Distortion (music)

Guitar distortion is a guitar effect applied to the guitar, the bass, and other instruments, particularly within hard rock, punk rock, and heavy metal. Guitar distortion can give the lead guitar a sustaining tone for playing solos or leads, or provide a rough, crunchy tone suitable for rhythm guitar.

Edge-of-breakup as the baseline rock sound

The central point of reference for dialing-in rock distortion sounds is not a clean signal, but rather, a tube power amp that is running on the edge of audible distortion, so that as the guitar strings are plucked harder, the amount of distortion and the resulting volume both increase, and lighter plucking cleans-up the sound. Special effects are then dialed-in to complement and preserve that baseline foundation of edge-of-breakup.

Guitar amp modelling is about various guitar-specific distortion qualities, rather than pure amplification or special effects. Amp modelling is about reproducing several popular varieties of distortion that serve as common points of reference. Guitar distortion is produced by using effects pedals in conjunction with a guitar amplifier, and thus bridges the two subjects, while also including guitar pickup selection and windings, guitar volume, and how the guitar is played.

Physical approaches to creating distortion

The earliest uses of intentional distortion that have been recorded were achieved through "doctoring" amplifiers and speakers. Guitarists would use a screwdriver or pencil to poke holes into their speaker cones to create a distorted sound. Later, distortion and fuzz effects were achieved through electronics.

Clipping in signal processing

In fuzzboxes and solid state distortions, the signal is boosted, and the tops of the waveform clipped off. This is usually achieved by either amplifying the signal to a point where it must be clipped to the supply rails, or by clipping the signal across diodes that short it to supply rails.

In vacuum tube distortion, or tube modelling distortion, the top of the wave form is compressed, thus giving a smoother distorted signal, that retains more of the original waveform. This is generally considered more pleasing to the ear (see tube sound). This is commonly referred to as overdrive, as it was originally (and often still is) attained by driving the tubes in an amplifier a little harder than they can handle without affecting the signal. Many solid state distortion devices attempt to emulate the valve sound of overdriven vacuum tubes.

If the overdrive effect is used at a high setting, the top of the waveforms clip off. This creates a "dirty" or gritty tone, which sounds like an amplifier "driven" to its limit. This can be combined with a tube amplifier (usually driven to a point short of distortion or overdrive) to create extremely thick and "heavy" distortion sounds. This is referred to as "crunch". Today, there are a series of distortion devices that try to recreate this sound.

Distortion voicing through alternating EQ and clipping stages

Rock guitar distortion is obtained and shaped throughout the standard signal processing chain, including multiple stages of preamp distortion, power tube distortion, power transformer distortion, and guitar speaker distortion. Much of the distortion character or voicing is controlled by the frequency response curve before and after each distortion stage; this dependency of distortion voicing on frequency response can be heard in the effect that a wah pedal has on the subsequent distortion stage, or by using an EQ pedal to favor the bass or treble components of the guitar pickup signal prior to the first distortion stage. Similarly, a guitar amp's tone controls shape a different power-tube distortion voicing if the tone controls are set to extremely emphasize the bass or treble. Extreme settings, either one way or the other, are most popular in heavy metal, which has long been known for high volume and distorted guitar tones. Increasing the bass and treble while reducing or eliminating the centre midrange (750Hz) results in what is popularly known as a "scooped" sound (since the midrange frequencies are said to be "scooped" out of the tone). James Hetfield of Metallica used this tone almost exclusively on Metallica's first four studio albums. Conversely, decreasing the bass while increasing the midrange and treble creates a harsher sound; Kerry King and Jeff Hanneman of Slayer have both used midrange-heavy tones since the mid-'80s.

Packaging of power-tube distortion capability

Power-tube distortion can be produced in a dedicated rackmount tube power amp. A modular rackmount setup often involves a rackmount preamp, a rackmount tube power amp, and a rackmount unit containing a dummy load and guitar speaker cabinet simulator filter (Palmer's PDI-03). A similar alternative is to use a rackmount dummy load (without a cabinet simulation filter), followed by additional line-level rackmount signal processing, and then use a rackmount solid-state amplifier to re-amplify the signal to drive a guitar speaker.

Some effects pedals internally produce power-tube distortion, including an optional dummy load for use as a power-tube distortion pedal. Such effects units use a preamp tube such as the 12AX7 in a power-tube circuit configuration (as in the Stephenson's Stage Hog), or use a conventional power tube, such as the EL84 (as in the H&K Crunch Master compact tabletop unit).

Obtaining preamp or power-tube distortion

Preamp distortion can be produced entirely within a distortion pedal, floor preamp/processor, or rackmount preamp/processor designed for guitar. Or, a non-distorting level booster such as an equalizer pedal can be used to push the guitar amp's preamp stages into distortion. Similarly, a floor guitar preamp/processor or an outboard rackmount guitar preamp/processor with built-in preamp distortion can be the sole origin of the preamp distortion.

During the 1960s to early 1970s, distortion was primarily created by overdriving the power tubes. During the 1980s and 1990s, the Master Volume feature was standard on almost all guitar amplifiers, enabling conveniently generating high distortion levels in the guitar amp's preamp section while blocking most of the resulting signal from going to the power tubes, keeping well within the linear region of the power tubes and thus keeping the sound level down to the desired level. A wide selection of distortion and overdrive pedals became available then, including preamp-tube based distortion boxes, so distortion was popularly created by preamp distortion.

Power attenuation for volume-independent power-tube distortion

Power-tube distortion is becoming available at lower volumes by using power attenuators, power-supply-based power attenuation, built-in attenuation in tube guitar amps down to the milliwatt level, lower-wattage tube amps (such as a quarter-watt or less), speaker isolation boxes, and low-efficiency guitar speakers.

Amp modelling for distortion emulation

Amp modelling, typically using digital signal processing, produces refined flavors of distortion that attempt to emulate the combined sounds of preamp, power-tube, and speaker distortion in famous guitar amplifiers. This digital signal processing to produce a wide range of famous distortion sounds can be in the form of realtime software running on a computer, or it can live in hardware such as a compact pedal, oversize pedal, rackmount processor, desktop or floor processor, or in a guitar amp head, including a tube amp. However, the sound these devices produce is not universally accepted as a proper substitute for the traditional tube sound.

See also

• Distortion
• Overdrive (music)
• Fuzzbox
• Guitar effects
• Effects unit
• Guitar pedalboard
• Power attenuator (guitar)
• Re-amp
• Valve sound

External links

• A Musical Distortion Primer (R.G. Keen) Article on the physics of distortion and a round-up of electronic techniques
• Guitar Distortion 101 Article on the physics of distortion, relationship of waveforms and harmonics
• GuitarSync.com Forums Online resource for acoustic and electric guitar players
• Amptone.com How to dial-in guitar sounds independently of volume level
• Tons of Tones !! Nonlinear distortion stages in amplifier models
• AX84 Free schematics and plans to help readers learn to build their own guitar amps, many low-wattage
• The Train Kept A-Rollin' The first rock recording (1956) to feature heavy guitar distortion