All audio and events that you generate in the Composer can be sent to Racks.
A rack is a collection of synth and effect modules that process your audio and events and route them to an output.
A rack also has an audio gain, pan and mute control that processes the audio just before the first post-fader plug-in. (more on this below)
Racks are very powerful and flexible!
For example, you can set up a rack "Bass" to have a nice bass synth plugged in, and also a subtle audio chorus plug-in that further processes the bass sound.
You can then set this "Bass" rack as the target of a bass sequence part in your composition, so that all events of that sequence go to the "Bass" rack, where the synth will make sound, and the chorus will fatten that sound 
It's important to understand that a rack can receive audio as well as events. It depends very much on what you plug in whether these audio and events are further processed.
In the above example, sending audio to rack "Bass" won't have much effect because generally a synth plug-in does not process audio input, but only event input, which it then turns into audio, i.e. the bass sounds.
On the other hand, you could setup another rack that receives events from a sequence part, with a MIDI effect plugged in (e.g. an arpegiator), and then sending the result to a hardware MIDI port so you can send the resulting events to one of your external synths. Now in this case, it won't make much sense to send audio to this rack, because neither a MIDI effect, nor the MIDI port can do anything with the audio.
So it all depends on what you input into a rack and which modules are in the rack. MU.LAB does not set any limits here, and thus you are free to make any combinations of plug-ins and routings of audio and event signals.
At first you might not always hear what you would expect, because audio or event signals don't come thru due to certain modules. But if you then precisely analyse how signals flow, you'll find the reason.
The good thing about this is creative freedom!
There are so many possibilities, some modules are just for audio, some other are just for MIDI events, and there are also modules that process both audio an events, or convert audio into events or vice versa.
MU.LAB's racks lets you PLAY with it
The Rack Desk shows all used racks.
Each rack can be shown in an expanded or collapsed version. This can be controlled by the little button at the top left of each rack. This button also has a context menu with extra functions.
Each rack can receive audio and events from any part in the composer, but can also receive audio and events from other racks! This is done by plugging in a "Send" from one rack to another.
For example: if you want to apply the same reverb on synth A (in Rack A) and synth B (in Rack B), then send the audio from the Rack A and Rack B to a common rack; then that common rack will process the audio from A and B thru its plugged in reverb.
Also check out the tutorial Using Insert And Send Effects.
All racks work in the same way:
- Click a rack's name to focus it.
- Double-click a rack's name to edit its name.
- Drag a rack's name to move the rack to the left or right.
- Also at the top left there is a little button that lets you collapse/expand the rack.
- [Shift]+clicking this collapse/expand button will collapse/expand all racks.
- There are multiple slots, in which you can plug in a synth or effect modules, or plug in a Send (cfr the example above).
- At the left of these slots, there is the Pre/Post Indicator, which you can move up or down by clicking at the left of the slots.
The name Pre/Post comes from the Pre-Fader/Post-Fader analogy.
The input signal (audio and/or events) is directly processed by all modules before the POST. Then the Volume/Pan/Mute is applied to the audio, events just pass thru. Then the signal(s) stream(s) thru the first Post plug-in, and thru the following ones.
- You can double-click a slot to open the plug editor. Or click the edit button at the right of it. Once a plug-in editor is open, you can also use Esc to close it.
- At the right of a Send is a little knob for controlling the send gain, i.e. for controlling how much is sent.
- You can rearrange the order of the slots: Drag the slot up or down. Or you can even move it to another rack!
- [Alt]+click a volume slider resets it to 0 dB.
- Right-clicking the output field lets you choose which type of output must be displayed: Audio or Event. Note that a rack always has an audio out and event out at the same time, but here you choose which one is displayed. Whether the audio out and/or event out is effectively outputting a signal of course depends on what signal(s) stream thru the rack.
- You can mute a rack via its mute button. Right-clicking the mute button pops up a context menu with Solo and Unsolo options. Note that the mute is applied just before the first POST plug. This means that e.g. any sends before this point are still sending signal.
- [Alt]+click the mute solos/unsolos a rack.
- When you hover a rack slot, a tool tip shows what's plugged in.
- The 'Pan' control has 2 aspects: Stereo Position and Stereo Width. Drag to change the Stereo Position, [Shift]+drag to change the Stereo Width. [Alt]+click to reset to center position and full width. When also holding [Cntrl] adjustments are more fine.
- You can popup the Rack Desk context menu by right-clicking the background of the Rack Desk itself, thus at the very right of the Rack Desk.
- You can add a new rack via the Rack Desk context menu -> "Add New Rack".
- The Rack Desk can be used in docked mode or in windowed mode. You can toggle between these modes via the Rack Desk context menu -> "Switch To Docked/Windowed Mode".
- You can drag & drop a VST plug-in file onto a rack slot. This will add that VST plug-in to MU.LAB's VST Plug-In Manager and it will also plug a new instance into that rack slot.
- The level meter shows 3 thin lines: Top = -6dB, middle = -12dB, bottom = -18dB.
- When the mouse hovers the volume slider, MU.LAB will show the level in a tooltip.
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