document Metronome effect

[ronso0]

https://mixxx.discourse.group/t/metronome-ignores-meta-knob-and-always-routes-through-master/25339/2
https://mixxx.zulipchat.com/#narrow/stream/109176-documentation/topic/Metronome.20effect.3A.20how.20is.20that.20supposed.20to.20work.3F

Yeah, unfortunately the Metronome effect is not documented.
I just tested it (with Mixxx main branc, unstable 2.4-alpha).

First of all, to make the Meta knob control effect parameters you need to define the Meta knob linking mode (for most built-in effects those are already set): 11. Effects — Mixxx User Manual

Admittedly, the Metronome effect is a bit special:

• it requires a track to be loaded into one of the assigned decks
• if the Sync button is active the Metronome will click only if a track is playing and the BPM are known
  with Sync disabled, you can link the Meta knob to ‘BPM’, then it’ll click independently from the assigned deck’s BPM
• Metronome is post-fader, i.e. the deck Volume fader has no effect. The Metronome gain can be controlled with the Fx units Dry/Wet knob

That said, you can get a controllable click sound two ways:

• disable Sync and use either the BPM parameter itself, or map the Meta knob and use that
• if you need to know the BPM you need to work with (metronome) Sync and a playing track:
  • load a track with constant BPM, set a loop (4 beats will do)
  • turn down the volume, play the track
  • adjust the click volume with Dry/Wet and the speed with the decks pitch fader