[question] Meaning of "time" for multiple tracks + tempo changes and conversion to absolute time in seconds?

[KoenT-SS]

Hi,
thanks for creating mido!
These are more questions than issues:

1. Could you please clarify: what is the meaning of the time attribute for events in a MIDI file track (talking about MIDI type 1 files here)?

I know that normally, time in MIDI streams is expressed as "delta time in ticks from the previous message".
But mido groups events per track, and it looks like the time attribute is then "delta time in ticks since the previous message on that same track".

Is this correct?
And also: is this then relative to note events only, or also relative to other events in that track?

For example, this is what I see for a simple note sequence of C D C D, where the C's are played on 1 track, and the D's on another track (note that I've also added a tempo change event somewhere in the middle of the first D note):

=== Track 0
MetaMessage('track_name', name='midi_tracks_test', time=0)
MetaMessage('time_signature', numerator=4, denominator=4, clocks_per_click=24, notated_32nd_notes_per_beat=8, time=0)
MetaMessage('set_tempo', tempo=500000, time=0)
MetaMessage('set_tempo', tempo=333333, time=1440)
MetaMessage('end_of_track', time=0)
=== Track 1
MetaMessage('track_name', name='track1', time=0)
Message('note_on', channel=0, note=60, velocity=100, time=0)
Message('note_off', channel=0, note=60, velocity=64, time=960)
Message('note_on', channel=0, note=60, velocity=100, time=960)
Message('note_off', channel=0, note=60, velocity=64, time=960)
MetaMessage('end_of_track', time=0)
=== Track 2
MetaMessage('track_name', name='track2', time=0)
Message('note_on', channel=1, note=62, velocity=100, time=960)
Message('note_off', channel=1, note=62, velocity=64, time=960)
Message('note_on', channel=1, note=62, velocity=100, time=960)
Message('note_off', channel=1, note=62, velocity=64, time=960)
MetaMessage('end_of_track', time=0)

2. Since my goal is to convert note on/off events to notes with an absolute start/end time in seconds, am I right that this is the way to handle this with mido:

• create a tempo map from the set_tempo events (each item has a time in ticks and the tempo value being set at that moment)
• start at time 0 s with the tempo of the first tempo map item (at tick 0)
• iterate over the note on/off events of each track while each time accumulating the time in ticks
• if a next tempo change is not reached, simply accumulate the time in seconds by using tick2second(delta_time_ticks, ppqn, tempo)
• if a next tempo change is reached, accumulate the time in seconds by using tick2second for all the tempo changes since the last one, taking into account the ticks between these tempo changes + the amount of ticks since the last note event in this track spent at each tempo
  For that 1st D note in my example, it would mean that its absolute duration in seconds is calculated:
• for the 1st part using tick2second with 960/2 ticks and the initial tempo
• for the 2nd part using tick2second with 960/2 ticks and the second tempo
  So essentially, do I need to do the bookkeeping myself of which ticks were spent at which tempo, on a track per track basis?
  Or is there already something available in mido that handles this for MIDI files?