It seems it is better a midi track to have its room: not too many other frequencies on its main frequencies.
It is the first time I do this work, so I please need some advices for the tools. Can someone tell me where to find :
A good mean to watch the spectrum of a midi track.
A good mean to manually set a filter on this track.
Does an automated mean exist, which mind about the change of musical notes in the track (and automatically adapts the filter to the new pitch) ? Is it a compressor ? Where can I find this mean if it exists ?
A MIDI track on its own doesn’t produce any sound so you can’t see its spectrum or use an audio filter on it.
The potential sound it produces comes from whatever synth or sample plugin you’re adding to the track.
To get a spectral representation of the synth sound you can use x42 Meter Collection
Ardour has a built in ACE-EQ filter plugin and you can also use EQs from x42, LSP or others.
If you only want to hear the main frequency you should set the synth to produce a sine wave.
It won’t be a particularly interesting sound but that’s what “main frequency” is.
All the interesting sounds you usually hear are combinations of the main sine wave and some or all of its even and/or odd harmonics.