A software sequencer arranges sounds in patterns to produce larger compositions. Sounds can either be recorded samples or synthesized notes, and some sequencers handle both, while others only do one or the other.
Jackbeat (v. 0.7.6)
Jackbeat is a sampled sequencer application. This means that there is no synthesizer function. Instead, Jackbeat relies on samples to produce sound. The UI layout is modeled after a drum machine, and Jackbeat can easily double as one, assuming you already have a number of drum samples.
The user interface contains just a menu bar, work area, and properties pane. Properties can be switched between track and sequence mode using buttons near the bottom of the window. Jackbeat defaults to four tracks, though this number can be adjusted in the sequence properties pane along with beat, measure, and loop. Pitch, octave, semitone, and fine-tune can be adjusted via the track properties pane.
Jackbeat is a pretty simple application to use, and its UI stays mostly out of your way and once you realize you need to provide your own samples. JACK connections are automatically made from Jackbeat directly to system playback.
MusE (v. 1.01)
MusE is a self-described “MIDI/Audio sequencer with recording and editing capabilities." This app sports real-time recording and playback, as well as support for LADSPA, DSSI, and VST plug-ins.
The user interface is heavily dated (think Windows 95-era). Although most of the application uses a single window, JACK Control and the MusE Marker window add two more. MusE 2.0 (upcoming) will offer an interface overhaul that should modernize this app. MusE does require JACK Control to start before it can function properly.
Overall, MusE is an older audio production application that has yet to fulfill its goal of becoming a fully-fledged DAW. However, development is ongoing, and adding new features is the focus. As a Linux-only app, future commitment to the platform is certain. MusE is one to watch.
Qtractor (v. 0.4.5)
Qtractor can open an unlimited number of tracks and has a matching unlimited undo history. LADSPA is supported, along with DSSI, VSTi, and LV2 plug-ins. The zoom function in Qtractor is split between horizontal and vertical zoom, allowing the user to independently change the level of detail for either.
This is another application that relies on JACK Control to work properly. Qtractor's unattractive interface can be completely rearranged to suit individual preferences. The move and selection functions are both accomplished by left-clicking and dragging within a track, whereas most other audio editors have different modes for each function. This, however, has drawbacks. Without any differentiation, we often accidentally moved a track when we had intended to select a portion of it instead.
Seq24 (v. 0.9.0)
Seq24 is a MIDI sequencer intended for live performances, although it is thoroughly capable in a studio environment as well. This application is totally dependent upon JACK Control for any type of playback.
The Seq24 interface is split into three editors, each housed in their own windows. The main window is home to the sequence editor. Starting a new project in Seq24 isn't the most obvious procedure. You must right-click somewhere within the sequence editor and select New from the pop-up dialog.
Doing this opens the pattern editor where notes are entered. The upper toolbar of the pattern editor has controls for beat, note length, output, MIDI channel, grid selection, and zoom. A piano roll occupies the left-hand side of the screen. Actually entering notes requires another not-so-obvious right-click procedure. Holding down the right mouse button switches the cursor into a pencil, and a left click while the pencil is shown enters notes.
The third window houses the song editor, where patterns created in the pattern editor can be organized and/or duplicated into larger creations. The toolbar here contains play controls, time signature, and grid options. If you can tame JACK, Seq24 is a popular tool for live sequencing.