Phase Nonsense!

In sound design and especially synthesis, phase is a fundamental concept that is essential for understanding and creating unique effects. The term “phase” refers to where a sound exists in a cycle, and is a core part of any sound along with frequency, amplitude, and timbre. In this article, I’ll discuss the characteristics and uses of phase in sound design, including phase scrambling, phase modulation, phase distortion, how phase modulators and allpass filters are similar, and how to use phase for spatial audio.

Phase scrambling is a technique used in audio processing to modify the phase relationship between different frequencies in a sound. This technique involves shifting the phase of various components of a sound, resulting in a scrambled audio signal that can both create unique and complex effects, or just fix asymmetrical signals to afford more headroom (as is the typical case in Izotope RX). I particularly enjoy using phase scrambling along with waveshapers because while you might start with an asymmetrical square sound, it can quickly transform into something else because shapers work on the physical signal and not the perceived sound.

Phase is an important concept in spatial audio as well: by manipulating the phase relationship between left and right (or any other channels really but we’re just sticking to stereo/binaural for now), we can create realistic spatial audio effects that simulate a 3D audio environment based on how we hear things. If you hear a sound to your left and slightly behind you, that’s partially because the sound is slightly delayed and lowpassed thanks to your skull and physical distance between your ears. That delay can be achieved via phase shifting. This technique is used in VR gaming extensively along with filtering and reverb to place a sound in space and move it appropriately. In film, it’s used similarly albeit less often as film is not typically in a first person perspective. It is still useful for creating things like the doppler effect of cars going by during a scene though.

Getting more synth-y, phase modulation is another technique used in sound design that involves changing the phase of a waveform to produce a new sound. This technique is used in what is erroneously called frequency modulation (FM) synthesis, a la DX7. In phase modulation, one waveform is used to modulate the phase of another waveform, and can do everything from analog exponential FM-like effects (hence the name) to brostep basses and even warping of samples and oscillators by modulating phase over time with carefully crafted shapes (usually via wavetable).

Phase distortion is a much less popular technique that pretty much was born and died within Casio’s ecosystem of CZ synths. Like phase modulation, phase distortion changes the phase of one signal with another. The catch is that phase distortion is more akin to modern wavetable warping, where the modulator effectively remaps the phase of the carrier. If using a linear ramp modulator on a sine carrier, for example, you’ll get a sine wave. As that linear ramp goes more logarithmic, that sine turns into a saw. This can be used to create a variety of interesting effects when samples are the carrier and wavetables are the modulator, as you can warp the sound to your whim to create everything from raspier vocal sounds to octave effects, sync effects, filtering effects, and more.

Finally, and why I wanted to write this post in the first place, let’s talk phase modulators and allpass filters. They’re both devices that are related to phase, but intrinsically differ on the simple fact an all-pass filter is a tool used as a phase modulator. The two are often conflated terms even in sound design tools, but  from a purely design perspective we’re more focused on the creative result of this process than the strict scientific terminology.

Phase modulators are used for audio synthesis primarily. Operators in an FM matrix can typically feed back on themselves, shifting their own phase until eventually you just get noise. Also, if you modulate a carrier by a modulator enough, eventually (by like the 900 degree point, where most DX synths limit themselves) you’ll just have noise as well. This is why most FM sound design involves tuning and subtle modulation amounts, but with large, complex envelopes controlling operator pitch and amplitude. It’s easy to create highly complex movements and sounds with a very simple 2-operator synth, but I prefer at least 4 operators with a variety of waveshapes so I can perform warping along with core sound generation.

Allpass filters are used for audio equalization and time delay effects. For example, a phaser effect employs multiple allpass filters in series to create a specific effect. A phase modulator can be described as a single stage of a phaser, which is useful for analog DX-like tones, introducing microdelays, and other fun effects. When you filter a sound, the phase moves by 45 degrees per pole at the cutoff frequency, approaching 90 degrees. This is an audible change, especially if you mix it with the original signal: with two poles this would cancel the specific frequency out, creating a highpass filter assuming the 2-pole filter is a typical lowpass. An allpass filter does the same thing as a normal filter, but without actually filtering: it changes the phase at the frequency, but doesn’t change the decibels. The actual shift occurs around a set cutoff frequency, where the frequencies above said cutoff are continuously shifted (all filters have a sort of “knee” which smooths around the cutoff frequency so it’s not a harsh change right at said frequency).

What’s really cool is when you employ this, but have each stage shift at a different frequency. Then you get into Disperser territory, which is a VST plugin that employs an array of phase modulators at set frequencies with an overall structure similar to a resonant lowpass filter. It literally spreads the frequencies such that each harmonic of the input signal is actually spread out (dispersed) across a range of frequencies. It’s useful for making impacts more impacty, but can also be used to apply a variety of neat phase-related effects from phasing to multi-spectral audio rate modulation, or just emphasis on certain frequencies (hearking back to old analog synths where resonance was called emphasis).

In conclusion, phase is an important concept in sound design that is used to create unique and creative audio effects. Techniques such as phase scrambling, phase modulation, and phase distortion are used to manipulate the phase relationship between different audio signals, while phase modulators and allpass filters are used to modify the phase response of audio signals. Finally, phase is an important concept in spatial audio, which is used to create realistic three-dimensional audio environments for virtual reality, augmented reality, and gaming applications.