Changing The Tempo of a MIDI file

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i am trying to play a midi file using DxVBLib with C# .
how can i change the tempo of the loaded midi file please .
here is my small code :

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
//using DxVBLib;
using System.Runtime.InteropServices;

using System.Threading;

namespace htcdemo
{
public partial class Form1 : Form
{
public AudioX7 mid;

public Form1()
{
InitializeComponent();
}

public class AudioX7
{
private static int port = -1;
public static int Port
{
get { return port; }
set { port = value; }
}

private DxVBLib.DirectX7 dx;
private DxVBLib.DirectMusicLoader loader;
private DxVBLib.DirectMusicPerformance performance;
private DxVBLib.DirectMusicPerformance performance2;
private DxVBLib.DirectMusicSegmentState segmentState;
private DxVBLib.DirectMusicSegment segment;
private bool fIsPaused = false;
private double dTempo;
private double seconds = 0;
private long startTime = 0;
private long mtTime = 0;
private double mtLength;
private long offset = 0;
private bool playing = false;
public double CurrentPosition
{
get { return this.GetCurrentPosition(); }
set { this.SetCurrentPosition(value); }
}
public double Duration
{
get { return this.GetDuration(); }
}

public AudioX7()
{
dx = new DxVBLib.DirectX7();
loader = dx.DirectMusicLoaderCreate();
performance = dx.DirectMusicPerformanceCreate();
//MessageBox.Show("");
performance.Init(null, 0);
performance.SetPort(Port, 16);
performance.SetMasterAutoDownload(true);

//Creating a Perf2 so that we can get all the segment information
//without having to play the segment
performance2 = dx.DirectMusicPerformanceCreate();
performance2.Init(null, 0);
performance2.SetPort(Port, 16);
performance2.GetMasterAutoDownload();

}

public void Open(string path, string filename)
{

int zero = 0;
loader.SetSearchDirectory(path);
segment = loader.LoadSegment(filename);
segment.SetStandardMidiFile();

// Play the segment just long enough to get the info
mtTime = performance2.GetMusicTime();
performance2.PlaySegment(segment, 0, (int)mtTime + 2000);
//MessageBox.Show(segment.ToString());
//GetTempo
dTempo = performance2.GetTempo((int)mtTime + 2000, ref zero);

// Get Length
mtLength = (((segment.GetLength() / 768) * 60) / dTempo);
// MessageBox.Show(dTempo.ToString());
}

public void Play()
{
if (segment == null) return;
if (fIsPaused)
{
offset = mtTime - startTime + offset + 1;
segment.SetStartPoint((int)offset);
segmentState = performance.PlaySegment(segment, 0, 0);
playing = true;
Thread.Sleep(90);
}
else
{
offset = 0;
if (performance.IsPlaying(segment, segmentState) == true)
performance.Stop(segment, segmentState, 0, 0);
segment.SetStartPoint(0);
segmentState = performance.PlaySegment(segment, 0, 0);
playing = true;
//MessageBox.Show("now pl");
Thread.Sleep(90);
}
fIsPaused = false;
}

public void Stop()
{
if (segment == null) return;
fIsPaused = false;
if (performance != null)
{
performance.Stop(segment, segmentState, 0, 0);
playing = false;
}
seconds = 0;
}

public void Pause()
{
//Console.WriteLine(performance.GetMusicTime());
//Console.WriteLine(performance.GetClockTime());
if (segment == null) return;
if (this.Playing())
{
fIsPaused = true;
mtTime = performance.GetMusicTime();
startTime = segmentState.GetStartTime();
performance.Stop(segment, null, 0, 0);
playing = false;
}
else
{
fIsPaused = false;
offset = mtTime - startTime + offset + 1;
segment.SetStartPoint((int)offset);
segmentState = performance.PlaySegment(segment, 0, 0);
playing = true;
Thread.Sleep(90);
}
}

public bool Playing()
{
//return (performance.IsPlaying(null, segmentState));
return playing;
}

public bool Paused()
{
return fIsPaused;
}

public void Close()
{
performance.CloseDown();
}

public double GetDuration()
{
return mtLength;
}

private double GetCurrentPosition()
{
if (performance.IsPlaying(null, segmentState))
{
// calculate the seconds
seconds = ((((performance.GetMusicTime() -
(segmentState.GetStartTime() - offset)) / 768) * 60) / dTempo);
}
return seconds;
}

public void SetCurrentPosition(double position)
{
offset = (long)((position * dTempo / 60) * 768 -
performance.GetMusicTime() + segmentState.GetStartTime()) + 1;
if (performance.IsPlaying(null, segmentState))
{
mtTime = performance.GetMusicTime();
startTime = segmentState.GetStartTime();
performance.Stop(segment, null, 0, 0);
playing = false;
fIsPaused = true;
this.Play();
}
}
}

private void button1_Click(object sender, EventArgs e)
{
mid = new AudioX7();

mid.Open("d:\\", "paleface.mid");
mid.Play();
//mid.SetCurrentPosition(20);
//MessageBox.Show(mid.GetDuration().ToString());
}

private void button2_Click(object sender, EventArgs e)
{
mid.Pause();
}
}
}