Aphaser is anelectronic sound processor used tofilter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs of the waveform being affected is typicallymodulated by an internallow-frequency oscillator so that they vary over time, creating a sweeping effect.
Phasers are often used to give a "synthesized" or electronic effect to natural sounds, such as human speech.
The electronic phasing effect is created by splitting an audio signal into two paths. One path treats the signal with anall-pass filter, which preserves the amplitude of the original signal and alters the phase. The amount of change in phase depends on the frequency. When signals from the two paths are mixed, the frequencies that are out of phase will cancel each other out, creating the phaser's characteristic notches. Changing the mix ratio changes the depth of the notches; the deepest notches occur when the mix ratio is 50%.
The definition of phaser typically excludes such devices where the all-pass section is adelay line; such a device is called aflanger.[1] Using adelay line creates an unlimited series of equally spaced notches and peaks. It is possible to cascade a delay line with another type of all-pass filter.[2] This combines the unlimited number of notches from the flanger with the uneven spacing of the phaser.
Traditional electronic phasers use a series of variableall-passphase-shift networks which alter the phases of the different frequency components in the signal. These networks pass all frequencies at equal volume, introducing only phase change to the signal. Human ears are not very responsive to phase differences, but this creates audibleinterferences when mixed back with the dry (unprocessed) signal, creating notches. The simplified structure of a mono phaser is shown below:
The number ofall-pass filters (usually calledstages) varies with different models, some analog phasers offer 4, 6, 8 or 12 stages. Digital phasers may offer up to 32 or even more. This determines the number of notches/peaks in the sound, affecting the general sound character. A phaser with n stages generally has n/2 notches in the spectrum, so a 4-stage phaser will have two notches.
Additionally, the output can be fed back to the input for a more intense effect, creating a resonant effect by emphasizing frequencies between notches. This involves feeding the output of the all-pass filter chain back to the input, as shown here:
The frequency response of an 8-stage phaser with or without feedback is shown. Note that the peaks between the notches are sharper when there's feedback, giving a distinct sound.
Astereo phaser is usually two identical phasers modulated by aquadrature signal; the output of the oscillators for the left and right channels are a quarter-waveout of phase.
Many modern phasers are implemented usingdigital signal processing, often emulating analog phasers. Phasers are mostly found asplugins for sound editing software, as a part of a monolithicrackmount sound effect unit, or asstompbox guitar effects.
The term was often used to refer to the original tapeflanging effect heard on manypsychedelic records of the late 1960s, notably "Itchycoo Park" (1967) by theSmall Faces.[3] TheEventide Instant Phaser from 1971 was one of the first studio devices to emulate the tape flanging effect (with all-pass filters instead of delay, thus being one of the first to distinguish phasing from flanging).[4] It was widely employed in the studio and in live settings by artists such asLed Zeppelin andTodd Rundgren. Phasing is a popular effect forelectric guitar. In 1968, Shin-ei'sUni-Vibe effects pedal, designed by audio engineer Fumio Mieda, incorporatedphase shift, soon becoming favorite effects of guitarists such asJimi Hendrix andRobin Trower.[5]
By the early 1970s, phasing was available as a portable guitar effect, the first being the Maestro Phase Shifter PS-1 designed byTom Oberheim.[citation needed] Unlike other phase shifters to follow, the Maestro PS-1 had three buttons to control the speed: slow, medium, and fast speed.[a] Notable users of the Maestro Phase Shifter wereJohn Paul Jones ofLed Zeppelin,Alex Lifeson ofRush,Waylon Jennings[6] andErnie Isley ofThe Isley Brothers. Another notable early example was theMXR Phase 90[7] which featured a control knob for speed control. From 1974,Steve Hackett of Genesis, in theSelling England by the Pound (1973) studio album and tour, used the MXR Phase 90 for his Les Pauls, and fromThe Lamb Lies Down On Broadway (1974) album and tour, used thephase filter section in hisElectronic Music Studios Synthi Hi-Fli. In country music, Waylon Jennings was a notable user of the phaser effect, particularly prominent on "Are You Sure Hank Done It This Way" (1975).[8] In the late 1970s,Brian May used large amounts of phasing, in such songs as "Sheer Heart Attack".[9] In the late 1970s and 1980s,Eddie Van Halen often used the MXR Phase 90 as part of his signal chain,[10] for example in the instrumental "Eruption" and on the song "Atomic Punk".[11]
Keyboard players also used phasing: in the 1970s,keyboard instruments like theFender Rhodeselectric piano, theEminent 310electronic organ, and theClavinet were commonly treated with a phaser, especially inavant-garde jazz.Bill Evans, for instance, used a Maestro phaser onIntuition (1974).[12] The phaser is also used to "sweeten" their sounds. Examples can be heard inBilly Joel's "Just The Way You Are",Styx's "Babe", andJean Michel Jarre'sOxygène (1976) on which he used an EHX Small Stone phaser extensively.Tony Banks (Genesis) used anMXR Phase 100 on his RMI 368x Electra piano from 1974 (later he inserted this effect, as well as the fuzzbox, into the Electra's panel); from late 1977 he also used the phaser (along with aBoss CE-1) on his Hammond organ to replace theLeslie's rotating effect.Daft Punk helped to re-popularize the effect in the 21st century, utilizing it on a number of tracks on theirDiscovery album in 2001.Richard Tee also used a phase shifter hooked up to hisFender Rhodes.[citation needed]
Inmotion picture ortelevision production, the effect created by a phaser is often used to imply that the sound is synthetically generated, like turning a natural human voice into a computer or robot voice. The technique works because the frequency filtering produces sound commonly associated with mechanical sources, which only generate specific frequencies, rather than natural sources, which produce a range of frequencies.Vocoder andring modulation are also used to create synthetic effects.
A specific type of phasing,flanging is a similar effect, in which the notches are linearly spaced. In a flanger effect, the notches are created by mixing the signal with adelayed version of itself. Flangers tend to sound more pronounced and natural, like the "jet plane whoosh" effect, whereas phasers tend to sound more subtle and otherworldly. For comparison of the two effects, checkFlanging § Comparison with phase shifting.
TheUni-Vibe is an early phase-shifting effect that usesphotoresistors for modulation.
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