How is Spira different from other filter effect pedals?

Spira started as a simple idea: Create a Shepard Tone illusion using an array of resonant bandpass filters. The Shepard Tone is a type of overlapping glissando, requiring many oscillators (or in our case resonant bandpass filters), that are spaced an octave apart, and sweep through the audio range at a constant rate per octave of sweep.

See this example spectrum image from Wikipedia:

In order for Spira to work correctly, the ramp oscillators that sweep each filter sweep at a constant rate per octave, not linearly. And they maintain an octave spacing throughout.

Each filter completes its sweep before starting at the beginning of the ramp again, and we have taken care to ensure the filters sweep the entire audio range. The amplitude tapers off at the beginning and end of the sweep to provide a smoother glissando effect, and to prevent audible discontinuity when each ramp resets.

In the early design phase we quickly realized that there needed to be at least enough filters to account for each octave in the audio range. We also wanted to implement the Diana Deutsch innovation where the filters are a tritone apart, requiring twice as many filters. We finally settled on a filter bank of 24 filters.

These design requirements, and the intent to produce the Shepard glissando effect, are what sets Spira apart from the many other filter effect pedals. However, Spira also offers many other features that make a versatile and capable filter effect.

Filter Slope

The Spira bandpass filters are State Variable Filters, which in bandpass mode naturally have a 6 dB rolloff. (SVF filters in high pass or low pass mode have a 12 dB rolloff.) We stack these, with up to three stages per filter to provide 12 dB and 18 dB rolloff.

Resonance

The resonance of the SVF bandpass filters can be adjusted from flat to near oscillation. This is not unique in itself, but it does greatly increase the utility of the effect. The Shepard effect relies on hearing the frequency center of each filter, and it is more pronounced when the filter resonance is higher.

Rate

A lot of review and testing went into what the range of the rate control should be. The classic Shepard illusion is best heard with a slower sweep - about 9 or 10 o’clock on the Spira. However, the tremolo range from 12 o’clock forward sounds great and is very musical. Fully counterclockwise, you can freeze the sweep and the effect can be compared to anything from comb filter to cocked wah, depending on the other settings. We did testing with higher rates that went all the way into ring mod territory, but ultimately felt those were less useful without other controls also geared toward ring moduation.

Blend

All bandpass filter effects will remove your high end and low end to some extent. The blend control lets you bring as much of this back in as you need to, to get the right amount of transparency for your situation.

Spira offers a number of ways to control the subtlety and transparency of the effect: the gentler 6 dB slope, lower resonance, and higher filter density (tritone spacing) all contribute to a more even frequency response throughout the audio spectrum. This is hard to accomplish with other filter pedal designs that use bandpass filters because the number and spacing of the (up to 24) filters across the entire audio band is part of how Spira gets this transparency while still presenting a distinctive and audible effect.

The result of this is that Spira offers some unique capabilities even when being used in other ways than its original intent. It turns out to be a very musical sounding filter modulation without needing to know anything about Shepard Tones or how it works under the covers.

For those who haven’t heard it yet, you can check out a demo here.