Review: Spitfire Audio BT Phobos

A leading soundware developer invents new ways to blend timbres
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BT (aka Brian Transeau) is a musician who continually pushes the boundaries of technology. Phobos is the personification of fear in classical Greek mythology and the larger of Mars’ two moons.

BT Phobos is a synthesizer plug-in that takes a unique approach to crafting genuinely original sounds and making music with them. Spitfire Audio, a leading developer of mostly orchestral soundware formatted for Native Instruments Kontakt, makes BT Phobos.

BT Phobos gives you instant access to the parameters you’ll want to edit most often.

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BT Phobos’ synthesis technique is sample playback, but with a twist that Spitfire Audio calls polyconvolution. Convolution, you may remember, is a technique that superimposes an impulse response (IR) from one sound on another sound. Polyconvolution is polyphonic convolution, which superimposes IRs from one or more samples on other samples. Every time you press a key, BT Phobos generates an impulse response from a sample called a convolver, which you can use like an effects algorithm to process sounds. The result often sounds like the original sound is vibrating at the convolver’s resonant frequencies. For example, if you route a drumbeat through a brass instrument, the beat becomes metallic.


In BT Phobos, any of the supplied samples can be used as either a sound source or a convolver, rather like an operator can be a modulator or a carrier in FM synthesis. BT Phobos divides its factory samples into two categories, BT Loops and BT Tonal. All samples are uncompressed 24-bit, 48kHz audio data in a proprietary format.

BT Loops are drum and percussion patterns that don’t transpose when you press different keys. Most are distinctive and unique, with an emphasis on intricate rhythms and beats suitable for EDM and other danceable musical genres. Because they’re unsliced audio loops, you can’t edit the patterns. I didn’t examine every loop, but I found two that weren’t perfectly spliced at the end, making their rhythm slightly off at the loop point.

BT Tonal sounds have pitch, and you can play them polyphonically. Most tonal sounds emphasize complex, engaging, synthesized textures and atmospheres. Although the pitched sounds are looped, too, the release of their envelopes lets you play them melodically.

Some of the sound-design work on these samples is just spectacular. Because BT Phobos supplies such a wealth of captivating sounds as raw material, different combinations of sources and convolvers yield a seemingly endless variety.


I’m grateful that BT Phobos’ GUI is resizable, because its default size displays text so small that it’s difficult to read. The main panel divides controls into distinct sections called units, with sound source units at the top and convolver units in the middle. At the bottom, an onscreen keyboard—so stylized I didn’t immediately recognize it as such—lets you specify pitch ranges for each source and convolver. In the center of the convolvers, the Convolution Triangle controls how the sources and convolvers interact.

Each of the four source units has identical user parameters, including a highpass filter, wet and dry level knobs, a mute button, and an ADSR envelope you adjust by dragging its breakpoints. Two parameters are labeled Random: Wave, which chooses a source at random from the sound library, and Param, which randomizes all parameters as much as 10 percent in either direction. All knobs display their precise values.

Highpass filters are necessary because many samples have low-frequency content that clashes with low frequencies in other samples. The HPF knob helps you avoid rumbling cacophony while also improving the clarity of higher-frequency material.

The three convolver units have the same controls as source units, with some exceptions. Instead of Wet and Dry knobs, convolvers have a single Output level knob. They also have a waveform display for viewing the convolver’s contour at a glance and a pitch knob for transposing in semitones or cents.

The Convolution Triangle contains four triangular pucks representing the four sound sources. Their placement determines how much of each source is sent to each convolver. Each of the triangle’s three corners represents a convolver, and a sound source’s proximity to a particular convolver determines the level of the source passing through it. For example, placing puck 1 in the upper left corner routes it fully through convolver W. Drag it to the triangle’s lowest point to crossfade to convolver Y, the upper right for convolver Z, and the center for all three convolvers equally.


Fig. 1. Using the Mappings panel, you can create and edit the behavior of the modulators you have routed.

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Another button that sound sources and convolvers have in common is Mappings (Figure 1). Click on the Mappings button to open the Mappings panel, where you can toggle between Mappings and Controls. Mappings is where you view, create, and edit tons of modulation routings. You can map well over 250 sources, including velocity, the envelope generator, and four LFOs, as well as global or channel-specific aftertouch, pitch bend, and MIDI CCs. In addition to filter, envelope, gate, and level parameters, targets (e.g., destinations) include speed, pitch shift, and send levels.

Fig. 2. The Controls panel reveals a host of additional parameters you can work with.

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The Controls panel furnishes knobs for making fine adjustments (see Figure 2). Its parameters include levels, panning, offsets, and gates, as well as highpass and lowpass filter settings. It also has knobs for adjusting envelopes that offer hold and curve parameters along with attack, decay, sustain, and release.

You can access global parameters using master controls on either side of convolver Y. These include knobs to determine BT Phobos’ overall gain, pitch, and MIDI channel, across from buttons that enable MPE (multidimensional polyphonic expression) and open panels for adjusting polyphony and LFO parameters. You can automate all parameters in your DAW, too. For creating, controlling, or customizing presets, BT Phobos gives you lots of control over lots of parameters.


BT Phobos comes with 716 presets and 2,381 samples, totaling almost 23 downloadable gigabytes. None of the sounds are multisampled, but you can define the splits and layers between them using controls that overlay the onscreen keyboard.

Clicking on the name of any source or convolver opens BT Phobos’ browser window. On its left is a list of samples, and on its right is a list of attributes. Dropdown menus let you display only tonal samples in a specific key (including clusters) or loops at a specific tempo. You can audition samples before you load them, which helps you locate sounds more quickly. If you find a sample you like but it’s not quite right, click on Find Similar to locate comparable samples.

The Presets menu categorizes most factory presets by creator and category. For example, BT’s presets are divided into Atonal, Harmonic and Melodic, Hybrid Melodic-Rhythmic, and Rhythmic categories. Christian Henson’s presets are categorized as Analogue, Convolution Synth, Drones and Pads, and Loops. Richard Divine’s presets are a simple list of 53 presets with no categorization.

BT Phobos displays the current preset’s name only in the browser menu, and that name disappears whenever you make the slightest change, which could be a problem. In Logic Pro X, it doesn’t even save the name when you close and reopen a DAW file. That leaves you guessing which preset you previously loaded.

The dynamic range of factory presets varies from subtle to aggressive. One preset choice is Default State, which doesn’t load any sound sources or convolvers. That proved extremely useful for building my own presets and for exploring and acquainting myself with BT Phobos.


BT Phobos gives you the means to combine and crossfade as many as seven sampled sounds in ways that often mask their origins and generate something new from the combination. Best of all, the variety of timbres it produces is astounding. Need phat, fresh beats? Coming right up. Ominous, otherworldly atmospheres? Piece of cake. Pads that sound like nothing else? No problem. If you’re constantly on the lookout for new and different timbres, BT Phobos may be right up your alley.

Inspiring factory content. All parameters can be automated. MPE support. Easy to create useable presets.

Preset name disappears when you make edits. Can’t edit loop patterns.


Writer, synthesist, and EM editor-at-large Geary Yelton lives in Asheville, North Carolina.