Allpass Filters

Allpass filters are weird. They don’t actually filter anything, but they do shift phase around. There’s also two types: allpass filters, that pass all frequencies but shift phase at the cutoff point like a normal filter, and pure phase modulators, which shift the phase of the entire spectrum. Both are useful in sound design for a variety of things, but I will mostly be speaking of phase modulators. Feel free to apply any of these techniques to any form of allpass filter – you’ll get slightly different effects for all of them!

Apply FM synthesis to any sound
Despite the name, FM synthesis does not modulate frequencies, but phase. It was named FM because at the time it was common for synthesizers to modulate frequency, and FMing two sines together sounds a lot like PMing two sines together – but it falls apart once you use other waveforms. As such, using a phase modulator will allow you to use one signal to modulate the phase of another. You can modulate a wavetable with a resonator, or noise with a saw wave, or literally any combination of two signals! And of course, those two signals can be summations of other signals, and you can set up matrices to get complex FM algorithms just like FM synthesizers – but way more complex and unique-sounding.

A second point of this is you can use phase modulation to create warping effects by modulating a signal with a precise and synchronised waveform. A rectified sine will cause the waveform to clump up on itself. A saw will create sync-like effects. A continuously variable inverting saw will create PWM-like effects. Very fun tools for sure.

Create Doppler effects
When creating the sound of a moving object, one must not only think of the horizontal axis (panning) but also height (filtering) and distance from the listener (doppler). That last one is caused by continuous change in phase over time: if you were to modulate panning with a simple linear ramp and a phase modulator with a synchronised triangle waveform, you would successfully recreate the doppler effect as heard in the real world. As an object approaches, it moves closer to your auditory center, and its sound gets compressed as it moves toward you, resulting in a seeming rise in pitch. as it moves away from you, it will sound as if it is lowering in pitch. This is the doppler effect.

Nudge sound slightly further ahead
Important in music and visuals as much as sound design, the ability to ‘nudge’ sound forward slightly can help greatly in giving multiple layers room in a mix. It can also be used at extreme lengths, usually in the form of a delay effect rather than phase shifter, to delay a sound perceptively, which can be useful for echo effects or repeating the same sound in a different place without retriggering it – especially useful if using a monophonic sampler or synthesizer to create multiple similar sounds in the same time span.

Create Haas-like effects
Usually performed with a short (10-20ms) delay, you can use a phase modulator to shift phase forward slightly on one channel of a stereo sound to add width. This is especially useful for first-person shots where you have two sound sources moving slightly on either side – just synchronise the phase modulator with the movement on-screen and you’re set!

Make a phaser
Using multiple phase modulators in series using the same modulation source (usually a sine or triangle LFO) and mixed with the original signal, you can create a typical phaser effect. However, a similar setup with a static modulation source can give you very unique comb-like filters, which I use regularly for processing noise into more natural sounds.

Make a comb filter (with feedback)
Speaking of comb filters, a phaser is essentially just a modulated comb filter with multiple phase modulation stages. A pure comb filter is very simply the original signal mixed with a phase-shifted version of itself, often with feedback. This creates a series of peaks and troughs in the spectrum due to phase differences colliding and cancelling/amplifying each other. With high feedback you can create karplus-strong-like resonating sounds.

Comb filters are also useful in spatialisation. Given different frequencies travel different distances through different mediums, different frequencies arrive at a given point at different times, creating phase clashes and other sounds that a light comb filter easily replicates (especially when used in conjunction with shallow LP/HP filters).

Build a reverb
Reverbs are actually very simple devices, incorporating a series of feedback/feedforward phase shifters or delays. Tom Erbe has an amazing writeup on how to further expand this concept: http://tre.ucsd.edu/wordpress/wp-content/uploads/2018/10/reverbtopo.pdf

I’ve built several reverbs using the techniques outlined in that document using Bitwig’s grid, my favourite of which being a reverb with a global feedback loop, inside which a simple frequency shifter is inserted. When modulated, it creates absolutely astounding sounds from the most mundane of sources.

Build your own Disperser
Finally, the effect that helps create hard-hitting kicks and gnarly basses, Disperser, is nothing more than a series of phase modulators with an integrated phase shift curve along the spectrum. Any sound that shifts in frequency, such as a swept noise swash, bass-heavy impact, or even a simple decay-modulated sine wave, can be further enhanced by a Disperser effect, which will accentuate the phase shift that occurs as the sound moves through the phase-shifted area of the spectrum.

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As you can see, allpass filters have many, many uses in a sound designer’s arsenal. The above are only a few common examples – I recommend going out there and exploring what phase modulators can add to your work!