Qtractor is an audio and MIDI multi-track sequencer application written in C++ with the Qt4 framework. It was written and is actively maintained by Rui Nuno Capela, also responsible for Qjackctl, Qsynth and Qsampler. It features a built-in connection patchbay control and persistence (much like Qjackctl), non destructive and non linear editing, unlimited number of tracks per session/project, unlimited number of overlapping clips per track, unlimited undo/redo, unlimited number of plug-ins per track or bus, a wide range of audio editing functions and tools, and extended midi editing capabilities (including scale presets).
The following audio file formats are supported: OGG (via libvorbis), MP3 (via libmad, playback only), WAV, FLAC, AIFF and more (via libsndfile).
It also supports LADSPA, DSSI, native VSTi and LV2 plug-ins.
Just emerge it:
root # emerge -a qtractor
Qtractor will open up with a relatively blank window. This is the canvas from where you will build up a project. At the bottom of the screen is a message window where you can view any/all status messages, including any errors you may receive from JACK_Audio_Connection_Kit, including any xruns. This can be hidden and reopened when needed by clicking on the button with the blue square at the top of the window. The other buttons in that part of the program have hover-over tooltips and should be reasonably self-explanatory.
To start creating a track simply right click on left hand side of the screen (under the rows of buttons) and choose Add Track (this can also be done from the Track menu option or from right clicking on the right hand side of the screen and going to Track>Add Track). Choose a name for the track, whether it is a midi or an audio track, and press ok. Any of the options here can be changed later by double clicking on the track on the left hand side of the screen.
If you click on the mixer button on the top of the window (F9) will be presented with a virtual mixer of the track you created and any other buses the project may have. Plugins can be added to a track by right clicking on the blank rectangle above the mixer controls and choosing Add Plugin. This window supports filtering based on what you type in the box and/or which type of plugin you select from the drop down menu on the right hand side.
Guides and Documentation
Most up to date official documentation (PDF)
Older manual for version 0.3.0 and before (PDF)