Bitbox Micro Firmware: The Compressor

It is on the sum of every pad routed to Out 1/2, including the delay and reverb returns. This is a mix bus compressor, not a channel strip, and it hears the whole thing at once. See where they sit for the chain, which it shares with the EQ.

Two things follow that matter here. Pads on Out 3–6 bypass it completely, which is your clean path and is useful below. And the safety limiter behind it means you can push the compressor hard without worrying that you will damage anything.

This guide is not the place to learn that, and there are far better resources than anything that would fit here. But a one-line version, so the controls below make sense:

The five controls decide how loud is loud enough to act on, how much to turn it down, how fast to react, how fast to let go, and how much to put back.

Threshold: how loud a sound has to get before the compressor pays any attention. Everything below it passes untouched. Bring it down and more of the music gets caught.

Ratio: how hard it clamps once the threshold is crossed. At 2:1, a sound going 10 dB over the threshold only comes out 5 dB over. At 10:1 it comes out 1 dB over. Low ratios shape. High ratios flatten.

Attack: how quickly it reacts. Fast attacks catch the very front of a transient and squash it. Slow attacks let the initial hit through and only then clamp down, which is what keeps drums punchy.

Release: how quickly it lets go again. Short releases pump and breathe with the track. Long releases are smoother and less obvious.

Makeup gain: how much level you put back afterwards, to compensate for what the compression took away.

You do not have to configure anything. Switch it on and you have gentle, musical bus compression, the sort that pulls a mix together rather than announcing itself. "Glue" is the usual word for it.

The starting values are deliberately close to the classic SSL bus compressor setting, which is the most-copied mix-glue recipe there is:

Three of those five are literally switch positions on an SSL: 2:1, a slow 30 ms attack, and a 300 ms release. The slow attack is the important one: it is what lets the front of every kick and snare through before the compressor clamps, and it is the difference between glue and mush.

Why −12 dB and 2:1, specifically. Threshold is the one setting that depends on how loud your material is, and a default has to work without knowing that. At 2:1, being wrong is cheap: a sound 10 dB over the threshold comes out only 5 dB over, so even if the threshold is far lower than ideal you get a gentle squeeze rather than a crushed mix. Pair that with −12 dB and you get roughly:

It does the right thing at sensible levels, stays musical if you run hot, and steps aside if you run quiet.

On input level and makeup gain. Aim to have your mix peaking somewhere around −6 dBFS before the compressor. That is good practice anyway. It is the level these defaults are built around. Makeup gain starts at 0 dB, so switching the compressor on can only ever make things slightly quieter, never louder, never clipping. Once you have set a threshold you like, bring makeup up until engaged and bypassed sound about the same loudness. That is the honest way to judge whether it is helping. Match the levels first. Then compare.

If you run much hotter than −6 dBFS, the compressor works harder. You will want more makeup. If you run cold, it will barely engage and you should bring the threshold down rather than reaching for makeup.

If your controller can send several CCs from one button press, set one up to send these six. This is the single most useful thing you can do with the compressor.

All six on MIDI channel 1, as ordinary 0–127 values. Include CC 40. Without it the button restores five settings into a compressor that may still be switched off, and you hear nothing change.

Each value is the nearest CC step to the figure beside it, which lands within a fraction of a dB, or a few milliseconds, of the target. Well under what anyone can hear. How It Arrives explains why these five.

Set the button to momentary, not toggle. A latching button holds its values down, and your encoders cannot move the parameters out from under it. Momentary sends the six once, on press, then gets out of the way. Controllers call it momentary, temporary or trigger, as against toggle or latch.

One consequence, and it is not a fault. After a press the module sits on the glue values while your encoders are still wherever you left them, so the two disagree. Turn one and the parameter jumps to meet it. If your controller offers pickup, also called soft takeover or catch, switch it on for these five. The encoder then waits until you sweep past the value the module is really on, and takes over from there.

It gives you a home to return to. Modify the settings as much as you like, and when you want the glue back, one press restores it instantly. Knowing the way back is what makes the rest worth trying.

If you want to hear what your module did before you patched it, a second button does that: threshold 115, ratio 20, attack 12, release 31, makeup 57. That is −4 dB, 4:1, 10 ms, 250 ms and −4 dB of makeup, which is a headroom trim rather than a musical setting. It is worth hearing once, to know what was changed on your behalf.

The module's screen: never shows any of this, and there is no gain reduction meter. That is true of everything this patch adds, the display does not redraw when a CC arrives. A compressor is invisible anyway. Your controller is the display. If you turn an encoder on the module itself, the module wins.

Finding the threshold: the only fiddly part. There is a technique:

The others are quicker to hear:

• Ratio: sweep it top to bottom on a busy loop. Transparent, then obviously squashed.
• Release: short, on a kick loop. It should pump hard. The most audible of the four by a wide margin.
• Attack: long, on the same loop. The click of the kick comes back.
• Makeup gain: level, and nothing else.

Nothing happens while the compressor is off. The five settings are remembered. But nothing applies them until it is running. So if you set everything up with it switched off and hear no change, that is expected: switch it on and it all arrives at once.

Makeup gain has two zeroes, and they are nowhere near each other. It runs from −36 dB to +36 dB, and it arrives at 0 dB, which is the middle of the throw. The bottom of the throw is −36 dB, which is silence. So if that encoder happens to be parked at the bottom when you first touch it, the sound vanishes and it looks like something has broken. It has not. Turn it up to halfway.

There is no sidechaining. You cannot duck the mix from a kick on another channel, or key the compressor from anything but the signal passing through it. The detector is wired to the audio it is compressing and there is no key input to redirect it, so this is a limit of the module rather than something the patch chose not to expose.

What gets you close: ordinary bus compression already ducks the mix when the kick lands, that is what glue compression does, and with a fast attack and a loud kick it pumps convincingly. The difference is that the kick is compressed too. And if you sequence CCs, you can draw a threshold envelope in time with the kick, which is automation rather than sidechaining but reaches a similar place.

There is no wet/dry mix either. The module cannot blend compressed and uncompressed signal: there is no mix control and no way to add one.

What you can do instead: pads routed to Out 3–6 skip the compressor entirely. Put the same sample on two pads with the same MIDI channel and Pad Note, route one to Out 1/2 and the other to Out 3/4, and mix them outside the box. Squash the compressed one hard and blend it underneath. That is parallel compression, using stock routing.

It costs a pad and an output pair, so most people will not bother, but it is there if you want it.

Your compressor settings survive a preset change, which nothing else in this patch does, and a power cycle puts them back to the glue values under How It Arrives. That behaviour is shared with the EQ and is set out in what they have in common.

Used well, this is the control that makes eight pads sound like one record. Everything else here shapes a sound; the compressor is what makes all of them sit together, breathe together and hit as a single thing. That is what glue means. It is why this is the addition worth learning properly.