The first Uni-vibes were an attempt to mimic a Leslie. So much so that Shin-Ei actually called them that, except with a Japanese pronunciation error. (
http://www.guitars-of-love.com/shin-ei_reslytone_tech_2.html )
Let's start from the beginning, though.
A rotating speaker (Leslie) produces what is called a "doppler effect" (
http://en.wikipedia.org/wiki/Doppler_effect). Put simply, since sound moves through air, and takes time to get where it is going, if the
source of the sound also moves, then it can result in cancellations for various frequencies, depending on the speed with which the source is moving. Those cancellations are what produces the characteristic sound a jet makes when it passes by at low altitude.
The basic uni-vibe is essentially a phase shifter, with components selected to produce shallow wide dips in the frequency response, rather than the deep focussed notches that a phase shifter normally produces. This can mimic some of the perceived characteristics of a rotating speaker, but not perfectly. But since those dips are not so deep or focussed, it produces a nice background "animation" to the sound instead of focussing your attention on where-the-notches-are-right-now, as a phaser does. I will add that since the effect IS subtler than a phase shifter, it generally becomes inaudible at very slow speeds. Finally, feedback/regeneration, such as you would find in a phaser or flanger, does absolutely NOTHING useful here.
A second attempt to mimic what a Leslie does came in the form of chorus. If the doppler effect depends on having a sound source that was over here and now its over there, then having two sound sources that come together and move farther apart in time, ought to do the same thing. And that's what a chorus does. The delay chip has its delay time lengthened then shortened then lengthened, etc. It catches up, falls behind, catches, up, etc. That mimics the illusion of several sound sources, that occurs with a Leslie, and also produces the cancellations that doppler effects do. Since it produces a LOT of cancellation points across the entire frequency spectrum, rather than simply two dips, it does a better job of mimicing the doppler effect from a Leslie than a UNi-Vibe does. And because those cancellations are distributed across the entire spectrum, rather than starting at this or that very obvious place (as what happens with a phaser or flanger), we tend not to notice where the cancellations are, but hear it as a pleasant kind of doubling.
Brief sidetrip: The falling behind, catching up, can produce an audible "pitch wobble", similar to a tape deck having inconsistent speed. The old Boss DC-2 chorus addressed this by having 2 delay chips moving opposite to each other, with one catching up while the other falls behind. There still is pitch wobble, but since the delayed sound is always sharp
and flat, you don't notice it.
Second brief sidetrip: When Boss introduced the CE-1 Chorus ensemble, the target market was very likely keyboard players who didn't want to drag around a big cabinet. I say this because the input stage of the CE-1 was most definitely NOT ideal for guitars, but it was well-adapted to the output levels and impedances of keyboards.
So what is
different between a Leslie, chorus, and vibe?
Certainly chorus and vibe pedals produce "swirl" similar to doppler effects, however there are a couple of important elements that they do not capture.
1) A Leslie is a sort of post-production effect, since it is applied at the very end of the signal path at the speaker. It accumulates all changes to the harmonic content of the signal, even up to output transformer distortion, and then applies the doppler effect to all of that. A chorus or vibe pedal still has the rest of the signal path to pass through.
2) Because, unlike a jet plane, a Leslie
rotates, in addition to the doppler cancellations it also produces slight dips in volume as the speaker faces away from you then toward you then away from you, etc. They're not strong enough to be an obvious tremolo, but they are there. And because the speaker horn faces away then towards then away, etc., there are also subtle changes to the treble content, with a gentle dulling when the speaker faces away, and you are relying on reflected sound to hear treble.
3) A Leslie is a mechanical device. I can produce near instaneous changes in speed, and a near infinite variety of them, with a knob twist on a chorus or vibe. A rotating speaker will have two speeds (fast/slow), and inertia being what it is, will take time to change over between them. That ramp-up/ramp-down is a very characteristic and signature feature of a Leslie. Many contemporary effects have incorporated this speed-changeover feature, including not only pedals that are deliberate Leslie simulators, but also things like the Keeley Phaser. I suspect that somewhere out there, someone is making chorus and vibe pedals with that feature as well.
4) Where the effect depth of a chorus or vibe can be adjusted so that it isn't as in-your-face, a true physical Leslie is either there, or it isn't. I imagine it is possible to use two amp/cabs, with one a Leslie, and adjust their relative volumes to make the Leslie sound more and less obvious.
5) Most people are used to using a vibe, and often even a chorus pedal, in mono, and the sound they associate is a mono sound. A true Leslie is inherently omni-directional. If you have never used one, you owe it to yourself to just try it once. I've told this before but when Tim Larwill, who makes the Retrosonic CE-1 clone, came over to my place a few years ago, I asked him if he had ever played through a rotating speaker. He hadn't. Keep in mind that the CE-1 (and his greatly improved version of it) were intended to
replace a rotating speaker. So, I hooked up my little Vibra-tone cab to my Princeton, and let him have a go. He had this "Holy s**t!" look on his face, and realized that as good as a chorus can be, there is something very special about having the sound swirl all around you. Those of us who own Leslie emulators routinely remind people that if you aren't using them in stereo, then you will likely think of it as a bland effect. Choruses sound much better in stereo, as the old Roland JC-120 and a bunch of other amps have so clearly indicated. But having one speaker sound the same and a second one fall behind, catch up, and fall behind, is not the same thing as having that sound move all around you.
6) The depth of the effect on a Leslie is limited by the size of the cabinet, and the radius of the moving horn. The shorter the radius, and smaller the circumference, the subtler the effect. A chorus or vibe can be set for effect depth that might only be possible if your horn were 10ft long! You mentioned Kurt Cobain's use of chorus. Many of those wobbly-pitch sounds would be completely uinattainable on conventional Leslies.
Having said all of that, vibes, choruses, and phasers can often do a pretty good job of replicating the bubbly sound of a fast Leslie, even if they don"t/can't do ramp-up/down. Again, the Leslie is tacked onto the end of your signal chain, so there is that difference too, but whatever it changes by being there will also change depending on guitar, pedals, amp, etc. The other night, I was laying at a friend's house, and plugged a light overdrive (Boss OD-1 clone, with more bottom) into an old flanger set for slow speed, followed by a a 4-stage phaser, also set to slow speed, and I was
very pleasantly surprised by the degree to which it emulated a Leslie slow speed. Not sure if I could replicate again, but the swirly growl was wonderful. I have a Small Clone I modded to straddle the range between chorus and flanger, and it does a nice slow Leslie emulation, too, again minus that slight volume and treble fluctuations.
So, in order of complexity: vibe-> chorus -> Leslie. Hope that answers your question. You're lucky I took the day off today.
) explains me the difference between the three? For me, there were the leslie that someone emulate via a vibe then someone bluid a chorus, an easiest way to do vibe. So Chorus, would be the third generation...Am I wrong?
