Flex Time algorithms and parameters in Logic Pro for Mac

Flex Time algorithms are track-based settings that determine how the timing or pitch of your audio material can be altered—using time compression or expansion, manipulating invisibly sliced segments, or speeding up or slowing down the material. You can choose an algorithm based on the type of audio material you’re working with. Each algorithm comes with its own individual parameters, located in the Track inspector.

You can also have Logic Pro for Mac choose the most suitable Flex Time algorithm based on an automatic analysis of your audio material.

Figure. Track inspector showing Flex algorithms and parameters.

Note: When you choose a flex algorithm for a track, the Freeze Mode parameter in the Track inspector switches from Pre Fader to Source Only. This freezes the track signal without any effects plug-ins. See Freeze tracks.

Have Logic Pro choose a Flex Time algorithm

In the Logic Pro Tracks area or Audio Track Editor, choose Flex Time - Automatic from the Flex Mode pop-up menu.
The resulting algorithm is chosen as follows:
- Monophonic for monophonic audio material—bass line, for example
- Slicing for percussive, non-tonal audio material—drums, for example
- Polyphonic for chords, or complex material—mixes, for example

Use the Slicing algorithm

In Logic Pro, choose Flex Time - Slicing from the Flex Mode pop-up menu.
Slicing cuts the audio material at transient markers, then shifts the audio while playing each slice at its original speed. No time compression or expansion is applied to the shifted audio. Any gaps that occur as a result of shifting the audio can be filled using the decay function. Slicing is a good choice for drums and percussion and comes with the following parameters:
- Fill Gaps: Turns the decay function on or off, allowing you to fill any gaps that occur between sounds as a result of shifting the audio.
- Decay: Sets the decay time between sounds, because no time stretching takes place to compensate for the gaps.
- Slice Length: Shortens each slice by a percentage value. Shortening slices can be helpful for removing unwanted pre-attack sounds from the following slice, or to create a gated effect.

You can slice an audio region at transient marker positions, splitting it into separate regions at each transient marker. You do this by Control-clicking the audio region, then choosing Slice at Transient Markers from the shortcut menu.

You can also slice an audio region at flex marker positions, splitting it into a separate regions at each flex marker you have previously placed. You do this by Control-clicking the audio region, then choosing Slice at Flex Markers from the shortcut menu.

Note: You can use Slice at Transient Markers and Slice at Flex Markers regardless of the flex algorithm selected.

Use the Rhythmic algorithm

In Logic Pro, choose Flex Time - Rhythmic from the Flex Mode pop-up menu.
Rhythmic time stretches the material, looping audio between slices in order to fill any gaps. This algorithm is most suitable for material such as rhythmic guitars, keyboard parts, and Apple Loops. Rhythmic comes with the following parameters:
- Loop Length: Sets the length of the looped section at the end of a slice that is used for time expansion.
- Decay: Defines a decay value for the looped area.
- Loop Offset: Allows you to move the looped area up to 100 ms to the left, preventing pre-attack sounds from the following transient to appear in the looped and crossfaded area.

Use the Monophonic algorithm

In Logic Pro, choose Flex Time - Monophonic from the Flex Mode pop-up menu.
Monophonic is designed to be used on melodic instruments only playing a single note at a time, and is therefore good for solo vocals and monophonic solo instruments—melody and bass lines, for example. When you use this flex algorithm, your recording should be relatively dry, without audible reverberation. Otherwise, you may want to try using Polyphonic. Monophonic comes with a single parameter:
- Percussive: Preserves the area around transient markers to protect the percussive part of the sound. If selected, allows better timing for all kinds of percussive monophonic tonal material, such as plucked strings (guitar, bass) or tonal percussion. If unselected, prevents glitches on transient markers in non-percussive tonal material, such as bowed strings or wind instruments.

Use the Polyphonic algorithm

In Logic Pro, choose Flex Time - Polyphonic from the Flex Mode pop-up menu.
Polyphonic time stretches material based on a process called phase vocoding—a process that uses phase information to time stretch an audio signal without touching its pitch. It is the most processor intensive of all the flex algorithms, but it delivers high sonic quality with suitable polyphonic material. It’s recommended for complex polyphonic material and is good for all kinds of chords—such as guitar, piano, and choir—and also complex mixes. Polyphonic comes with a single parameter:
- Complex: Activates more internal transients in the audio material.

Use the Tempophone algorithm

In Logic Pro, choose Flex Time - Tempophone (FX) from the Flex Mode pop-up menu.
Tempophone emulates the effect of an historical tape-based time-stretching device known as a tempophone, and results in a mechanical sound with many artifacts similar to those produced with granular synthesis techniques. Tempophone is intended to be used creatively for special effects and comes with the following parameters:
- Grain Size: Sets the size of the grains that are played or repeated in their original speed and crossfaded to create time compression or expansion.
- Crossfade: Adjusts the crossfade length from 0.00, which produces hard artifacts, to the full grain length of 1.00, which tends to sound softer.

Use the Speed algorithm

In Logic Pro, choose Flex Time - Speed (FX) from the Flex Mode pop-up menu.
Speed time stretches material by playing the source material faster or slower, including pitch change. Because of the pitch shifting, this flex algorithm is mainly useful for percussive material, but it can be used on all material for interesting creative effects.

Download the guides:

Logic Pro User Guide: PDF

Logic Pro Instruments: PDF

Logic Pro Effects: PDF

Control Surfaces Support Guide: PDF

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Logic Pro User Guide for Mac