Phaser Effect

The Phaser effect combines the original signal with a copy that is slightly out of phase with the original. This means that the amplitudes of the two signals reach their highest and lowest points at slightly different times. The timing differences between the two signals are modulated by two independent LFOs. In addition, the Phaser includes a filter circuit and a built-in envelope follower that tracks volume changes in the input signal, generating a dynamic control signal. This control signal alters the sweep range. Sonically, phasing is used to create whooshing, sweeping sounds that wander through the frequency spectrum. It is a commonly used guitar effect, but it is suitable for a range of signals.

Phaser Feedback Section
  • Filter button: Activates the filter section, which processes the feedback signal.
  • LP and HP knobs and fields: Set the cutoff frequency of the filter section’s lowpass (LP) and highpass (HP) filters.
  • Feedback slider and field: Determines the amount of the effect signal that is routed back into the input of the effect.
Phaser Sweep Section
  • Ceiling and Floor sliders and fields: Use the individual slider handles to determine the frequency range affected by the LFO modulations.
  • Order slider and field: Allows you to choose between different phaser algorithms. The more orders a phaser has, the heavier the effect.
  •  The 4, 6, 8, 10, and 12 settings put five different phaser algorithms at your fingertips. All are modeled on analog circuits, with each designed for a specific application.
  • You are free to select odd-numbered settings (5, 7, 9, 11), which, strictly speaking, don’t generate actual phasing. The more subtle comb filtering effects produced by odd-numbered settings can, however, come in handy on occasion.
  • Env Follow slider and field: Determines the impact of incoming signal levels on the frequency range (as set with the Ceiling and Floor controls).
Phaser LFO Section
  • LFO 1 and LFO 2 Rate knobs and fields: Set the speed for each LFO.
  • LFO Mix slider and fields: Determines the ratio between the two LFOs.
  • Env Follow slider and field: Determines the impact of incoming signal levels on the speed of LFO 1.
  • Phase knob and field: Available only in stereo instances. Controls the phase relationship between the individual channel modulations.
  • At 0°, the extreme values of the modulation are achieved simultaneously for all channels. 180° or −180° is equal to the greatest possible distance between the modulation phases of the channels.
Phaser Output Section
  • Output Mix slider and field: Determines the balance of dry and wet signals. Negative values result in a phase-inverted mix of the effect and direct (dry) signal.
  • Warmth button: Enables a distortion circuit, suitable for warm overdrive effects. 
Source : http://documentation.apple.com/en/logicexpress/effects/index.html#chapter=9%26section=6