Several times over the past few months we've briefly mentioned the idea of modulation as an effect parameter. Now it's time to take a closer look. Modulation is often applied to delay for creating a wide variety of time-based effects, such as flange and chorus. The word "modulate" means to change, and in this case, we are referring to a change in delay time.
It might help to think of modulation in terms of analog tape (Sherman, set the Wayback Machine for 1972.) Let's say that we are playing a voice recording on a reel-to-reel tape machine, and the tape is moving at 15 IPS (Inches Per Second). Let's also suppose that this tape machine has a pitch control. When the pitch control is "Off," or at "0," the tape moves at precisely 15 IPS (or so the manufacturer's specs tell us). When we adjust the pitch control, the tape speed changes and we hear a variation in the pitch of the recording. If we simply move the pitch control to a positive value, pitch increases. If you sneakily did this to an unsuspecting listener when they walked out of the room, they might not notice the difference when they came back in (depending upon the acuity of the listener's ear, and the severity of the pitch change). That's because the rate of the pitch change is too slow to notice. If the listener sat there while you rigorously moved the pitch control back and forth, they would hear the pitch wobble, or "modulate."
Rate and Depth
In our tape machine analogy the number of times you move the pitch knob every second is the modulation rate, while the severity of the change is modulation depth. Both rate and depth are required to modulation; rate without depth (or vice-versa) generally does not create an audible change. Thus we find many digital delay algorithms include depth and rate parameters.
Depth: Depth controls the amount that the delay time is changed. Let's say that our delay is set to 100 mS (milliseconds). If we use a small amount of Depth (1 percent, for example), our delay time might vary between 99 mS and 101 mS. If we use a high depth value, our delay time can vary more widely, say from 85 mS to 115 mS. Keep in mind that if we don't add rate, the change in delay time never happens, so we don't hear any difference.
Rate: Rate is how fast the delay time is varied. If our delay time varies between 99 mS and 101 mS, the rate is the number of times per second this variation happens and is usually expressed in Hertz (cycles per second).
Revisiting the tape machine analogy: if you were to move the pitch control back and forth, how far you moved the knob would set the modulation depth while how fast you moved the knob would set the modulation rate. Subtle amounts of modulation sound best when combined with the original "dry" signal, so pay attention to the mix control. Also the delay feedback can have a dramatic impact on modulation.
The cool thing about modulation is that it adds motion to an effect. Take, for example, Avid's standard Medium Delay plug-in (Fig. 1). Delay time is set to 5 milliseconds, while rate, depth and feedback are set to 0. If we set the mix to 49 percent (almost equal wet-to-dry signals) you'll hear a weird, hollow timbre to the sound, almost as if you were listening through the end of a vacuum hose. This is boring.
Look at Fig. 2, showing the same plug-in with depth at 55 percent and rate at 3.02 Hz. This means that the original delay time (5.01 milliseconds) has a 55 percent depth of modulation – meaning that the delay time will vary from about 2 to 7 milliSeconds, at a rate of 3 times per second. Congratulations, you have now achieved ‘wobble.' Depth is the severity of the wobble; rate is the speed of the wobble. Now that we have wobble, this is starting to sound like a flange, and we can have some fun.
The Big Three
There are at least three very useful, popular effects based on delay plus modulation. The basic differences between them are the delay time and amount of modulation. They can be created with most simple delays whether software- or hardware-based.
1. Flange
If you don't know what a Flange sounds like check out an old Jimi Hendrix record or listen to the guitar in the intro of "Breakout" from Foo Fighters. Think "this sounds like an airplane taking off." Flange can be created with delays from about 1 to 10 milliseconds (YMMV). Set the depth around 50 to 60 percent and keep the rate slow, under 0.5 Hz. To really get the ‘whoosh' of the plane taking off, you need to set the feedback high, around 50 to 75 percent. Once you get into delay times from roughly 10 to 20 milliseconds you are in Chorus territory.
2. Chorus
The Chorus effect gives a shimmery, subtle thickening or possibly a tremolo type effect, depending upon the parameter settings (check out the guitar in the intro to "Paradise City" from Guns ‘N Roses). For a traditional chorus try a 13 mS delay, 15 percent depth, 0.75Hz rate and a feedback of 10 percent. This is my favorite for thickening up backing vocals, and clean electric or acoustic guitar. To make the chorus out-of-tune set the depth around 60 percent. Tremolo effects can be attained with delay times in the vicinity of 17 mS, depth from 10 to 30 percent, rate around 1.5 to 2 Hz and feedback in the 50 percent area.
3. Doubling and Fake Stereo
Delay times from 30 to 70 mS get you into the territory of doubling, and you can do some interesting things, such as pan the original sound hard left and pan the effect hard right to add some width to a sound. (This works great with synth pads where you are looking to add size and a sense of ambiance). Adding a lot of feedback will produce a bounce on the tail of the effect (akin to a spring reverb) and make it sound more out-of-tune. Once you get into delay times beyond approximately 70 to 80 mS, you'll start to hear a slap echo.
Modulating Longer Delays
{mosimage}No one ever said you can't add modulation to longer delays, a technique that produces interesting results. Take a look at Fig. 3 showing a delay time of 233 mS with a lot of modulation and feedback. Using these settings, each echo is slightly pitch-shifted relative to the original so that, as the sound repeats, the pitch changes. This adds a lot of dimension to the sound, but be careful of letting it go too far out of tune.
Those of you who prefer hardware may have tried these tricks with the Lexicon PCM41 or PCM42 – wonderful delay units. If you're on a tight budget, you can get similar results out of the DigiTech RDS series of digital delays – very underrated units with wonderful modulation. I sold my RDS1900, and wish I never had!
