问题

过去的几天里，我一直在尝试用Java编写一个节拍器作为练习。我使用了 javax.sound.midi 库来制作一个简单的4/4节拍，程序通过这个库播放节拍。我的主要问题是，Sequencer似乎会在第一个节拍上播放失调。如果我设置循环序列，这只会在第一次循环时发生。另外，如果我更改音轨的bpm，它会在第一次循环后重置。此外，我尝试了多个MIDI文件，以防我创建的MIDI文件有问题，但所有的测试结果都相同。

这是处理MIDI播放的代码部分：

public class MidiHandler 
{
    private Sequencer sequencer;
    private Sequence seq;
    private float newTempoFactor;
	
    public MidiHandler()
    {
        try 
        {
            sequencer = MidiSystem.getSequencer();
            if (sequencer == null)
            {
                System.err.println("Sequencer not supported");
            }
            sequencer.open();
        } 
        catch (MidiUnavailableException ex) 
        {
            Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
    
    public void setAudioTrack(String filePath)
    {
        try 
        {
            seq = MidiSystem.getSequence(new File(filePath));
            sequencer.setSequence(seq);
        } 
        catch (InvalidMidiDataException | IOException ex) 
        {
            Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
    
    public void playTrack(float bpm) throws InterruptedException
    {
        try 
        { 
            seq = editEvents(); // editEvents()方法将所有MIDI事件向前推进100个ticks
            sequencer.setSequence(seq);
        } 
        catch (InvalidMidiDataException ex) 
        {
            Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
        }
        sequencer.setLoopCount(Sequencer.LOOP_CONTINUOUSLY);
        sequencer.start();
        // sequencer.setTempoInBPM(bpm);
        newTempoFactor = bpm / 120;
        sequencer.setTempoFactor(newTempoFactor); // 默认的节拍是120bpm --> 节拍因子 = 1

        sequencer.setLoopStartPoint(100); // 将循环的起始/结束点向前推进100个ticks
        sequencer.setLoopEndPoint(seq.getTickLength());
    }    

    public Sequence editEvents() 
    {
        Sequence seq = this.seq;
        try 
        {
            seq = MidiSystem.getSequence(new File("res//myTrack.mid"));
            for (Track track : seq.getTracks()) 
            {
                for (int i = 0; i < track.size(); i++) 
                { 
                    MidiEvent event = track.get(i);
                    event.setTick(event.getTick() + 100);  
                }
            }
        } 
        catch (InvalidMidiDataException | IOException ex) 
        {
            Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
        }
        
        return seq;
    }
}

我的主类：

public class main 
{
    public static void main(String[] args) 
    {
        try 
        {
            MidiHandler mh = new MidiHandler();
            mh.setAudioTrack("res//myTrack.mid");
            mh.playTrack(120f);
        } 
        catch (SecurityException | InterruptedException ex) 
        {
            Logger.getLogger(main.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

public class MidiHandler 
{
private Sequencer sequencer;
private Sequence seq;
private float newTempoFactor;
public MidiHandler()
{
try 
{
sequencer = MidiSystem.getSequencer();
if (sequencer == null)
{
System.err.println("Sequencer not supported");
}
sequencer.open();
} 
catch (MidiUnavailableException ex) 
{
Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
}
}
public void setAudioTrack(String filePath)
{
try 
{
seq= MidiSystem.getSequence(new File(filePath));
sequencer.setSequence(seq);
} 
catch (InvalidMidiDataException | IOException ex) 
{
Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
}
}
public void playTrack(float bpm) throws InterruptedException
{
try 
{ 
seq=editEvents();//editEvents() method pushes all midi events 100 ticks forward
sequencer.setSequence(seq);
} 
catch (InvalidMidiDataException ex) 
{
Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
}
sequencer.setLoopCount(Sequencer.LOOP_CONTINUOUSLY);
sequencer.start();
//sequencer.setTempoInBPM(bpm);
newTempoFactor=bpm/120;
sequencer.setTempoFactor(newTempoFactor);//Default tempo is 120bpm --> Tempo factor =1 
sequencer.setLoopStartPoint(100);//Shift the loop start/end by 100 ticks
sequencer.setLoopEndPoint(seq.getTickLength());
}    
public Sequence editEvents() 
{
Sequence seq= this.seq;
try 
{
seq = MidiSystem.getSequence(new File("res//myTrack.mid"));
for (Track track :  seq.getTracks()) 
{
for (int i=0; i < track.size(); i++) 
{ 
MidiEvent event = track.get(i);
event.setTick(event.getTick()+100);  
}
}
} 
catch (InvalidMidiDataException | IOException ex) 
{
Logger.getLogger(MidiHandler.class.getName()).log(Level.SEVERE, null, ex);
}
return seq;
}
}

public class main 
{
public static void main(String[] args) 
{
try 
{
MidiHandler mh = new MidiHandler();
mh.setAudioTrack("res//myTrack.mid");
mh.playTrack(120f);
} 
catch (SecurityException | InterruptedException ex) 
{
Logger.getLogger(main.class.getName()).log(Level.SEVERE, null, ex);
}
}
}

答案1

得分: 1

你的代码看起来没问题。

最初的节拍不准确通常是由连接的外部 Midi 设备引起的（在我的情况下，当我使用外部 USB Midi 声卡时出现这种情况）。如果可以的话，尝试使用不同的 Midi 设备。

如果不能更换设备，一个解决方法是将创建的“Sequence”的所有“MidiEvents”整体后移，比如说后移 4 拍，然后使用“Sequencer.setLoopEndPoint(long tick)”和“Sequencer.setLoopStartPoint(long tick)”来使循环从新的起始点开始。

关于开始后的节奏变化，这是一个 JDK 的 bug。解决方法是在“Sequencer.start()”后立即调用“Sequencer.setTempoInBPM()”。