending some control signals through a digital delay and it worked. I sent an envelope through the delay and then into a filter on my MS20 and the envelope went though it’s stages and then repeated until the delay faded out. I tried it with an LFO and that worked too, but it amounted to a sort of extra warbling (because the LFO was repeating out of sync with the delay).
So, pretty cool.

5

you both had some interesting ideas for me to think about, here’s my 2 cents:

everything i’ve read says that audio and cv signals are the same, the only difference is that most lfo’s and other cv’s are below the range of our hearing. because the frequencies of audible signals are so high they really don’t make for useful modulation sources (at least most of the time). so an audible signal at 500 hz has too fast of a cycle to be useful in a lot of ways (like eric mentioned about getting ring mod-esque sounds) while an inaudible signal at .5 hz has more ways to be utilized.


do you think that using a delay to process cv signals without a mixer could damage the mf’s? if you use a delay that repeats once could you end up doubling the voltage amount (say from 5 to 10 volts if the peaks of the original waves overlay)?


anyone know for sure how the cp’s lag accumulator works? i thought that it slowed the rising or falling rate of a waveform (and so the waveform’s shape changed), but didn’t actually ‘delay’ the entire waveform so that it was the same shape only later in time. i’m not sure though, i never thought about it like eric said.

6

I’m probably not spot on about the lag processor. But this is how I understand it:
You’re right about what they do to wave forms. But, if I’m not off, it’s the Keyboard CV for the Oscillator when put through a Lag results in portemento on the Osc, whereas the Osc. audio signal put through the lag results in a different waveform. And the same is true of LFOs, only, I think instead of portamento you get a delayed LFO… maybe.

As far as damage to the circuits from doubled voltage, I don’t think this is likely. If the delay was in sync with the lfo wave, it probably would have a summing effect. But even if it reached 10 volts, it shouldn’t have any negative effects on the circuit, because I know that many gate voltages, for instance, are up to 10 volts or more. i’m pretty sure that it’s completely safe to connect any Mooger input to any output. It’s just not recommended to put more than one output into and an input without mixing, but I don’t think that delay patch described above qualifies.

One other note that comes to mind on this subject are the Buchla and Serge Modulars. On the Buchla and Serge, all of the inputs and outputs can be used for any type of signal. This may be true on every synth, but it’s explicit in the Serge philosphy. So alot of Serge modules have multiple functions like an Envelope/Sequencer and so on. Buchlas too. I’ve never played either. They’re both still made but super expensive; particularly the Buchla.

7

can you use the lag processor to change the timbre of audio signals?

8

Control and audio signals are one and the same with regard to analog synthesizers. An audio signal can be used as a control signal. To distinguish them, we usually categorize signals below 20Hz as control signals since either we cannot hear them or if we do, they seem unpitched.

The lag processor smooths out signals. You can use it on either control or audio signals, although, as I said, these are used interchangeably. The effect is actually similar to filtering: it removes harmonics. A square wave when lag processed will have its edges smoothed. Control signals will also sound more tame. You need to get a ‘feeling’ for modules: it seems odd to think that an envelope (usually considered a control signal) can be filtered, but they can! Oscillators can be used as filters, filters can be used as oscillators, oscillators can be used as envelopes, almost anything goes!

Yannis

9

Yannis, thanks for all the info. One thing though. How is an oscillator used as a filter? Do you send an audio signal into the CV input or something? isn’t that a modulator? --or is it just a matter semantics?

One other thing, on some sythesizers, aren’t there dc and ac voltage sources/destinations?

10

  1. Oscillators can be used as filters when they are in sub-audio range, ie functioning as LFOs. However, this effect is very subtle. Take a look at the STS Dual Slope Generator to see how an oscillator can also function as a filter.

  2. In some modular systems (like Serge for instance) ac and dc are used interchangeably. In others manufacturers use dc and ac because they probably finetuned their systems to function in this way.

Y.

11

Well, DC voltages are control signals and audio, right?

So, what are AC voltages used for?

12

DC = direct current, so the potential (voltage) does not change with time.
AC = alternating current, so the potential does change with time.

Say a knob is on “0” and you twist it to “10.” On a regular vintage analog synth, this would represent a change from 0 to +5 Volts. As long as the knob stays on “10,” the voltage will be a constant, DC +5 volts.

Say you turn the knob to 5 = +2.5 volts, but now use an LFO with 100% modulation with that knob as a destination. As a function of the LFO waveform (i.e. the change matches the LFO shape and frequency/rate), the voltage will alternate between 0 (2.5 - 100%*2.5) and +5 (2.5 + 100%*2.5) volts. So now we have an alternating current.

Audio signals are AC. Oscillator voltages are transduced into audio signals (vibrations of air/other media) by vibrating a speaker. When the oscillator audio signal is in the positive phase, this represents the speaker moving in one direction (e.g. out). In the negative phase, the speaker moves in the opposite direction (i.e. in).

Try sending a DC signal to a speaker (NOT your $1000 studio monitors!). This can be done by placing a 9V battery over the wire connectors. You hear a “pop” and then the speaker will stay in one position. Reverse the battery and the speaker will move in the opposite direction and stay there.

So an audio signal must be AC, otherwise we wouldn’t hear anything!

13

wow thanks dude

i understand now, well kind of

so things like knob and epression pedal voltages are DC, but VCO/audio/noise/lfo are AC

what about envelopes, if they sustain then you have a direct current (?) for a shout period of time?

and one more thing
what about the different voltage ranges used by different synths, like:
-15 to +15 volts on the system 700
also it says in the moog manuals that you can plug any 0 to 5 volt source into the jacks but what if you go over 5? can you damage your pedal? and about negative voltages? the cp251 has an inverter so doesn’t that give out a negative voltage?

I’m assuming that 0 to 5 really means -5 to +5
and that +5 and -5 are really the same thing only opposite phase?


thanks for all the info

14

I wouldn’t call an envelope signal AC or DC, because it only triggers when you give it a gate signal. No gate signal = 0V envelope CV! Maybe if you were to trigger one regularily (with a clock etc.), then the wave might be considered AC.

Different instruments use different CV, the most common being 0 to +5V. One exception is the Voyager’s Pan CV, which is -2.5 to +2.5V

I was told by Moogmusic that a voltage over +5V will be current limited, so will not damage the circuits. Be careful though! You can do interesting things by going outside the 0 to +5V window though (see later).

Negative voltages give interesting results! Patch the ENV. OUT of the MF-101 to the 251 to get an inverted envelope signal. You may wish to attenuate the signal, so you can control the envelope amount. Patch the inverted envelope back into CUTOFF on the 101. Now when you trigger the envelope, the cutoff frequency will be lowered according to the amount you set on the attenuator. This is in contrast to the normal operation of the pedal. Remember, if you are lowering the voltage, you must first start with a higher frequency, so turn the CUTOFF knob up so that its voltage may be lowered by the inverted envelope.
You could also try patching the same inverted envelope to MF-104(X) TIME input. Then the repeat rate would increase as the envelope is triggered.

If the normal range (0 to +5V) is exceeded, this will sometimes do nothing, but on LFOs the range of the LFO’s rate is increased. I guess you could call it under or over-clocking. A negative (e.g. -1V) applied to a rate input will decrease the rate accesible (i.e. provide lower frequencies) than the knob allows normally. Similarly, but opposite, when +5V is exceeded.

These are dependent on the knob position and are additive. The inverted envelope lowers the CUTOFF voltage, but not below zero; e.g. CUTOFF normally at +4V, recieves a -1.5V signal, drops to 2.5V. Underclocking an LFO means going below zero Volts. It just depends how the CV destination works.

0 to +5 is not the same as -5 to +5, as the latter represents a change in voltage of 10V, the former 5V. However, a change in voltage from 0 to either +5 or -5V represents a change of 5V, so if the functions with respect to time are the same, they are opposite signals (you would get the other by placing one through an inverter circuit).

These signals actually have the same phase but opposite amplitute. This is because, if they start at the same time and have the same frequency, they peak at the same time. If we added these together, we would get 0V (any number plus its opposite = zero):

---------------------------------- +5 Volts
//////////////
-----------------------------------0 Volts
//////////////
---------------------------------- -5 Volts

Say both signals had the same amplitude (0 to +5V):

///////
---------------0 Volts and at the same time:

///////
---------------0 Volts

These are in phase, but have the same amplitude, so when we add them together, the signal doubles, going from 0 to +10 V (at the peak we have +5 + +5 = 10V)

Now say we have two signals out of phase (by 180 degrees), with the same amplitude:

////////
------------------0 Volts and

////////
------------------0 Volts

Now, the resultant voltage will always be +5V. Look at the signals, when one peaks at +5, the other is 0V and vice versa. Somewhere in the middle, the two will add to give +5V. (This is the principle behind a bridge diode, it converts AC to DC).

And now +5 and -5V DC:

------------------------------- +5 Volts
------------------------------- 0 Volts
------------------------------- -5 Volts

All the way along we have +5 and -5V, so add them together to get zero.

(Two times +5V = +10V, if you had two +5V running together.)

So +5 and -5 are opposite in magnitude, only wave functions (voltage/vibrations etc. changing with time) can be out of phase.

I hope that explains it.



I’m going to go and play the Voyager…

15

+5 and -5 voltages do not have the same phase and opposite amplitudes. They have opposite phases and same amplitudes.

You can think of DC voltages as straight lines. Actually this is what you will see if you plug a DC source in an oscilloscope as opposed to AC where the voltage is alternating up and down.

You can think of an envelope as a multiplier (also known as a gain controller): When you apply an envelope to ANY signal, that signal gets a contour through multiplication. For instance if you have a VCO at 440Hz and you apply an ADSR, you actually multiply the ADSR numbers with 440. This is an extremely important concept in my opinion. If you try to visualize what happens to sound when you apply a simple modulation, you can get a different ‘feel’ of what various modules can do.

Y.

16

Wow, I can see I’ve really opened up a can of worms here. I’m going to have to read back through these responces when I’m sober… I just got home… this all seems quite complicated right now…

17

Sorry, you’re quite right! :blush:

I completely contradicted myself. This is why I do chemistry and not physics! Of course amplitude is an absolute value, here peak to peak it’s +5V

Sorry for any confusion.

18

If I understand this correctly, direct currents which are straight lines and the kind of current that knobs output, as in a knob at “10” outputing a steady 5V voltage, is probably similar to Keyboard voltage output, which is like a quantized knob that outputs discrete voltage amounts, right? So, keyboard Voltage is DC also, right?

I know on an oscillator module, or filter module, you can input DC keyboard voltage or
AC LFO voltage (for instance). So, isn’t it true that AC and DC currents are interchangeable on all synthesizers? Not just on the Serge? Or am I getting confused about things again.

I think I’ve got it now. I don’t know of a situation where it would be neccesary for me to consider these principles or to distinguish between AC and DC when using the synthesizer. But I like to have a better idea of what’s going behind the controls.

19

Sounds correct to me. Think of an AC signal supplied to somewhere as a modulation source. If you apply a non-modulated voltage, you are giving the module or whatever a direct current, so the controller position is essentially static.

20

I can’t understand why the voyagers pan input would be - 2.5 to + 2.5

I you wanted to modulate the pan with an LFO then that would give a positive signal right? (unless inverted by the 251, but even then it would be negative not both)

from what i can tell pluggign an LFO into the pan control would only turn one channel up and down and not cross between them.

and if the lower part of an LFO cycle is what goes into the negative range then how can an LFO that goes between positive and negative, have an effect on a frequency input that only goes from 0 to +5 volts

no no i get it the negative voltage just takes away some of the positve voltage thus rendereing the freq lower by whatever the range of the lfo is set to.

is this right?

so do LFOs output signals from (- 2.5 to + 2.5) or (- 5 to +5)?
I assume that VCOs would have the same voltage range as LFOs