Cong Burn Stacks

Stacks is a busy environment visually, but easy enough to understand, with no cryptic labelling. This is the VST3 version, with its resizable window, and with three looper tracks loaded.

If you want proof that original independent music software is still flourishing, you need look no further than Stacks.

Stacks is a new virtual instrument for macOS, Windows and iPadOS by Cong Burn, the software development arm of British electronic artist and programmer John Howes. It follows on from Strokes, which I reviewed in the June 2024 issue of SOS, and shares a similar left‑field, minimalist, modular‑inspired character, and is similarly affordable.

If forced to summarise what Stacks actually is, you’d have to say it’s a four‑track looper with an onboard synth, audio input, step sequencers, modulation sources and effects. Particularly in its standalone app form, on the iPad, it has sufficient scope to create exploratory long‑form pieces all by itself, and perhaps even a longer ‘set’: sequencers and modulators drive the synth, the output of which (or another incoming live signal) can be captured and layered in the loopers.

The same and more is possible with Stacks running in a DAW or other host, in plug‑in form (AUv3 for iPadOS, AU for Logic, and VST3 for everyone else on macOS or Windows). However, Stacks is open‑ended, and I think it’s likely some users will have it for its multitrack looping alone. It’s also not strongly deterministic, in the way a linear timeline‑based DAW is, but more of a platform for experimentation and for generating that old friend, the happy accident.

Stacked

A visual scan of Stacks’ main interface is potentially a bit overwhelming at first — it’s a very ‘busy’ design — but the main sections are easy enough to spot. Two step sequencers (one primarily for notes, the other for modulation) towards top left. Synth and effects top right. Various modulation sources plus mode/view buttons (and, on the standalone app, transport controls) at bottom left. And then at bottom right the four parallel loopers. However, this is only the ‘Main’ layout, and others reconfigure the interface, opening up new panels or increasing the size of existing sections. For example, the Loopers mode increases the size of all four looper tracks and hides the sequencers and the synth almost entirely. The way elements interrelate remains very clear at all times though.

While the default Main window layout gives an overview of all four looper tracks, an alternative view opens up all the parameters for pairs of tracks.

Such is the depth of functionality in Stacks I dare not try to describe all its features. But I’ll focus in on a few to hopefully give a flavour of the what, how and why.

Let’s take the uppermost step sequencer, Seq A, for starters. Its main role is to send notes to the onboard synth (and, for good measure, across virtual MIDI too). You specify note pitch by dragging faders, with the option to generate single notes or many types of chords.

An eight‑step design might look rather limiting, but these are actually eight ‘stages’, individually adjustable in length and with varying numbers of notes triggering (like a ratchet) within that length. Ensuing rhythmic subdivisions can range from downright predictable to exotically discombobulating. There’s even a way for the note triggers within a stage to be skewed, for a ‘bouncing ball’ effect. Equally, you can dial in rests, or skip stages completely. An overall Steps parameter sets the ultimate musical length of the loops you create here. It doesn’t alter the number of stages but forces a reset to stage 1 every time the whole cumulative caboosh of note triggers and durations plays out, helping to maintain useful musical structures — or not... Finally, Seq A can run forward, backwards, in palindrome fashion, or completely randomly.

Notes from Seq A feed into a pitch quantiser section to the right. This constrains raw note data to any musical key and one of many preset scales (or indeed to a live MIDI input), all of which means you needn’t be deadly accurate with note programming in the sequencer. The quantiser will also alter chord inversions and voicings, apply pitch offsets, allow for control and modulation of velocity and note length, stereo spread, and much more.

Stabs

Stacks’ synth is essentially a subtractive design, but has many unconventional tricks up its sleeve. The Oscillator section gives you 26 wavetables to choose from, and within them there’s proper smooth interpolation of waveforms via two control knobs. FM, AM and phase modulation are available, as well as a noise source and sub‑oscillator. It’s a potent harmonic palette, unashamedly digital‑leaning.

The filter section offers a HPF/LPF combo with either a notch or digital degrader stage, and drive. A ‘rubberband’ option links the cutoffs, for a sweepable band‑pass response, or you can switch the whole thing to become a six‑band filter bank, with control over the balance and spread of centre frequencies.

The final Amp stage is also unusual. It defaults to a Buchla‑esque Low Pass Gate, which is to say it’s an amp and filter combined, with level and resonance called Ping and Bite. With only a decay envelope it works best for percussive sounds. A Clean mode is more familiar, and offers a fuller ASD envelope. Tucked away here is also a compressor and a really nice‑sounding chorus with three different depths.

Loops

For getting non‑synth audio into Stacks’ looper tracks there’s an Audio In option in the Oscillator section.I expect for some users the loopers will be the main draw of Stacks. There are four of them, capable of running in musically coherent sync’ed fashion, or as independent entities.

It’s possible to load each track with an audio file: mono or stereo WAV, AIF and OGG worked for me, not FLAC or MP3. They’ll also record and overdub material in real time, either from Stacks’ internal synth or live‑input oscillator, or the output mix of the loopers themselves, allowing resampling and Portastudio‑style three‑into‑one bouncing. By default, loop recording and playback is quantised to a four‑beat bar but that can be changed to longer or shorter time divisions, or indeed quantising turned off altogether.

Using default settings you can, if you wish, use these loopers in a traditional, orthodox way. A MIDI configuration tab, in fact, is completely given over to setting up MIDI control for conventional record, stop, play and overdub commands, from a foot controller perhaps.

A whole other deeper, weirder character lies just underneath the surface, though. Each looper can work in one of three modes. The first, Reel, recreates both the sound and varispeed characteristics of magnetic tape. Grain and Slice can essentially resample material in the loop buffer, and are capable of scanning through it forwards or backwards at different rates, freezing it in time, and transposing it. These modes are similar, but where Slice keeps generated material sync’ed to clock, Grain is more nebulous. Neither, incidentally, performs any sort of transient detection or formal ‘beat‑slicing’, but with some material it can sound like they do.

All modes offer a lot of creative playback parameters, alongside basic level, pan and sends to the effects. So much so that it’s easy to end up with audio looping around that bears little or no resemblance to what seeded it in the first place. Essentially the loop tracks become tone generators — granular oscillators. What’s more, there’s a way for their pitch parameters to follow the current sequencer pitch quantising, so they can be heavily modulated but retain musical relevance. Stacks allows for a drag‑and‑drop from a looper track to a DAW audio track, to ‘print’ its sound, and intriguingly, usefully, the resulting audio file includes any results of modulation. So you can use a looper as a laboratory, and capture your work as you go. There’s not the grain density of a dedicated granular synth, but some pretty remarkable sounds can be produced.

Stacking Up

You know you’re in for a mind‑expanding experience with a Cong Burn product, and Stacks is no different. It’ll take time — hours, perhaps a few days — to absorb all the possibilities on offer and find fluency, and let’s just say that Stacks might not be ideal for a user taking first tremulous steps beyond GarageBand, for example... But if you’re an experimental type who relishes getting stuck into a Eurorack case full of funky modules, or an out‑there Max for Live patch, this is going to be right up your street.

For what is such a unique instrument, clearly a labour of love by a brilliant, free‑thinking developer, I hardly feel I should criticise anything in Stacks. But there are a few things... Even on an 12.9‑inch iPad Pro the user interface is so crammed that it’s sometimes difficult to operate. That’s especially true for knobs and controls right near the edge of the screen, which can end up triggering system gestures if you’re not careful. Testing the VST3 (in Studio One 7 on macOS) some pop‑up menus took a few clicks to open (though I would fully expect glitches like this to get ironed out). Amongst the multitude of oscillator waveforms I could see no way of generating simple narrow pulse waves, and hence any kind of PWM tones. The reverb control scheme is a bit odd currently: workable, but just not very intuitive. I also wish the loopers’ Reel mode Pitch control wasn’t quantised to semitones, to allow for really gloopy pitch‑bends. Finally, I’d love to see a looper track parameter reset, so it’s possible to return to normal playback after localised, temporary destruction, as it were. The current Recall button does something else.

Stacks is unusual and idiosyncratic, but it’s also brilliant and unique, inspired and inspiring in equal measure.

These are all rather trifling concerns, however, and in every other way there is absolutely masses to enjoy in Stacks, whether you’ll use all it has to offer or only bits. Interesting things seem to flow out of it whether you’re approaching it as a completely self‑contained creative ecosystem or just as a scratchpad to generate or deconstruct material to be assembled elsewhere. The loopers alone, with their ability to support both utterly mainstream and many radically creative approaches, are worth the price of admission. Yes, Stacks is unusual and idiosyncratic, but it’s also brilliant and unique, inspired and inspiring in equal measure.