如何准确设置midi事件的时间戳？

Question

我正在解析一个midi文件，然后使用MIKMIDI将midi事件发送到连接的合成器，但我会将事件的时间戳四舍五入为整数秒，而不是更具体的毫秒。我需要帮助准确地计算时间。谢谢。//之前定义的类备注{ var symbol:String var octave:Int var midiValue:Int var duration:Float }

let BPM:Double = 75
// Ode to joy simplified
var song:[Note] = [
  Note(symbol: "E", octave: 5),
  Note(symbol: "E", octave: 5),
  Note(symbol: "F", octave: 5),
  Note(symbol: "G", octave: 5),

  Note(symbol: "G", octave: 5),
  Note(symbol: "F", octave: 5),
  Note(symbol: "E", octave: 5),
  Note(symbol: "D", octave: 5),

  Note(symbol: "C", octave: 5),
  Note(symbol: "C", octave: 5),
  Note(symbol: "D", octave: 5),
  Note(symbol: "E", octave: 5),

  Note(symbol: "E", octave: 5, duration: 3/8),
  Note(symbol: "D", octave: 5, duration: 1/8),
  Note(symbol: "D", octave: 5, duration: 1/2)
]
let now = NSDate()
let totalDuration:Double = 0
for note:Note in song {
  let fullNoteValue:Double = (60000 / BPM) * 0.001 * 4
  let noteDuration:Double = fullNoteValue * Double(note.duration)
  let timestamp = now.dateByAddingTimeInterval(noteDuration + totalDuration)
  MidiDevice.sharedInstance().playNoteOn(note.midiValue, withTimestamp: timestamp)
  totalDuration += noteDuration
}

MidiDevice playNoteOn定义

- (void)playNoteOn:(NSInteger)note withTimestamp:(MIDITimeStamp)timestamp {
  MIKMutableMIDINoteOnCommand *noteOn = [MIKMutableMIDINoteOnCommand commandForCommandType:MIKMIDICommandTypeNoteOn];
  noteOn.note = (NSUInteger) note;
  noteOn.velocity = 100;
  noteOn.timestamp = timestamp
  // noteOn.midiTimestamp = timestamp;

  NSMutableArray *destinations = [NSMutableArray array];

  for (MIKMIDIDevice *device in [[MIKMIDIDeviceManager sharedDeviceManager] availableDevices]) {
    for (MIKMIDIEntity *entity in [device.entities valueForKeyPath:@"@unionOfArrays.destinations"]) {
      [destinations addObject:entity];
    }
  }

  //  NSArray *destinations = [self.device.entities valueForKeyPath:@"@unionOfArrays.destinations"];
  if (![destinations count]) return;
  for (MIKMIDIDestinationEndpoint *destination in destinations) {
    NSError *error = nil;
    if(![self.deviceManager sendCommands:@[noteOn] toEndpoint:destination error:&error]) {
      NSLog(@"Unable to send command %@ to endpoint %@: %@", noteOn, destination, error);
    }
  }
}

如何将此时间戳更改为可以使用MIDISend()或MIKMIDI发送的准确时间戳？

Answer 1

您正在使用不兼容的时间度量。首先，你这样说：

let now = NSDate()
let timestamp = now.dateByAddingTimeInterval(noteDuration + totalDuration)

然后你把它交给midi设备：

 MidiDevice.sharedInstance().playNoteOn(note.midiValue, withTimestamp: timestamp)

但这里的时间戳应该是一个MIDITimeStamp：

(void)playNoteOn:(NSInteger)note withTimestamp:(MIDITimeStamp)timestamp {

MIDITimeStamp不是NSDate！MIDITimeStamp基于mach_absolute_time()，这是一种完全不同的度量。NSDate是以NSTimeInterval度量的，它是某个固定参考日期-时间的重复计数秒数。马赫绝对时间是一个巨大的整数，从计算机开机开始计算纳秒！

因此，要成功调用此方法，您需要去掉NSDate，并在MIDI时间内思考。