Modulation of Time and Frequency – by Mike Overly

ModulationIn the last post we learned the history of guitar effects and how manipulating volume alters the sound of our guitar. Now, let’s take a look at ways to make our guitar sound even weirder by exploring effects that modulate time and frequency. Simple defined, modulate is to change or vary certain characteristics of sound, such as amplitude, frequency, and time.

Among the first effects guitarists were attracted to were reverb and echo. From the very first tape-based echo units to the most advanced looping pedals of today, nothing has done more to change the sound of the guitar than time shifting. Here are some of the most classic time modulated effects: Delay and Echo, Reverb, Looping.

Delay pedals send back the original audio signal in milliseconds, ms. A millisecond is one thousandth of a second, or 0.001 second. As a general thought, below 20ms is called reverb, doubling is between 20-30ms, and above 30ms is known as echo. Delay effects units produce a repeating effect by adding a duplicate audio signal to the original signal at a varied time-delay. The effect can either be very fast reverb, a single slapback echo, or multiple echoes that last for a very long time. In fact, a delay may last so long that you can play new musical material over the top of the original material, see looping below. It’s also possible to reverse the delay to produce a variety of very interesting effects.

Reverb is very fast delay which has each repeat of the original audio signal, known as iteration, decaying at different rates. Spring reverb systems, which are often used in guitar amplifiers, use a transducer to create vibrations in a spring. Plate reverb systems use a transducer to send vibrations across a plate of metal. Most modern reverb effects use digital signal processing algorithms, or DSP, to create a reverb effect by applying multiple feedback delay circuits. Without a heavy use of reverb, there would be no surf guitar!

Echo is a slower delayed effect with each echo decaying at a different rate. Echo effects make you sound like you’re playing inside a very large empty room. Vintage analog echo effects, such as the 1959 Echoplex and the 1973 Roland Space Echo, are highly prized and are still in use today. The first digital delay pedal was the Boss DD-2 in 1984.

Loop effect units, also known as phrase samplers, are simply delay pedals with extra long iterations and no decay. The first loop effects were created with reel-to-reel tape using a tape loop. Today however, this looping effect is created digitally by using a DSP loop pedal. These loop pedals enable a guitarist to record and then replay a phrase or musical fragment with endless repeats. Loops can be created on the spot during a performance, or they can be pre-recorded. Some loop effect units allow a performer to create multiple loop layers which makes for some interesting musical textures. Modern looping pedals enable the traditional singer-songwriter to add a new dimension to the live performance.

Alright, now that you know how to modulate time, let’s learn how to modulate frequency!

Effects that modulate frequency produce some of the most interesting and bizarre sounds available for guitar. Modulation effects work by affecting an audio signal’s frequency/pitch to create everything from classic vibrato to outer space-like sounds. Most modulation effects split an instrument’s audio signal in two, altering one portion of the signal and mixing it with the unaltered portion called a carrier wave to create sounds with unusual tonal properties. Here are some of the most classic frequency-based modulation effects: Chorus, Phaser, Flanger, Tremolo and Vibrato.

Chorus pedals mimic the very slight out-of-tune differences in pitch that choirs and string orchestras produce naturally. A chorus effect electronically splits the instrument-to-amplifier audio signal into two parts and adds a slight delay and frequency variation to one part of the signal while leaving the other part unaltered.

Phase shifting pedals split the audio signal in two and then play back these two signals at different frequencies to produce a spacey, swishing sound. A phase shifter creates its rippling effect by altering the phase of one of the signal paths. Here’s something interesting. In the 1940s, Don Leslie developed a true analog phase-shifting speaker cabinet that split and sent an audio signal to a low-frequency 15-inch drum speaker and a high-frequency horn. The drum and horn rotated at different speeds in opposite directions, producing sounds which oscillated from slow to shimmering fast as the speakers rotated in and out of phase with each other.

Flanger pedals are a lot like phasers, but with more of a sweeping, Doppler-like jet plane sound. The first flanger effects were the result of attempts to reproduce phaser effects in the studio by the use of manual tape flanging. Manual tape flanging is produced by recording a track on two synchronized tapes and periodically slowing one tape by pressing the edge, or flange, of its reel. When the two tapes’ audio signals are mixed together, a comb-filter flanging effect is heard.

Although you may think they sound similar, tremolo and vibrato are two completely different modulating effects.

Tremolo is actually a dynamic effect produced by variations in signal volume, which results in a wavy, in-and-out sound. Electronic tremolo units create their effect by mixing an instrument’s audio signal with a sub-audible carrier wave that generates amplitude variations in the sound wave. And here’s something worth knowing. What a guitar player calls a tremolo-bar is really not a tremolo bar, but rather a vibrato-bar, or whammy bar. This is true because the whammy bar is a mechanical device attached to the guitar bridge that loosens the strings thereby lowering the pitch. This pitch-bending effect should be correctly called vibrato.

Vibrato modulates a pitch higher and lower to a greater or lesser degree. A guitar equipped with a vibrato bar, also known as a whammy bar, enables the player to produce vibrato manually. Electronic vibrato effect units allow for the control of the rate of speed at which the pitch variations occur and to set the depth, which is the difference in pitch. The vibrato effect unit enables the guitarist to imitate the micro-tonal nuances produced naturally by singers and violinists as they sustain a single pitch. Setting a vibrato unit to an extreme depth setting, a half step or more, will produce a dramatic, undulating sound. Vibrato effects produce vibrato by mixing the instrument’s audio signal with a modulating carrier wave to create frequency variations in the sound wave. As was said earlier, the terms vibrato and tremolo are often mislabeled.

Octave pedals output an audio signal which is an octave higher or lower. In 1967, Roger Mayer made the first octave effect, the Octavia. Upon first hearing this pedal, Jimi Hendrix went into the studio and used it to record the guitar solos on “Purple Haze” and “Fire.”

Ring Modulators produce a variety of resonant noises ranging from dissonance and grinding, to metallic bell-like sounds. Ring modulators mix the instrument’s audio signal with a carrier wave generated by the device’s internal oscillator to create tones that are mathematically generated. The original sound wave is suppressed and replaced by a ring of inharmonic pitches known as sidebands. Ring modulators were used by avant-garde musicians in the 1950s as well as the guitar solo on Tony Iommi’s heavy metal super-hit Paranoid.

As we have learned, changes in time, frequency and amplitude can greatly affect the way a sound is perceived. So, now that you’ve modulated a signal’s volume, manipulated with its frequency and made it repeat itself… what’s left to explore? In a word ~ filters!

Filters alter the frequency content of the audio signal that passes through them by either strengthening/boosting or cutting/weakening specific frequencies or frequency regions. Filtering effects affect the sound waves’ quality in a number of ways, from a wah-wah pedal, that can make your guitar sing or cry, to tweaking EQ so as to make your signal sound as tiny as an old AM radio or as bass-heavy and muffled as the club next door. Here are a few filtering effects to get you started: Equalizer, Pitch shifter, Harmonizer, Feedback, Sustainer, Wah-Wah, Envelope Follower, Talk Box, Simulator and Modeling.

Equalizer/EQ pedals use a variety of filters which allows the player to shape the guitar’s sound. An equalizer is a set of linear filters that boost/strengthen, or cut/weaken, specific frequency regions. And while it’s true that guitars and amps have built-in equalization, tone control knobs that manually adjust highs, mids and lows, an EQ pedal allows the player to shape the guitar’s tone much more precisely. EQ pedals are very useful in compensating for a wide variety of signal problems in live situations.

Pitch shifters modify frequency by transposing the pitch of an audio signal higher or lower by a pre-set interval. For example, a pitch shifter may be set to increase the pitch by an interval of a fourth which will raise each pitch played a Perfect 4th above the original frequency played. Simple pitch shifters raise or lower the pitch within an octave, while more sophisticated and expensive devices offer a greater range of interval selections.

Harmonizers are a type of pitch shifter that combines the original pitch with an altered pitch to create harmony of two or more pitches heard at the same time. Some harmonizers are able to create chorus-like effects by adding micro-tonal pitch shifts.

Feedback is an effect produced when an amplified sound is picked up by a microphone or the electric guitar’s pick-up and played back through an amplifier, initiating a feedback loop. Feedback, is generated by playing an electric guitar directly in front of an amplifier set to a high volume. This seemingly primitive technique of volume and guitar position relative to the amp’s loudspeaker can be hard to find, however, when it’s found, the thrill of high-pitched sustained overtones is an experience not so forgotten!

Sustainer effects produce true undistorted sustain. One example is the EBow, which is a small, handheld electromagnetic string driver that consists of two coils: one that picks up and amplifies the guitar string vibration much in the manner of a guitar pickup, and one that acts as a magnetic string driver vibrating the string. The EBow vibrates one string at a time. There are also guitar pickups that have electromagnetic string drivers built into them that are capable of producing feedback/sustain at low volumes.

Wah-wah pedals create vowel-like sounds by filtering out certain frequencies while compressing others. The wah-wah pedal is operated by a foot treadle that opens and closes a potentiometer which alters the guitar’s frequency spectrum in what is known as a spectral glide or sweep. The first wah-wah pedal, The Clyde McCoy, was manufactured by Warwick Electronics in1967. In 1968, Univox began marketing its Uni-Vibe pedal, an effect that mimicked the odd phase shift and chorus effects of the Leslie rotating speakers we learned about earlier. Wah-wah pedals are the favorite modulating effect of many guitarists.

Envelope Followers activate an envelope filter once a designated amplitude/volume is reached. One effect that uses an envelope filter is the auto-wah, which produces a wah effect depending on how loud or quiet the incoming audio signal is played. Auto-wah pedals are wah-wah pedals you can set and forget.

A Talk Box directs the audio signal from the guitar into the mouth of the performer via a plastic tube. This enables the sound to be shaped into vowels and consonants of human speech. The modified guitar sound is picked up by a microphone and heard through a PA, Public Address system. The Talk Box really does make the guitar talk! Some famous uses of the talk box include Bon Jovi’s “Livin’ on a Prayer,” and Peter Frampton’s “Show Me the Way.”

Simulators enable electric guitars to imitate the sound of other instruments such as acoustic guitar, electric bass or a sitar. Simulators can make guitars with single-coil pick ups replicate the sound of guitars with humbucker pick ups, or vice-versa. Boss makes an Acoustic Simulator pedal which enables electric guitarists to realistically approximate acoustic guitar tones. A de-fretter effect simulates the sound of a fretless guitar by using an envelope-controlled filter and a voltage-controlled amplifier to soften the pick attack and change the timbre, which is the quality of the sound. A rotary speaker simulator mimics the doppler sound of a vintage Leslie speaker system by replicating its volume and pitch modulations, overdrive capacity and phase shifts.

Modeling, also known as amp modeling, is a digital effect that reproduces the sound of various amplifiers, most often analog tube amps. Sophisticated modeling effects can simulate various amplifier heads, speaker cabinets and microphone recording techniques.

One of the problems of using many individual effect pedals is that you can only change one effect at a time. Therefore, manufacturers began producing all-in-one pedal board solutions. In other words, pedal boards which were designed to change a variety of different effects at the same time with only one tap of the foot. The Boss ME-70 Guitar Multiple Effects Pedal Board and the Line 6 Pod are two popular multi-effect pedal boards that can simulate classic amplifier sounds, as well as offer a variety of different effects.

Okay, now that you know what effects pedals do, it’s time to put them to work. In the next post, The Art of Chaining, we’ll learn how to connect these effects together so that you may find that allusive “perfect tone.”

So, ‘til next time have some modulating fun… I’ll be listening!

One Response to Modulation of Time and Frequency – by Mike Overly

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