MidiFile : changing tempo during playback

[mungewell]

I am playing with scripts to play Midi drum loops from file, and would like to be able to adjust the tempo during the playback.

I can (easily) adjust prior to start, ie before each 4 bar loop... but this does account for session tempo changing mid loop.

        # scan for characteristics
        for msg in mid.tracks[0]:
            if msg.is_meta:
                print(msg)
                if msg.type == 'set_tempo':
                    if force_bpm:
                        mid.tracks[0].remove(msg)
                        value = mido.bpm2tempo(force_bpm)
                        mid.tracks[0].append(mido.MetaMessage('set_tempo', tempo=value, time=0))
                        bpm = force_bpm
                        force_bpm = None

Playback is then done as simply as possible, this is presumably where I need to add code....

        while not force_bpm and not event_stop_player.is_set():
            # play 'file'
            start = time.monotonic()
            for msg in mid.play():
                outport.send(msg)

For those interested force_bpm is set as the AbletonLink session changes, using this 'connector':
https://github.com/square/pylink

[mungewell]

Found this comment in the code:
https://github.com/mido/mido/blob/main/mido/midifiles/midifiles.py#L416

        By default the system clock is used for the timing of yielded
        MIDI events. To use a different clock (e.g. to synchronize to
        an audio stream), pass now=time_fn where time_fn is a zero
        argument function that yields the current time in seconds.

AbletonLink counts 'time' in fractional beats, and does a 'weirdness' where the reference start time is re-calculated when the BPM of a session is changed.
https://github.com/Deep-Symmetry/carabiner#beat-at-time

I guess it may be possible to use now function to tap into this...

[mungewell]

hmm, that gives error:

Exception in thread Thread-2 (player):
Traceback (most recent call last):
  File "/usr/lib/python3.11/threading.py", line 1038, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.11/threading.py", line 975, in run
    self._target(*self._args, **self._kwargs)
  File "beat_play.py", line 85, in player
    for msg in mid.play(now=player_time):
               ^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: MidiFile.play() got an unexpected keyword argument 'now'

This is on Xubuntu 23.04

$ dpkg --list | grep mido
ii  python3-mido                          1.2.10-1                                all          Python 3 module for working with MIDI messages and ports

[satoer]

Hi, probably to late to help you but I was faced the same problem. I created a playfunction that where I can adjust the tempo at the start, But if you make the tempo_multiplier a global and thread the function you can probably change the tempo on the fly also.
(Using pygame to play the notes)

def playMidiTempo (filename, tempo_multiplier=1.0):
    try:
        pygame.midi.init()
        player = pygame.midi.Output(0)
        player.set_instrument(0)

        with mido.MidiFile(filename) as midi_file:
            adjust_tempo(midi_file, tempo_multiplier)
            ticks_per_beat = midi_file.ticks_per_beat

            for msg in midi_file.play():

                if msg.type == 'note_on':
                    player.note_on(msg.note, msg.velocity, msg.channel)
                elif msg.type == 'note_off':
                    player.note_off(msg.note, msg.velocity, msg.channel)
                elif msg.type == 'control_change':
                    player.write_short(0xB0 + msg.channel, msg.control, msg.value)

                if msg.time > 0:
                    adjusted_time = int(msg.time * tempo_multiplier)
                    time.sleep(adjusted_time / ticks_per_beat)

    except KeyboardInterrupt:
        print("\nExiting...")
    except Exception as e:
        print(f"An error occurred: {e}")
    finally:
        pygame.midi.quit()