A phaser is an audio signal processing technique used to filter a signal by attenuating a series of notches in the frequency spectrum. The position of the notches typically varies over time (modulation), and phasers usually include a low frequency oscillator for this purpose. A stereo phaser is usually two identical phasers modulated by a quadrature signal, i.e., the output of the oscillators for the left and right channels are a quarter-wave out of phase.

Traditional electronic phasers use a series of variable all-pass phase-shift networks. This will not alter the sound by itself, but will alter the phases of the different frequency components in the signal, which will yield constructive and destructive interference when it is mixed with the dry (unprocessed) signal. Additionally, the output can be fed back to the input to create a more intense effect. Most modern phasers are a part of a digital signal processor, often trying to emulate analogue phasers. Phasers are mostly found as plugins for sound editing software, as a part of a monolithic rackmount sound effect unit, and as "stompbox" guitar effects.

A flanger is a type of phaser where the notches are uniformly spaced, in particular, where the notches are created by mixing the signal with a delayed version of a signal. Flangers tend to sound more natural, like the "jet plane whoosh" effect, whereas phasers tend to sound more unnatural.

Uses

• Phasing is often used to make a natural sound seem artificial, such as making a human voice sound like a computer or robot voice. A phaser was used on C-3PO's voice in Star Wars, for example.
• Phasing is a popular effect for electric guitar. Eddie Van Halen often used a phaser as part of his signal chain, after his distortion effects, including the amplifier itself: Van Halen used a power attenuator to bring the amp's output down to line level so he could put effect boxes after it.
• Many electronic keyboard instruments are commonly treated with a phaser to "sweeten" their sounds - the Rhodes and the Clavinet being popular choices. Examples can be heard in Billy Joel's "Just The Way You Are" and Paul Simon's "Still Crazy After All These Years." Led Zeppelin also used a phaser on "No Quarter" to get a very mysterious and spooky sound with the Rhodes.

External links

• Big Muff & Guitar Effects Museum
• Phase shifting article on Harmony Central

Audio effects | Effects units