Modulation Effects – A quick look at design principles
Modulation effects are a pretty wide category of devices and algorithms that alter sound signals in somewhat different ways – yet the all have a little bit in common. They all add life and vibrance and fascination to the sound of a musical instrument. An interesting aside is that, while guitars and many other instruments can benefit from modulation effects, vocals can be ruined by modulation.
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So what effects fit into the category known as modulation? This category includes chorus, flanging, phasing, vibrato and tremolo. Tremolo can take the form of either a filter-based wah-wah or a volume tremolo. Then there are some hybrid effects out there such as the Leslie speaker effect. Let’s have a look now at each of these and how these effects are created.
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Basic Concepts
Low Frequency Oscillator
One concept running throughout the subject of modulation is that of the low frequency oscillator (LFO). In analog electronics this is a circuit which generates a constantly changing voltage. The voltage of the signal rises and falls in a regular, geometric pattern. The signal is then sent to another circuit as a control signal to alter the behaviour of that circuit in some way. In a digital circuit, a chip generates a sequence of numbers, the value of which rises and falls. You will see as you read on, what parameters are altered and to what effect. The regular rising and falling of the voltage or numeric value is what we refer to as modulation. The user typically can select the rate and depth of modulation and sometimes the geometric pattern, or wave shape, as well.
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Delay
The manner in which a sonic signal is changed in a modulation effect nearly always involves some use of delay. With the exception of tremolo, that applies even if the effect doesn’t sound anything like there’s an echo or resonance of any kind. How the delay is achieved depends on whether you are using an analog or digital solution. In a digital circuit this is very simple. The input signal, which after digitisation is simply a series of numbers called samples, is mixed, sample by sample, with samples that have been played a certain time previously to the present. To achieve this, the prior samples must still be stored in memory. Mixing is done by averaging the values of the present and past samples, with a divisor applied to the prior sample value so that it (the wet signal) is quieter than the dry signal. It’s possible to have just one repeat like that, by reading prior sample values from the dry signal, but a naturally repeating and decaying echo is achieved by reading from the wet signal instead, technically creating an infinite loop. The loop audibly decays until nothing is left as, thanks to the divisor, the regenerating sample values diminish until they contribute nothing to the sound. Digital electronics achieves this directly through software or firmware programming.
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To do this in an analog circuit, a delay is created with a special type of capacitor. While capacitors normally are used in audio electronics for filtering or equalisation, this special type only does a tiny bit of filtering, but have the special ability to delay the signal. These capacitors are built into bucket brigade delay chips and are arranged in series, each increasing the delay. It is questionable whether this really is analog as a chip is involved, but this is the manner in which it is done, except that these chips contain capacitors instead of just logic gates. Bucket brigade delay (BBD) chips are ubiquitous throughout all analog circuitry involving a delay. It is the capacitors which give an analog echo effect its slight mellowing attribute as the repetitions continue. The one other essential element is a feedback loop. After the BBD comes a signal split between the audio output and a circuit branch returning the delayed signal to a mixer which mixes the delayed signal with the input signal and adjusts the volume down.
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Tremolo
The simplest tremolo effect is volume tremolo. A tremolo pedal which does this has an amplifier built in, the output level of which is controlled by an LFO. A filter tremolo is comparable in sound to a wah-wah effect, except that the filter cut-off frequency is controlled not by an expression pedal, but by an LFO for constant modulation.
Vibrato
The vibrato effect is another, like the above, that has a very long history as effects go. While this can be achieved in synthesizers and organs by modulating the pitch of sonic frequency oscillators where the sound signal originates, as an effect the simplest solution is rather more sophisticated. It’s best if we approach this from the digital angle first. If the sound has been digitised with an analog to digital converter, then it can be slowed and sped up either by altering the playback rate sample by sample, or by resampling while maintaining the same playback rate. Resampling is a complicated mathematical procedure beyond the scope of this article, but it is something used normally to reduce audio file sizes, or to allow higher quality recording or mixing. Either way, at a low trough in the LFO wave, there are fewer samples per second, making a lower pitched version of the input signal, and at a high peak in the LFO there are more samples per second, making a higher pitched output.
As an analog effect, vibrato is even more complicated. As described above, a bucket brigade delay chip captures the sound signal. The length of the delay is then altered so that through a doppler-like effect, the pitch is altered. Increasing capacitance results in a slower delay and decreasing capacitance reduces the delay. Changing the delay time changes the playback rate, so that the pitch is altered. If you fiddle with the echo rate of an ordinary echo pedal you will observe changes in the pitch of the wet signal. There is a certain number of wave cycles between two regenerations of a sound, and lengthening the period between the regenerations lengthens the period of each wave cycle. What you notice also, is that the change in pitch only occurs while the delay rate is still changing, and the pitch returns to normal when the rate ceases to change. Therefore the trick is to maintain a constant rate of change in delay and then reverse the direction of change.
Chorus
The next conceptual step after vibrato is to mix the pitch-modulated signal with the dry signal. This creates a very pretty and lively-sounding interaction between the signals which is used frequently in popular music. Not only guitar, but keyboard as well, can sound a lot nicer with a chorus effect. As also with a simpler effect known as detune, the interaction of two closely pitched musical sounds is very attractive. By using vibrato instead of a simple detune, any impression of being off-pitch is eliminated and the enlivening effect of vibrato is added.
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The delay used in chorus is normally just a single tap. That’s all you need to introduce a second modulating signal. To avoid any annoying secondary audible repletion, the delay has to be very short, so much as to create a secondary, but fortunately unnoticeable, phasing effect. If it was a decaying echo, this would create high pitched flanger-style resonance which would be an undesirable distraction in a chorus.
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However, the subtle nuances and naturalness of the original sound may be required in a recording or performance. In such a case chorus should be minimal or absent. In some genres of music or in some production styles, chorus is surprisingly destructive and can sound plasticky or confected. Attention to good taste is not always obvious with chorus, but it’s always needed.
Flanger
It is tempting to describe a flanger as being similar to a chorus, but that is deceptive. It is principally deceptive because a flanger always has a decaying echo and the delay time is long enough to produce resonance at a bass to midrange audio frequency. Repetitive echo taps create a metallic ringing sound because it mimics the behaviour of metal objects when struck. Thus a metallic tone is one of the distinguishing hallmarks of the flanger sound. The other is the resonance which can be dialled up from nothing to dominate the sound through reducing the divisor used in dampening echo repetitions, down almost to 1.0. The pitch of the resonance depends on the rate of repletion; so too does the tone of the wet signal even without audible resonance.
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A flanger uses an LFO to sweep the delay rate back and forth through a sonic frequency band set by the user and so to alter the pitch of the echoing wet signal. This has a corollary effect of sweeping through a continuum of metallic tones. Higher pitches sound small, airless and closed-in, while lower pitches sound expansive and even watery. At the low end of the4 spectrum and echoey effect can be discerned.
Another attribute of a standard flanger is a background hissing sound. The hiss is a nuisance, but some flanger effects come with built in noise reduction such as a pre-filter raising the treble and a post-filter reducing the treble commensurately. You can also use a noise reduction pedal after the flanger. Make sure there is no compression or overdrive after a flanger without noise reduction or the problem will get out of control. The hiss comes from the basic nature of the effect, even if you are producing it digitally.
Phaser
Phasing can be achieved by taking two identical signals (that is, by splitting the input) and delaying one by a fraction of a millisecond. Mixing the two again into one output results in the soundwave being averaged out moment by moment into something with a very different wave shape and frequency spectrum. If you can imagine two sine waves at close phases (let’s say just one degree apart) they combine to create a wave with large peaks and troughs, but with an odd overall shape. A wave with an altered shape contains harmonics or partials that were not part of the original. As you change the delay, or as you change the frequency of the input with a constant delay, you get different alterations in the harmonic content. In a more complex sound, there are many frequencies occurring simultaneously, which are affected differently by the one phase-shifted signal. Graphing these frequency responses gives a frequency response graph with many peaks and troughs, like a comb. For this reason we also refer to a phaser also as a comb filter.
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In practice, however, in a classic type of phaser, there is no time delay a phaser effect uses all-pass filters which alter the phase of given frequency bands rather than delaying the wet signal. The amount of phase shift is controlled by an LFO. Using a digital system, though, another possibility emerges with the use of time stretching without pitch alteration. If the wet signal is stretched or contracted in time duration a constantly changing delay is created and this results in a seamless modulation in comb filtering. There is no need for direct phase manipulation. The result is acoustically crystal clear, almost like a chorus effect. Repeated stretching and contraction in time results in complete cyclic modulation.
Leslie Speaker effect
The Leslie speaker is a product from way back specially designed for use with electric organs. Having used one in a studio, I can attest they are very heavy to lug around. They consist of a large woofer speaker and a cone tweeter. The tweeter is the special part, rotating during operation. As the tweeter spins it creates a doppler effect, so that the treble, not the bass, alters in pitch and volume and pans from left to right. The treble pitch is rising as it grows louder and moves to the right. The pitch then falls as it gets to the middle and becomes loudest. Moving to the left, the treble pitch is dropping and the volume fading. It is a pleasing, lively stereo effect highly sought after by enthusiasts of rock organ, and useful for guitar. What is really nice about it is a footswitch that can be used to engage and disengage the rotation of the tweeter, which gains and loses speed very slowly. On paper that might seem like a non-event of a feature, but it is sheer brilliance to hear it in operation. A fine example you might be able to bring to mind is the keyboard part of Pink Floyd’s Another Brick in the Wall Part 2. This is cool stuff. Rather than explaining here how a guitar pedal can emulate this effect, just use some common sense. The only thing to note is that the wet signal is all treble, so a filter must be used at the signal split stage.