Bitbox Micro Firmware: The Compressor

It acts on the whole Out 1/2 mix, delay and reverb returns included, after the Character chain and the EQ. See where they sit. Pads on Out 3–6 skip it, and that comes in useful below.

Here it is in one line:

• Threshold: how loud a sound has to get before the compressor acts. Bring it down and more of the music gets caught.
• Ratio: how hard it clamps above the threshold. At 2:1, a sound 10 dB over 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 squash the front of a hit. Slow attacks let it through first, which keeps drums punchy.
• Release: how quickly it lets go. Short releases pump and breathe. Long releases are smoother.
• Makeup gain: how much level you put back afterwards.

Switch it on and you get glue: gentle bus compression that pulls a mix together. The starting values are close to the classic SSL bus compressor setting.

The slow 30 ms attack matters most. It lets the front of every kick and snare through before the compressor clamps. That is the difference between glue and mush.

How much it does depends on how loud your mix is:

Aim for peaks around −6 dBFS going in. Makeup starts at 0 dB, so switching on can only make things slightly quieter, never louder. Once you like the threshold, bring makeup up until the compressor sounds equally loud on and bypassed. Match the levels first. Then compare.

If your mix runs much hotter, the compressor works harder and you will want more makeup. If it runs cold, bring the threshold down rather than reaching for makeup.

If your controller can send several CCs from one button, set one up to send these six. Then experiment as much as you like. One press brings the glue back.

Send all six on MIDI channel 1. Include CC 40. Without it, a press can restore five settings into a compressor that is still off, and you hear nothing change.

Set the button to momentary, not toggle. Some controllers call it temporary or trigger. A latching button holds its values, and your encoders cannot move them.

After a press, the module sits on the glue values while your encoders are wherever you left them. 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. Each encoder then waits until it passes the module's value.

To hear what your module did before you patched it, set up a second button: 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. It is a headroom trim, not a musical setting.

CC 47 selects the compressor type, and each press of CC 48 steps to the next. Stock is the module's own compressor, described above. The other three are ports of Chris Johnson's Airwindows compressors:

• Logical4: a punchy bus compressor with some saturation.
• VariMu: a valve-style compressor that gets thick and heavy when pushed.
• Thunder: a one-knob compressor that adds weight to the low end.

All four use CC 41 to 45, for different controls:

CC 40 turns the selected type on and off. The module starts on Stock, and the glue settings and Glue on a Button above apply to Stock. Each type remembers its own settings. Switching is click-free. Only the selected type uses any processing power.

VariMu crushes hard at high Intensity. For weight without the crush, set Dry/Wet to about half.

Auto makeup sets the makeup gain for you, so the level stays the same as you compress harder. Each press of CC 46 on channel 1 steps it through Off, Normal and Hot. It works on every type.

• Normal: for a mix with headroom.
• Hot: for a mix that is already loud. On a quiet mix it adds too much gain.

While it is on, CC 45 is ignored, including the makeup a glue button sends. The pop-up says Makeup Auto if you move it. Switch auto off and the makeup stays where auto left it until you move CC 45.

There is no gain reduction meter. Your ears are the meter.

To find the threshold, bring it down slowly while something busy plays. At first nothing happens. Then the loud parts start to lean back a little. Leave it just past that point. If it sounds squashed or breathless, you have gone too far.

The others are quicker to hear:

• Ratio: sweep it on a busy loop, from transparent to obviously squashed.
• Release: set it short on a kick loop. It should pump hard. Release is by far the most audible.
• Attack: set it long on the same loop, and the click of the kick comes back.
• Makeup gain: it changes the level, and nothing else.

Nothing happens while the compressor is off. It remembers the five settings and applies them all when you switch it on.

Makeup gain runs from −36 dB to +36 dB and starts at 0 dB, in the middle. The bottom, −36 dB, is silence. If your encoder is parked at the bottom when you first touch it, the sound vanishes. Nothing is broken. Turn it up to halfway.

There is no sidechain. The module has no key input, so you cannot duck the mix from a kick on another channel. Bus compression with a fast attack and a loud kick already pumps convincingly, though the kick is compressed too. If you sequence CCs, you can also draw a threshold envelope in time with the kick.

Stock has no wet/dry mix, though Logical4 and VariMu do, on CC 44. For Stock, use the outputs instead. 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 first hard and blend it underneath. That is parallel compression, at the cost of a pad and an output pair.

Settings survive a preset change. Through power-off, the module also keeps the type, the Airwindows types' settings and auto makeup. Stock returns to the glue settings above. See what they have in common.