Gamelan is a good-enough soft real-time event scheduler especially for music applications. It exposes a simple API for executing Ruby code at a required time. Uses include sending MIDI or OSC messages to external applications or hardware.
Gamelan also makes life easier by supporting logical time. Logical time is reflected in the scheduler's phase. The unit in logical time is the beat, and the Scheduler's phase will increment by 1.0 with every beat.
Logical time varies with real time according to the tempo, which is specified in bpm. For example, the Scheduler's phase will increment by 2.0 for every second that elapses when using the default tempo of 120bpm. Applications are free to alter the tempo at any time, including from within tasks.
The author admits that Ruby is not at all friendly to realtime applications. No guarantees are made about the scheduler's performance. It will not drift (it will always stay in sync with the system clock), but jitter is inevitable. This is minimized by using a hybrid spinlock implementation to wait between dispatches, and by using a reasonably efficient priority queue to store Tasks.
The design is an elaboration of Topher Cyll's Timer implementation from his book, Practical Ruby Projects, and the Priority Queue implementation comes from Brian Amberg.