Timing in MusPy

In MusPy, the metrical timing is used. That is, time is stored in musically-meaningful unit (e.g., beats, quarter notes). For playback ability, additional resolution and tempo information is needed.

In a metrical timing system, the smallest unit of time is a factor of a beat, which depends on the time signatures and is set to a quarter note by default. We will refer to this smallest unit of time as a time step.

Here is the formula relating the metrical and the absolute timing systems.

\[absolute\_time = \frac{60}{tempo \times resolution} \times metrical\_time\]

Here, resolution is the number of time steps per beat and tempo is the current tempo (in quarters per minute, or qpm). These two values are stored in a muspy.Music object as attributes music.resolution and music.tempos.

The following are some illustrations of the relationships between time steps and time.

When reading a MIDI file, music.resolution is set to the pulses per quarter note (a.k.a., PPQ, PPQN, ticks per beat). When reading a MusicXML file, music.resolution is set to the division attribute, which determines the number of divisions per quarter note. When multiple division attributes are found, music.resolution is set to the least common multiple of them.