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Effects Explained: Modulation—Phasing, Flanging, and Chorus

Dave Hunter
Adapted from the book Guitar Effects Pedals: The Practical Handbook (Backbeat Books)

PhasersIn Part 4 of Gibson’s Effects Explained series we’re going to look at modulation effects. This group includes phasing, flanging, chorus, vibrato and tremolo, rotary speaker effects, and octave dividers, the latter of which I have loosely grouped in here because … well, they don’t fit in overdrive or delay, do they? Later analog versions of the first three of these—phasing, flanging, and chorus—do, as a matter of fact, use much of the same technology as echo and delay units, although with chips having shorter delay times, but it makes sense to include them here because their obvious sonic characteristics are of a type with other units made from very different kinds of circuits. Most such effects were developed in an effort to add depth, dimension and movement to the guitar’s natural sound without necessarily distorting it, strictly speaking. A few noteworthy types also developed from effects that were in use on the electronic organ. This is another big category, so we’ll split it into two chunks.

We call these boxes “phase shifters” because they split the guitar signal and shift one path out of phase by from 0 to 360 degrees through the entire range of the frequency spectrum, and blend it back with the dry path so the moving in-phase/out-of-phase relationship can be heard. When the two signals are totally out of phase—at 180 degrees (or, technically, 540 degrees or 900 degrees, etc, because the shift keeps moving)—they cancel each other out, creating what we call a “notch”. But a number of factors interact to give a phaser its characteristic “swooshing” sound. I will explain them in relatively simple terms, but in many units some pretty clever and complex electronics going into making all this happen. When a notch in the frequency response is swept across the frequency spectrum, the most dramatic sonic effect occurs at the peaks between the notches, where both paths are completely in phase, and we have a full-strength signal. Leaving it there, however, would repeatedly emphasize the same low, middle and high-frequency notes—and delete the same notes at the notches—so the phaser circuit also employs an oscillator to continually move (or “shift”) the point at which these notches and peaks occur, so that different frequencies are emphasized and de-emphasized at each pass, at a rate determined by the unit’s “speed” or “rate” knob.

I say from 0 to 360 degrees because that’s the simplest way to envision the “full circle” of the phase relationship, and in reality you can’t walk further around anything than a full circle. Unless you’re a phase shifter. For these pedals, designers talk in terms of a continual shifting of the phase relationship from 0 to, in theory, infinity, depending upon how many shifting stages the circuit contains. The phase shifts by 180 degrees for each stage, so for the typical simple phaser with four stages, we’re talking from 0 to 720 degrees, with three peaks and two notches along the way. Phasers with six, eight and even ten stages have been built for use with guitar. But for many, the simple four-stage circuit is most appealing.

Univox Uni-vibeMuch of this is probably thanks to an outdated a pedal with a two-function switch that is labeled “Chorus” and “Vibrato.” These words will trigger a sigh of awe and wonder from many a guitarist because, of course, they are the labels on the mode switch of the famous Univox Uni-Vibe. This pedal is a good place to start because it was one of the first of the transistorized effects of this type to become widely available, and it occupies a patch of ground all its own in the world of things that go “swoosh”. That said, and despite the name and switch labeling, the Uni-Vibe is more akin to a four-stage phaser than what we today consider to be a chorus pedal, even if that’s the label on its most-loved setting. The deception is forgivable when you remember that the Uni-Vibe’s intention was to reproduce the chorus-type sound—or “chorale” sound, as it was often labeled—produced by a Leslie rotating speaker cabinet used with a Hammond organ. Also, the unit existed before there was much categorization of such things: it was a guitar effects footpedal, it had its own sound… and that was all anyone needed to know. The Uni-Vibe—and the better clones that have followed it down the years—is based around a discrete transistorized circuit with four sets of light bulbs and light cells and a low frequency oscillator (LFO) which does the shifting work to move the peaks and notches. Unlike the drawing-board phaser discussed above, however, the frequencies of each stage of the Uni-Vibe are set differently, so it could be argued that there is indeed more of a chorusing of the sound.

Electro-Harmonix’s Small StoneOther early phase shifters used field effect transistors (FETs) to control each phasing stage in place of the light bulbs in the ’Vibe, and certainly later units employed opamps with variable resistors (six TL072 dual opamps or similar in the MXR Phase 100, for example). Electro-Harmonix’s sweet little Small Stone, on the other hand, has a more unusual design that employs five CA3094 type Operational Transconductance Amplifiers (OTAs). The results are similar, but subtly different. Many phasers—such as MXR’s Phase 45 and Phase 90, and E-H’s Small Stone—carry nothing but a speed control, plus a “Color” switch in the case of the latter. Others have Depth, Mix and Resonance controls. The latter appears on many units with internal feedback loops (the Small Stone and most phasers before it lack this circuitry), and allows the player tweak the degree to which the portion of the signal fed back enhances the frequency peaks.

The gray area between different types of phasers and chorus pedals—and phaser-style chorus pedals versus delay-based chorus pedals—arises probably because designers and manufacturers really have followed two distinct paths in this field. Some phasers have sought to approximate the Uni-Vibe’s approximation of a Leslie cab, and some so-called choruses have done much the same. Other phasers have been designed from the ground up more purely from the perspective of the principles of phase shifting in itself, rather than in an effort to sound like a whirling speaker or any other electromechanical device that has come before. The result means the field is broad and varied, and different phasers (or their related effects) can often have different voices with characteristics more distinct than, say, two delays from different makers.

Usually considered the big brother to the phaser, the flanger is indeed related in a sense, but achieves its heavier, some would say more oppressive sonic results by imposing more control over its placement of the notches created by the phase relationship, rather than spacing them evenly as the phaser’s sweep does. Much of the basic circuitry behind flanging, very simply put, follows the template as given above, but requires far more complex engineering to take it where it’s going. Pedal-sized units designed to replicate the sound of two big reel-to-reel tape machines sliding in and out of sync weren’t made possible until larger, more complex ICs became available to help do the job. This extra technology is needed to harmonically tune the out-of-phase notches, and therefore, relative to these, the peaks, and it’s this harmonic spacing of the spread that can make a genuine flanger pedal sound almost like it’s actively participating in the note selection of a sequence you are playing. Whereas phasers have from four to ten stages, the individual chips within proper flangers may carry hundreds of stages in themselves. Dizzying stuff.

A/DA FlangerNot the first commercially available flanger pedal, but probably the first that really worked properly with the full sonic depth of the effect, the A/DA Flanger was made possible by the advent of the SAD1024 chip (others were used through its lifetime, depending upon availability). It hit the shelves in 1977, and was shortly followed by Electro-Harmonix’s equally beloved Electric Mistress, and MXR’s big, gray Flanger. Nearly every big maker slapped its own flanger on the butt and sent it toddling down the pike within the next few years. These were serious pedals in their day, the big boys requiring their own regulated onboard AC power stage to run all that thirsty silicon. Most required a serious investment on behalf of the impoverished guitarist, too. A/DA’s unit retailed at $199.95 when introduced, and others went for even more. Not a steal, when you consider that minimum wage was $2.30 an hour in 1977.

The sound blew away guitarists when units first popped up in guitar stores. If the dizzying harmonic swirl didn’t just make you puke, it really sent you tripping. Interestingly, many tired of it a lot quicker than they did the phaser’s subtler, less imposing “swoosh”, and consequently it’s difficult today to name a fraction as many great guitar tracks with flangers slapped all over them as with phasers. For the latter, we’ve got the Stones’s “Shattered” (or just about anything from Some Girls), the Clash’s “Lost in the Supermarket” from London Calling and loads from Sandinista, and heavier rockers from early Van Halen to recent Foo Fighters. In the flanging corner, we’ve got The Pat Travers Band’s “Boom Boom, Out Go The Lights” and… well, I’m sure there’s another somewhere. Okay, maybe the intro lick to Heart’s “Barracuda” redeems it some.
I don’t mean to be unfair to the effect (and theoretically, this should be an article devoid of opinion). Flanging is impressive stuff. It’s just that, used heavily—where it best shows off its massive harmonics-plinking capabilities—it can become too imposing a sound for a guitarist to easily play with, which relinquishes it to the realm of background effects and early-’80s electro-pop. Still, plug in and send your brain to space and back.

While modified four-stage phasers like the Uni-Vibe and other effects before it had sought to evoke a choral-like sound, the chorus pedal as we know it didn’t arrive until affordable short-delay chips became widely available in the late 1970s. This in itself implies that chorus is a type of delay, and indeed it is, but the sonic results of the ways in which these short delays are transformed lands the effect firmly in the modulation camp, so it’s easiest to consider it as such (the function of these delay chips will be explained in more detail in the Echo & Delay section down the road). IC-based chorus effects work in a manner very similar to flangers, detailed above. The main difference, put simply, is that the short delays used to create harmonically spaced notches and peaks in the frequency spectrum are manipulated to modulate more tightly above and below specific frequency ranges, rather than shifted to swoosh broadly up and down the entire spectrum. The result, and intention, is a sound like a group of singers or similar instruments, where some voices/strings inevitably waver slightly from those beside them, producing a quivering clash of harmonics that evokes space and dimension when used subtly. The effect is possibly best heard—as pure effect, at least—in stereo, and the nature of the beast lends itself readily to stereo outputs. The broad soundstage and Doppler-like movement a good stereo chorus can spread out before you between a pair of spaced amps or L-R headphones make it one of the most spacious, three-dimensional effects available, short of long delays or cavernous reverb settings.

Boss’ CE-1 Chorus Ensemble was the first of these types commercially available, and is the best remembered of the company’s now-archaic looking early range of die-cast metal pedals. The unit was an instant success when it Boss CE-1hit the market in 1976, and was quickly snatched up by a range of major players. Andy Summers used the CE-1 with the Police in the late 1970s and early 1980s, and it’s probably most famously heard on the band’s big 1979 hit “Message In A Bottle,” though others made creative use of it too. Shortly after the Boss, Electro-Harmonix offered both its Memory Man Stereo Echo/Chorus—which featured a very good, spacious chorus setting that a lot of player’s loved—and smaller, stand-alone Small Clone chorus. Like the Small Stone phaser before it, the Small Clone had a softer, subtler sound than many of the chorus pedals that would soon flood the market, and it too was a huge hit. Kurt Cobain’s use of the pedal on “Smells Like Teen Spirit” and Chris Novoselic’s bass part on “Come As You Are” from Nirvana’s Nevermind album shows off how it can add a rich, moving, liquid texture to both clean and distorted parts. MXR, DOD and Ibanez all offered popular early IC-based analog chorus pedals, and today every major mass-manufacturer has a unit on the market.

As these same makers ramped up for digital production, digital choruses naturally joined the team. The effect as produced digitally might sound broadly similar, but it is done differently than in the analog circuits. Digital chorus pedals double a signal and add delay and pitch modulation to one path, the latter wobbling below and above the pitch of the unadulterated signal, which produces an audible out-of-tuneness when the paths are blended back together. It’s hard to fault the power and range of control that digital technology affords, and this version of the effect has been hugely successful, but many guitarists still prefer the subtle, watery shimmer of the analog version. Conversely, the same ears often find the digital variety makes them a little queasy.

Next installment: vibrato, tremolo, rotary speaker, and octave divider effects.

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