Difference between revisions of "Modulation Effects"

Revision as of 13:14, 24 September 2014

Modulation Effects cover a variety of effects that use a low-frequency oscillator (LFO) to alter the tone over time in a cyclic manner.

Effect Types

Phaser

Phasers use a (often variable) number of all-pass filters (called stages) to reverse the phase of certain frequencies, combining this phase-altered signal with the original signal to create nulls throughout the frequency spectrum. An LFO modulates how the stages operate and at what frequencies the nulls occur, sweeping them back and forth. Phasers can also use feedback on the phase-altered line to alter the make the nulls wider and less sharp. Some additionally have a mix parameter to determine the proportion of the dry (unprocessed) signal vs. the phase-altered signal.

Popular Models

• MXR Phase 90

Flanger

Flangers provide a very similar sound to phasers, but use delay and comb filters to create the nulls. The signal is split into 2 lines, with one line being delayed by a variable amount. When the lines are recombined, some frequencies will be out-of-phase, creating nulls at regular intervals (a comb filter). The delay time is linked to an LFO that changes the amount of delay cyclicly, so that the nulls sweep back and forth along the frequency spectrum.

Flangers often have the following controls:

• Depth/Width - The maximum range the LFO will alter the delay time from its baseline setting. Ex. The baseline delay is 500ms. Depth at 100% causes the LFO to change the delay time from 0 - 1,500ms, +/-500ms from the baseline. 10% depth ranges the delay from 450 - 550ms, only 50ms from the baseline. Note, these are hypothetical - actual values will depend on the specific device.
• Rate/Speed - The speed at which the LFO manipulates the delay time. Starting at the baseline delay, the LFO will lower the delay time to the min setting, then reverse and raise it to the max, then return to the baseline - that is considered one cycle. Rate is measured in cycles per second, or Hertz (HZ). The lower the rate, the slower the flanger's sweeping sound.
• Feedback/Resonance/Regen - Allows the delayed line to feedback, enhancing the sweeping at the expense of making the tone more artificial-sounding.
• Mix/Intensity - The proportion at which the dry (uncprocessed) signal is mixed with the delayed and feedback-adjusted signal.
• Manual/Pre-Delay - This sets the baseline for the delay. It is called manual, because if you set the LFO depth to 0%, you can actively use this control to manually sweep the frequencies.

The most important thing to be aware of is the manual control - properly setting this parameter is crucial to getting the sound you want.

Popular Models

• Boss BF-2
• Boss BF-3
• MXR EVH-117
• Electro-Harmonix PolyChorus

Chorus

Chorus effects provide a very spacious, slightly out-of-tune sound. They operate in one of two ways (or both). One way operates exactly like a flanger; however, it uses longer delay times and no feedback. Sometimes this is taken a step further by using stereo output, with both signals being delayed at different rates, creating a very spacious sound. In the case of stereo, there is no need to mix the signals back together, which can create a more natural sound.

The other way uses pitch modulation without delay to create a similar effect.

Some choruses may combine both techniques.

Popular Models

• Boss CE-1
• Boss CE-2

Rotary Speaker Simulator

Many devices attempt to simulate the Leslie Rotary Speaker enclosure, which rotated a speaker on a motor to provide a shimmery sound by cyclicly slightly pitch-shifting the audible sound via the doppler effect. While these devices commonly utilize the same techniques as phasers, flangers, chorus, or some combination of those, I want to mention them separately in case you didn't realize that. Also, more modern simulations may be more faithful to the acoustic principles of the original Leslie.

Popular Models

• Univox Uni-vibe

Tremelo

The tremelo effect alters volume via an LFO, causing the sound to shimmer in and out. Generally, the only controls on these effects relate to the LFO, altering its rate, depth, or waveform. The waveform generally ranges from a Triangle or Sine wave where the sound fades in and out, to a square wave that sounds like someone is toggling an on/off switch quickly alternating between silence and full volume.

Auto-Wah

Auto-Wah is simply wah pedal circuitry attached to an LFO, generating a constant wah-wah sweep. It is used to get wah movement that is generally faster and more rhythmically consistent than you'd want to produce manually with a traditional wah pedal.

Pre vs. Post Placement

Modulation effects do not have a typical placement like most other effects. They are often placed before the amp (pre) as well as in the effects loop (post).

Flangers, phasers, and chorus are often placed before the amp to have the swooshing frequency movement feed into the amp's distortion. This creates a somewhat similar effect as a wah-pedal, with the distortion tone changing with the sweep. It is particularly prominent on palm mutes. However, pre placement tends to dilute the overall effect compared to post placement, where the swoosh and phase-y-ness of the effects are more clear.

Tremelo, like a volume pedal, has a different effect in pre vs. post. Pre changes the level of distortion achieved more than the final volume, sounding like you are turning the guitar volume knob or amp gain knob back and forth. Post configuration has complete control over final volume (other than compression that occurs further down the signal chain such as power amp distortion or DAW/PA-based compression).

Auto-Wah is used mostly like traditional wah - at the very front of the signal chain after the guitar. Post configuration will sound out of control, as the wah heavily filters the entire frequency spectrum other than the resonant frequency, and the resonant frequency is in constant motion. It is recommended to always use auto-wah in pre config.