Univibe modulation waveform

[Steben]

Is there someone who actually studied and recreated the actual resulting modulation wave the original univibe has?
I mean, the LFO + bulb + LDR combo.
A vibe IS a 4 stage phaser but controlled with a non-linear wave.

[ElectricDruid]

Is there someone who actually studied and recreated the actual resulting modulation wave the original univibe has?
I mean, the LFO + bulb + LDR combo.
A vibe IS a 4 stage phaser but controlled with a non-linear wave.

Oh, *lots* of people, but most of them are now making expensive Univibe clone pedals and don't want to tell you what they learned!

I don't know of that information being in the public domain anywhere yet, no.

[Mark Hammer]

Is there someone who actually studied and recreated the actual resulting modulation wave the original univibe has?
I mean, the LFO + bulb + LDR combo.
A vibe IS a 4 stage phaser but controlled with a non-linear wave.

Ah, but how do you know the resulting modulation isn't a fairly mundane waveform that results from how the quirks of a filament bulb offset the quirks of an ldr?

[ElectricDruid]

Ah, but how do you know the resulting modulation isn't a fairly mundane waveform that results from how the quirks of a filament bulb offset the quirks of an ldr?

Quite! My money would be on a basic sine wave with some X or Y distortions. Nothing you couldn't copy and paste into a waveform data table on a PIC LFO, for instance...!

[Rob Strand]

Is there someone who actually studied and recreated the actual resulting modulation wave the original univibe has?
I mean, the LFO + bulb + LDR combo.
A vibe IS a 4 stage phaser but controlled with a non-linear wave.

Oh, *lots* of people, but most of them are now making expensive Univibe clone pedals and don't want to tell you what they learned!

I don't know of that information being in the public domain anywhere yet, no.

There's actually quite a few papers on the Univibe out there.     I went through some of my old notes and this one is fairly representative of the univibe.   There's more in the references.   Unfortunately I couldn't compare the results with my archived stuff.

https://dafx2019.bcu.ac.uk/papers/DAFx2019_paper_31.pdf

Quite! My money would be on a basic sine wave with some X or Y distortions. Nothing you couldn't copy and paste into a waveform data table on a PIC LFO, for instance...!

IMHO, that's going to be pretty close.

There's a few angles to take:

You can try to come up with models for the LDR and the Bulb based on physics.  Then, work out the how the all-pass filters are modulated.   The non-linearity from the small bulbs is where the uncertainties come in.   How much non-linearity depends on how deep you run the bulbs into the cold region.   You can run the bulbs cool and close the LDRs or far and a little hotter yet have the LDR cover the same end-point resistances.  (I've actually got some physics based models.  One uses Planck Blackbody radiation for the lamps!)

FWIW, the old Electronic Australia phaser/vibrato project from 60's/70's (March 1969) mentioned the lamp+LDR's non-linearity but only in passing.   There's a copy of the article in here,
https://worldradiohistory.com/UK/Bernards-And-Babani/Babani/200-Babani-Handbook-of-practical-electronic-musical-novelties.pdf

As far as the univibe goes, the sine modulation is clearly a difference to most phasers.

If you look at the leslie speaker the modulation is sine, (the univibe suppose to emulate a leslie),
https://www.researchgate.net/publication/2568326_Doppler_Simulation_And_The_Leslie

Another big difference to other phasers is the all-pass frequencies:   Most people think of the univibe as a 4-stage phaser.  It is, but if you look at Figure 9 in the DAFX paper the lower notch is pushed way down.  For the most part the upper notch is doing the sweep.   It's almost like a 2-stage phaser with a wide sweep.   That characteristic adds to the pulsating quality the univibe has.

If you look at some modern DSP papers they try to extract the characteristics from actual pedals.   There's a lot of misadjusted phasers out there.   Over 50 years, every tech has fiddled with the trimpots and some are way off factory settings.

In reply #30 of this thread is a graph of the lamp output (lux) with different drive levels,
https://www.diystompboxes.com/smfforum/index.php?topic=129457.msg1251344#msg1251344

It doesn't go way down to low voltage/current but you can see that the light output is a non-linear function of drive level (for reasons mentioned later in the thread).

[Steben]

Is there someone who actually studied and recreated the actual resulting modulation wave the original univibe has?
I mean, the LFO + bulb + LDR combo.
A vibe IS a 4 stage phaser but controlled with a non-linear wave.

Oh, *lots* of people, but most of them are now making expensive Univibe clone pedals and don't want to tell you what they learned!

I don't know of that information being in the public domain anywhere yet, no.

There's actually quite a few papers on the Univibe out there.     I went through some of my old notes and this one is fairly representative of the univibe.   There's more in the references.   Unfortunately I couldn't compare the results with my archived stuff.

https://dafx2019.bcu.ac.uk/papers/DAFx2019_paper_31.pdf

Quite! My money would be on a basic sine wave with some X or Y distortions. Nothing you couldn't copy and paste into a waveform data table on a PIC LFO, for instance...!

IMHO, that's going to be pretty close.

There's a few angles to take:

You can try to come up with models for the LDR and the Bulb based on physics.  Then, work out the how the all-pass filters are modulated.   The non-linearity from the small bulbs is where the uncertainties come in.   How much non-linearity depends on how deep you run the bulbs into the cold region.   You can run the bulbs cool and close the LDRs or far and a little hotter yet have the LDR cover the same end-point resistances.  (I've actually got some physics based models.  One uses Planck Blackbody radiation for the lamps!)

FWIW, the old Electronic Australia phaser/vibrato project from 60's/70's (March 1969) mentioned the lamp+LDR's non-linearity but only in passing.   There's a copy of the article in here,
https://worldradiohistory.com/UK/Bernards-And-Babani/Babani/200-Babani-Handbook-of-practical-electronic-musical-novelties.pdf

As far as the univibe goes, the sine modulation is clearly a difference to most phasers.

If you look at the leslie speaker the modulation is sine, (the univibe suppose to emulate a leslie),
https://www.researchgate.net/publication/2568326_Doppler_Simulation_And_The_Leslie

Another big difference to other phasers is the all-pass frequencies:   Most people think of the univibe as a 4-stage phaser.  It is, but if you look at Figure 9 in the DAFX paper the lower notch is pushed way down.  For the most part the upper notch is doing the sweep.   It's almost like a 2-stage phaser with a wide sweep.   That characteristic adds to the pulsating quality the univibe has.

If you look at some modern DSP papers they try to extract the characteristics from actual pedals.   There's a lot of misadjusted phasers out there.   Over 50 years, every tech has fiddled with the trimpots and some are way off factory settings.

In reply #30 of this thread is a graph of the lamp output (lux) with different drive levels,
https://www.diystompboxes.com/smfforum/index.php?topic=129457.msg1251344#msg1251344

It doesn't go way down to low voltage/current but you can see that the light output is a non-linear function of drive level (for reasons mentioned later in the thread).

Let's say we add a sine LFO to a Phase 45...

[Kevin Mitchell]

I've scoped the univibe lfo a while back. It's a super clean sine wave.
However the modulation itself reacts a bit different due to the use of a incandescent lamp where it takes longer to dim than it does to light which results in more of a ramping rev-sawtooth modulation. And that's not considering the way the photocells react which changes things even more.

I've compared these observations visually and audibly with other common LFOs. I recall that it had increased my appreciation of the Easyvibe's lfo design.

Just some notes from me 

It will be a challenge to smooth things out in a way to replicate a photocells reaction into current for an OTA.
Or... perhaps you  could use an LDR for modulated current. -I've thought about this myself.

[Rob Strand]

I've scoped the univibe lfo a while back. It's a super clean sine wave.
However the modulation itself reacts a bit different due to the use of a incandescent lamp where it takes longer to dim than it does to light which results in more of a ramping rev-sawtooth modulation. And that's not considering the way the photocells react which changes things even more.

Yes, exactly that.   The LFO is very clean.  Even the lamp current is fairly clean.

At mild modulations the modulation is smooth but the lamp and LDR time response can skew things a bit. 

You can see from figure 9 in the DAFX paper, that the falling edge of the notch is slow which skews the waveform.
https://dafx2019.bcu.ac.uk/papers/DAFx2019_paper_31.pdf

Here's the pic (DAFX site is coming up with security issues),


Here's some plots based on modelling the physics of the bulb and LDR.  I've taken into account:
- LFO is a real sine, not from the circuit.
- non-linear bulb resistance
- how the brightness of the bulb varies
- how the sensor only picks up part of the light spectrum.
- the LDR non-linearity with light level
- *** I have not include the time response of the lamp filament or the LDR ***

In the first plot I'm plotting the upper notch frequency over time.  The frequency is on a linear axis.

As the bulb is dimmed the power to bulb is decreased but at the same time the lamp spectrum becomes more red and the sensor sees even less light.   There are two effects dimming affecting what the LDR sees.   So the transfer function from LFO to frequency becomes quite non-linear.

A point to be made here is the non-linearity  from brightness effect is *way* *way* stronger than the non-linearity from the LDR.   In fact LDRs sense a bit more red than the eye and if you factor that in the LDR is nearly linear (that's conductance vs Light level).

[read right axis in kHz not kV]


If we plot the notch on a log axis (which is how we perceive frequency) then you can see all the non-linearity from the bulb translates to a fairly sinusoidal looking notch variation.   This is very close to figure 9 of the DAFX paper.

What's missing is the stretching out of the falling edge. Which is when the lamp cools and the LDR opens up, which are slow processes (in this case it's going to be the LDR).



The above represent the leslie type sound from the Univibe.

The throbbing sound comes form more intensity and the notch starts to follow the time response of the lamp + notch.
Here's how the notch frequency maps out on a device which has this behaviour,



Here's a few more quantities posted on the same plot (linear axis),


Apologies, fixed a few errors.

[Rob Strand]

Let's say we add a sine LFO to a Phase 45...

It needs a bit more to mimic the bulb non-linearity, see prev post.

I suspect a reasonable approximation can be made using diodes and LEDs.
John Hollis' univibe does that and there was something on Run-off Groove
or GGG that had something along the same lines.

Another option is to use the two transistor exponential circuit like that on the
Boss BF2.

All pushing it close but it's perhaps the fine details aren't close enough without
spending a lot time on it.   IIRC the Jim Dunlop UV-1 added a few caps to
coerce the time response a bit.  The idea looks OK but it's the fine details
that make it work.