What’s So Special About the Uni-Vibe? - Eventide Audio

What’s So Special About the Uni-Vibe?

Over the long history of guitar effects there are a few that have achieved a sort of mythical status (looking at you cloven hooved distortion pedal). The Uni-Vibe certainly falls into that category. Whether it’s because of the countless great records and performances it appears on, the fact that it sounds like its own special style of effect, or the fact that the origin story involves trying to copy the sound of distorted Russian radio waves bouncing off of the atmosphere (hint: varying delay plays a key role here), we’ll never truly know. What we do know is that it sounds freakin’ awesome. 

The Uni-Vibe has long been of interest to Eventide. Not least because it was the #1 most requested effect on H9 user forums. Sadly, the DSP chip at the heart of the H9 simply doesn’t have the cycles to do justice to the analog hardware and so we’re not able to fulfill that request. The Riptide is another story; it’s based on a powerful ARM processor. 

How did we know what it would take to convincingly emulate a Uni-Vibe? Well, 5 years ago we were able to create what we think is a pretty amazing emulation of that iconic effect thanks to stellar work from our intern Champ Darabundit, our sound designer extraordinaire Pete Bischoff, and head of audio development Russell Wedelich. The technical details were so compelling that they were published in the proceedings of DAFx 2019 titled “Digital Grey Box Model of the Uni-Vibe.”

Fig 1. Math! When combined these two transfer functions represent the digital model of each Uni-Vibe phase shifting section. If this looks fun to you – read the paper!

If you want all the details, including the results of some listening tests where we stack ourselves up against an original Uni-Vibe and some of the best clones out there, please give that a read. If you just want the short version, we’ll highlight some of the really good bits without all the math below. All of this great work has been available on our flagship processor (the H9000) for a few years, on the H90 since its launch, and now gets to shine in its own piece of hardware — Riptide

Riptide features two unique Uni-Vibe voicings

Aspirationally All-pass

A lot of people think of the Uni-Vibe as its own brand of effect, but it’s a unique sort of analog phaser which uses a special filter called an all-pass filter to apply a phase shift to the incoming audio. The filter circuit combines a resistor (R), a capacitor (C) and some form of amp (transistor, op amp) connected in such a way that all frequencies pass through without any change in level. Based on the values of the Rs and Cs, however, some narrow range of frequencies are ‘delayed’ passing through. When the output of the all-pass filter is combined with the original input, notches appear at certain frequencies. If you move those notches around – viola! You’ve got yourself a phaser.  

How do you move those notches? By moving the range of frequencies being delayed which depends on the values of the R and C – changing either will do the trick. Changing a capacitor’s value would require doing physical harm and would not be reproduceable. Fortunately, by the late 60’s, technology blessed us with a couple of components that behave like variable resistors; the LDR (resistance varies based on light) and the FET (resistance varies based on voltage).  

Our own legendary phaser, the Instant Phaser, uses specific pairs of capacitors and op-amps to create its all-pass filters and voltage-controlled FETs (Field Effect Transistors) move the notches. By combing a number of all-pass filters in series, each with a different C, you can get notches to sweep across most of the audio spectrum. If you’d like to learn about the Instant Phaser you can watch me talk about it here.

But, back to the Uni-Vibe. The Uni-Vibe certainly does fit this pattern, although in its own unique way. For one thing the capacitor values were picked seemingly arbitrarily (perhaps based on sound alone), and the filters were built out of discrete Bipolar Junction Transistors (BJTs). The result? Unevenly spaced notches in frequency, and an all-pass filter that isn’t quite ‘all-pass’.  

Because of the way the Uni-Vibe phase shifting stages are constructed each stage can add a variable amount of gain to the high or low frequencies depending on the resistance of its LDR (we’ll talk about the LDR later) and component tolerances. The difference is subtle, perhaps no more than a dB or two between high and low frequencies, but it certainly adds to the sound. Almost as if there’s a modulating filter, or even frequency dependent tremolo, in addition to the phasing. As we keep learning over and over again when modeling these older effects — sometimes it’s the things that aren’t intended that are the most important to get right. That’s what we made sure to do with our recreation of these almost all-pass filters. 

Fig 2. A single stage of the Uni-Vibe circuit. The output of this should be an all-pass filter, but if the gains at the nodes labeled Vc and Ve aren’t equal you end up with a phase shifting shelving filter.

Moving at the Speed of Light

As mentioned above, if you were designing a guitar pedal in the late 1960s your options for components were relatively limited. Creating an effect with automatically moving filters requires automatically changing one of the resistances that make up that filter. One of, if not the only readily available component that could do that at that time was a Light Dependent Resistor (LDR) whose resistance is proportional to the amount of light it is exposed to. Now, when we think of light the first word that comes to mind probably isn’t precise. Nevertheless, that’s what the designer of the Uni-Vibe, Fumio Saeda, had available to him to move the all-pass filters that make up the Uni-Vibe. Four LDRs, one per phase shifter section, surround a single incandescent bulb in a reflective metal enclosure. The interactions of the positions of the 4 LDRs relative to the bulb, the dimming of the bulb, and the reflections of the light off of the enclosure all contribute to the complex, but organic movement that we associate with the Uni-Vibe. Trying to mathematically model that would probably take a supercomputer, and doctorates in physics we are not (although we do have a few PHDs on the payroll).  

Fig 3. An opened up Uni-Vibe (left) showing the reflective enclosure and the 4 LDRS and lamp that it holds (right).

What we can do is precisely measure the output of each LDR to determine its resistance and from that determine how each all-pass filter moves. This presents its own challenges though because not only does the response of each LDR depend on the Intensity knob, but the speed at which the bulb is changing its brightness. As we discovered, the faster the bulb changes its brightness, the more asymmetrical the modulation shape becomes. That means lots and lots and lots of measuring. The result of all this measuring is a three-dimensional wavetable containing 640 unique LDR resistance curves for each LDR, we’re up to 2,360 now by my calculations, representing every combination of Speed and Intensity settings on the Uni-Vibe. Maybe one day we’ll be able to fit a supercomputer in a guitar pedal, but for the time being we think it’d be hard to get closer to capturing the natural, organic movement of that LFO than we did. 

Wrap Up

Eventide has always been a company that’s looking forward. Trying to come up with new effects, or new ways of doing the classics. Sometimes the best way to put yourself in a position to come up with something truly new is to learn about what’s come before. That’s one of the many reasons these modeling projects are fun. It gives us a chance to learn. Maybe that modulation sounds better if you don’t get from point A to point B in such a straight line. Maybe that filter that doesn’t work perfectly does something extra cool to the sound. All of these ideas are things that we can take from this project and think about going forward. Hopefully what you take from this post is a little insight into how we go about approaching a project like this, and some of the very cool things that we learned along the way. We also hope that this work has resulted in an effect that’s as fun to play as it was for us to make. Thanks for reading. 

— Woody Herman, Senior DSP Engineer and Fan of the Rad Sh*t His Coworkers Make