Tremolo is technically known as amplitude modulation, and it’s one of the first effects ever built into guitar amplifiers. Both Gibson and Danelectro pioneered the technology of tremolo in their designs of the late 1940s. Slightly more famously, Fender’s Tremolux (circa 1955) was the first guitar amp manufactured under the Fender name to employ tremolo.
Tremolo creates a cyclic or recurring level variation in amplitude or loudness. A simple example of this would be rhythmically turning the volume up and down on an electric guitar. Aside from amps, there are plenty of effects that will simulate tremolo, available both as external hardware processors or as software. Universal Audio’s Nigel, McDSP’s Chrome Tone, and Soundtoys’ Tremolator are good examples of plug-ins that offer this on your desktop. Tremolo sounds killer on almost anything — but is quite effective on electric piano, voices, synth pads, and of course, guitars. And a very cool thing about plug-ins is that you can often sync the tremolo speed to the host tempo.
Now let’s get technical. Figure 1 shows a basic sine wave. The frequency is constant, yet the amplitude varies. Remember, tremolo is a variation of amplitude (volume) applied to a sound of constant frequency.
There are two classic controls you’ll find on most guitar amplifiers and/or plug-ins outfitted with tremolo. The first is speed, which determines the rate (frequency) of the LFO (low frequency oscillator) modulating a gain stage. The LFO’s basic function here is to produce a periodic voltage variation at sub-audio frequencies, usually between 1–15Hz, which helps create slow level changes. The higher the speed control setting, the faster the tremolo effect.
The depth control varies the intensity of the effect. By turning it up, the level changes from completely on to completely off, which in turn creates a more intense effect. Turning it down simply creates less of a difference between the maximum and minimum levels.
Interestingly, early guitar amplifier manufacturers employed a light source and photoresistor combination (in a light-tight case) to achieve the tremolo effect. The LFO drove the lamp, making it brighter and dimmer. As the light brightened, the photoresistor’s resistance became smaller, allowing more of the guitar signal to pass and thus increasing amplitude or volume. As the light dimmed, the resistance of the photo resistor became larger, allowing less of the guitar signal to pass — thus decreasing amplitude or volume. Pretty cool stuff. What’s more, photoresistors have a natural decay time. So, as you turned up the speed, the tremolo effect would generally become less pronounced without having to actually change the depth.
Next up: vibrato — also known as frequency modulation. It’s based upon variation of pitch and frequency, as applied to a sound of constant amplitude or volume. As a player can create vibrato manually, it predates electronics, with roots dating back to use on early classical instruments and voice.
Magna Electronics Incorporated, which started making Magnatone amplifiers in the 1950s, was the first manufacturer to design, and build, real vibrato into their product. Even Fender’s line of famous Vibrolux amps did not have true vibrato — it was simply a modified tremolo circuit. Don L. Bonham is actually the pioneer behind the vibrato design. In 1961, he was awarded one of many United States patents for his work, but his filed designs started as early as 1954.
Referring to Figure 2, we see a basic sine wave. The frequency varies, yet the amplitude is constant. This can also be considered a shift in phase — or phase shifter.
Even though vibrato circuits are more complex than tremolos, they both share similarities, such as a low frequency oscillator (LFO) and a light source/photocell combination to achieve the desired effect. Magnatone later moved onto using Voltage Variable Resistors — or Varistors — in place of the Light Dependent Resistor (LDR) combination.
Vibrato can be created electro-acoustically through a Leslie’s Doppler effect, which simultaneously produces tremolo. A Leslie cabinet’s velocity and rotation will raise and lower the pitch of the tone produced through it. Vibrato can be realized through various methods you already know, such as the human voice, string instruments and once again, the guitar (think Angus Young here). Modulating a delay line can also produce vibrato, and plug-ins such as Waves’ Mondo Mod can create vibrato effects, while Line 6’s Amp Farm has vibrato on the Fender Amp simulations. Ah, but is it true vibrato? Now you’ll be able to know. . . .