Modulation effects are a great way to create movement within a mix. This article will explain the different types of modulation effects available for mixing.
- Tremolo
- Vibrato
- Flanging
- Phase Shifting or Phasing
- Chorus
All of these effects are built around a
Low Frequency Oscillator more commonly referred to as just an LFO. An
LFO is an audio signal usually less than 20Hz that creates a pulsating
rhythm rather than an audible tone. These are used for manipulate
synthesizer tones, and as you will see, to create various modulation
effects. All the effects listed use Sine wave as the waveshape for the
LFO.
Tremolo is an effect where the LFO is modulating the volume of a signal. The signal attenuation amount is controlled by the depth and the rate adjusts the speed of the LFO cycles.
Vibrato is an effect where the LFO is modulating the pitch
of a signal. This is accomplished by delaying the incoming sound and
changing the delay time continually. The effect usually not mixed in
with the dry signal. The depth control adjusts the maximum delay time,
and rate controls the lfo cycle.
Flanging is created by mixing a signal with a slightly delayed copy of itself, where the length of the delay is constantly changing.
Historically this was accomplished by recording the same sound to two
tape machines, playing them back at the same time while pushing down
lightly on one of the reels, slowing down one side. The edge of a reel
of tape is called the flange, hence the name of the effect.
Today the same effect is accomplished
in a much less mechanical way. Essentially the signal is split, one part
gets delayed and a low frequency oscillator keeps the delay time
constantly changing from 1-10ms. Combining the delayed signal with the original
signal results in comb filtering, notches in the frequency spectrum
where the signal is out of phase.
A flanger will usually have depth and rate controls.
The depth adjusts how much of the delayed signal is added to the
original, and the rate controls how fast it will change.
Phase shifting or phasing
is a similar effect to flanging, but is accomplished in a much
different way. Phasers split the signal, one part goes through an
allpass filter then into an LFO then recombined with the original sound.
An allpass filter lets all frequencies through without attenuation, but
inverts the phase of various frequencies. It actually is delaying the
signal, but not all of it at the same time. This time the LFO changes which frequencies are effected.
Phase shifters have 2 main parameters,
Sweep Depth: how far the notches sweep up and down the frequency range.
Speed/Rate: how many times the notches are swept up and down per
second.
Chorus is created in nearly the same way as flanging, the main difference is that chorus uses a longer delay time,
somewhere between 20-30ms compared to flanging which is 1-10ms. It
doesn’t have the same sort of sweeping characteristic that flanging has,
instead is effects the pitch.
Again the LFO is controlling the delay time. The depth control affects
how much the total delay time changes over time. Changing the delay time
up and down results in slight pitch shifting.
You may have noticed that the majority of
effects here involve delay. It's possible to recreate most of the effects by
using a digital delay with rate and depth controls, such as the
ModDelay2 included with Pro Tools.
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