Classic Sounds: Phase Shifting

Welcome to the first installment of our Classic Sounds series. In this edition we are going to touch upon the history of products that first implemented the effect known as phase shifting and how you can achieve similar sounds with the Digital Filter.

Technology

Traditional phase shifting is accomplished by passing an input signal through a set of All-Pass filters in series and then summing the output with the original input. All-Pass filters do not attenuate the input but like Low- and High- Pass filters, they shift the phase of specific frequencies. Wherever the output is inverted or 180 degrees out of phase with the input signal, a notch will occur in the frequency spectrum. Modulating the corner frequency of these notches creates the effect we know as phase shifting. With two All-Pass filter stages in series, we expect one notch to be created at the corner frequency. As we increase the number of stages by 2*n, we can expect to add n more notches.

Fg. 1: All-Pass Filter Phase Response
Fg. 1: All-Pass Filter Phase Response
1968: Breaking New Ground

The first popular effect to make use of phase shifting was the UniVibe (Shin-ei). Released in 1968, this circuit design is the most unique of the products we considered. This effect has four filter stages but is not typical in that each stage is tuned differently. Furthermore, the signal used to modulate the filters is not a standard waveform. This was due to the use of light-dependent resistors (LDRs) and the associated limitations with turning the light source on and off.

Though three years later, Maestro is credited with releasing the first official phase shifter, the PS-1 (1971). This effect has six stages and uses field effect transistors for variable resistance to modulate the filters. Musitronics (Mu-Tron) was inspired by this design and created the Phasor (1972), built with Operational Transconductance Amplifiers (OTAs) which eliminate the need for FETs as they can be directly modulated via bias input.

1974: Year of the Phaser

The next most popular effect is the Phase 90 (MXR). Released in 1974 with just one knob, a product for musicians couldn’t get any simpler. This effect has four stages and later introduced an internal feedback resistor which added a mild distortion and increased filter resonance. MXR also released the Phase 45 (2-stage) and the Phase 100 (10-stage). The Phase 100 had a few additional features including applying modulation to a subset of the filter stages.

In the same year, Mu-Tron released the Bi-Phase (1974), a configurable series/parallel combination of two six-stage filter banks. Each modulation source is selectable between sine and square waveforms and can be synchronized.

Also in that year Electro-Harmonix released the Small Stone (1974). This effect has four stages and was technically the first to introduce feedback, controllable by a switch. Electro-Harmonix then released the Bad Stone (1975), revised from an earlier design. This effect has six stages, a dedicated feedback control, and a manual-mode with dedicated control of corner frequency.

1979-Present: Coming Up To Date

Later, A/DA released the Final Phase (1979), with six stages and steeper notches for a more pronounced effect. The Final Phase had an additional feature that added a second low(er) frequency oscillator to create more complex modulation schemes similar in concept to the Bi-Phase.

Fast forward to modern times and the release of the Moogerfooger MF-103 (1999), a selectable 6/12-stage filter, and the Boss PH-3 (2000) a selectable 4/8/10/12-stage filter with additional features including rise/fall and step modulation, and that sums up the history of phase shifting as we know it.

Not so fast, with the growing interest in Eurorack Modular gear, it’s worth mentioning Pittsburgh Modular and the release of their Phase Shifter (2014). This effect has sixteen stages and brings the experimental nature of modular synthesis along with it, providing custom ins and outs, switchable changes in configuration, and multiple taps.

Fig 2: Vital Statistics
Fig 2: Vital Statistics
Digital Filter

Though the Digital Filter is not a phase shifter by design, we were able to overcome these limitations by combining the output of both HP and LP filters. There was enough difference in phase to generate notches or cancellations at the appropriate locations in the frequency spectrum when summed with the original signal.

We built up a set of patches based on classic designs, including the unmistakable sound of the UniVibe, the dual configuration of the Bi-Phase, and the simple but effective Phase 90. The intent is that this is a starting point for experimentation. We hope you find something useful and inspiring.

Download the Phase Shifter Refill

Sources

• Hughes, Tom. Analog Man’s Guide To Vintage Effects. For Musicians Only Publishing, 2004. Print.
• Thompson, Art. Stompbox: A History Of Guitar Fuzzes, Flangers, Phasers, Echoes & Wah. Miller Freeman Books, 1997. Print.
The BOSS Book: The Ultimate Guide to the World’s Most Popular Compact Effects for Guitar. Hal Leonard Corporation, 2001. Print.
http://www.geofex.com/article_folders/univibe/univtech.htm
http://www.pedalarea.com/small_stone.htm