python - 如何正确地将 MIDI 节拍转换为毫秒？

Question

我正在尝试将 MIDI 节拍/增量时间转换为毫秒，并且已经找到了一些有用的资源:

1. MIDI Delta Time Ticks to Seconds
2. How to convert midi timeline into the actual timeline that should be played
3. MIDI Time Code spec
4. MTC

问题是我认为我没有正确使用这些信息。我试过应用 Nik 展开的公式:

[  1 min    60 sec   1 beat     Z clocks ]
| ------- * ------ * -------- * -------- | = seconds
[ X beats   1 min    Y clocks       1    ]

使用来自 this test MIDI file 的元数据:

<meta message set_tempo tempo=576923 time=0>
<meta message key_signature key='Ab' time=0>
<meta message time_signature numerator=4 denominator=4 clocks_per_click=24 notated_32nd_notes_per_beat=8 time=0>

像这样:

self.toSeconds = 60.0 * self.t[0][2].clocks_per_click / (self.t[0][0].tempo * self.t[0][2].denominator) * 10

这最初看起来还不错，但后来似乎发生了变化。这是一个使用 Mido 的基本可运行示例和 pygame (假设 pygame 回放正确):

import threading

import pygame
from pygame.locals import *

from mido import MidiFile,MetaMessage

music_file = "Bee_Gees_-_Stayin_Alive-Voice.mid"

#audio setup
freq = 44100    # audio CD quality
bitsize = -16   # unsigned 16 bit
channels = 2    # 1 is mono, 2 is stereo
buffer = 1024    # number of samples
pygame.mixer.init(freq, bitsize, channels, buffer)
pygame.mixer.music.set_volume(0.8)


class MIDIPlayer(threading.Thread):
    def __init__(self,music_file):
        try:
            #MIDI parsing
            self.mid = MidiFile(music_file)
            self.t = self.mid.tracks

            for i, track in enumerate(self.mid.tracks):
                print('Track {}: {}'.format(i, track.name))
                for message in track:
                    if isinstance(message, MetaMessage):
                        if message.type == 'time_signature' or message.type == 'set_tempo' or message.type == 'key_signature':
                            print message

            self.t0 = self.t[0][3:len(self.t[0])-1]
            self.t0l = len(self.t0)
            self.toSeconds = 60.0 * self.t[0][2].clocks_per_click / (self.t[0][0].tempo * self.t[0][2].denominator) * 10
            print "self.toSeconds",self.toSeconds
            #timing setup
            self.event_id = 0
            self.now = pygame.time.get_ticks()
            self.play_music(music_file)
        except KeyboardInterrupt:
            pygame.mixer.music.fadeout(1000)
            pygame.mixer.music.stop()
            raise SystemExit

    def play_music(self,music_file):
        clock = pygame.time.Clock()
        try:
            pygame.mixer.music.load(music_file)
            print "Music file %s loaded!" % music_file
        except pygame.error:
            print "File %s not found! (%s)" % (music_file, pygame.get_error())
            return
        pygame.mixer.music.play()
        while pygame.mixer.music.get_busy():
            # check if playback has finished
            millis = pygame.time.get_ticks()
            deltaMillis = self.t0[self.event_id].time * self.toSeconds * 1000
            # print millis,deltaMillis
            if millis - self.now >= deltaMillis:
                print self.t0[self.event_id].text
                self.event_id = (self.event_id + 1) % self.t0l
                self.now = millis
            clock.tick(30)

MIDIPlayer(music_file)

上面的代码应该做的是根据 midi 文件在正确的时间打印正确的歌词，但它会随着时间的推移而漂移。

将 MIDI 增量时间转换为秒/毫秒的正确方法是什么？

更新

根据 CL 的有用回答，我更新了代码以使用 header 中的 ticks_per_beat。由于只有一个 set_tempo 元消息，我在整个过程中都使用这个值:

import threading

import pygame
from pygame.locals import *

from mido import MidiFile,MetaMessage

music_file = "Bee_Gees_-_Stayin_Alive-Voice.mid"

#audio setup
freq = 44100    # audio CD quality
bitsize = -16   # unsigned 16 bit
channels = 2    # 1 is mono, 2 is stereo
buffer = 1024    # number of samples
pygame.mixer.init(freq, bitsize, channels, buffer)
pygame.mixer.music.set_volume(0.8)


class MIDIPlayer(threading.Thread):
    def __init__(self,music_file):
        try:
            #MIDI parsing
            self.mid = MidiFile(music_file)
            self.t = self.mid.tracks

            for i, track in enumerate(self.mid.tracks):
                print('Track {}: {}'.format(i, track.name))
                for message in track:
                    # print message
                    if isinstance(message, MetaMessage):
                        if message.type == 'time_signature' or message.type == 'set_tempo' or message.type == 'key_signature' or message.type == 'ticks_per_beat':
                            print message

            self.t0 = self.t[0][3:len(self.t[0])-1]
            self.t0l = len(self.t0)
            self.toSeconds = 60.0 * self.t[0][2].clocks_per_click / (self.t[0][0].tempo * self.t[0][2].denominator) * 10
            print "self.toSeconds",self.toSeconds

            # append delta delays in milliseconds
            self.delays = []

            tempo = self.t[0][0].tempo
            ticks_per_beat = self.mid.ticks_per_beat

            last_event_ticks = 0
            microseconds = 0

            for event in self.t0:
                delta_ticks = event.time - last_event_ticks
                last_event_ticks = event.time
                delta_microseconds = tempo * delta_ticks / ticks_per_beat
                microseconds += delta_microseconds
                print event.text,microseconds/1000000.0
                self.delays.append(microseconds/1000)

            #timing setup
            self.event_id = 0
            self.now = pygame.time.get_ticks()
            self.play_music(music_file)
        except KeyboardInterrupt:
            pygame.mixer.music.fadeout(1000)
            pygame.mixer.music.stop()
            raise SystemExit

    def play_music(self,music_file):
        clock = pygame.time.Clock()
        try:
            pygame.mixer.music.load(music_file)
            print "Music file %s loaded!" % music_file
        except pygame.error:
            print "File %s not found! (%s)" % (music_file, pygame.get_error())
            return
        pygame.mixer.music.play()
        while pygame.mixer.music.get_busy():
            # check if playback has finished
            millis = pygame.time.get_ticks()
            # deltaMillis = self.t0[self.event_id].time * self.toSeconds * 1000
            deltaMillis = self.delays[self.event_id]
            # print millis,deltaMillis
            if millis - self.now >= deltaMillis:
                print self.t0[self.event_id].text
                self.event_id = (self.event_id + 1) % self.t0l
                self.now = millis
            clock.tick(30)

MIDIPlayer(music_file)

我根据转换为毫秒的时间打印消息的时间看起来好多了。然而，几秒钟后它仍然漂移。

我是否正确地将 MIDI 节拍转换为毫秒并跟踪更新 while 循环中经过的毫秒数？

转换是这样进行的: self.delays = []

    tempo = self.t[0][0].tempo
    ticks_per_beat = self.mid.ticks_per_beat

    last_event_ticks = 0
    microseconds = 0

    for event in self.t0:
        delta_ticks = event.time - last_event_ticks
        last_event_ticks = event.time
        delta_microseconds = tempo * delta_ticks / ticks_per_beat
        microseconds += delta_microseconds
        print event.text,microseconds/1000000.0
        self.delays.append(microseconds/1000)

这就是随着时间的流逝检查是否遇到“提示”的方式:

millis = pygame.time.get_ticks()
            deltaMillis = self.delays[self.event_id]
            if millis - self.now >= deltaMillis:
                print self.t0[self.event_id].text
                self.event_id = (self.event_id + 1) % self.t0l
                self.now = millis
            clock.tick(30)

我不确定此实现是否错误地将 MIDI 增量节拍转换为毫秒，是否错误地检查了基于毫秒的延迟是否通过或两者兼而有之。

Answer 1

首先，您必须合并所有轨道，以确保正确处理速度变化事件。 (如果您首先将增量时间转换为绝对刻度值，这可能会更容易；否则，每当在另一个轨道的事件之间插入一个事件时，您都必须重新计算增量时间。)

然后您必须为每个事件计算到最后一个事件的相对时间，如以下伪代码所示。重要的是计算必须使用相对时间，因为节奏可能随时改变:

tempo = 500000        # default: 120 BPM
ticks_per_beat = ...  # from the file header

last_event_ticks = 0
microseconds = 0
for each event:
    delta_ticks = event.ticks - last_event_ticks
    last_event_ticks = event.ticks
    delta_microseconds = tempo * delta_ticks / ticks_per_beat
    microseconds += delta_microseconds
    if event is a tempo event:
        tempo = event.new_tempo
    # ... handle event ...