DirecX.Capture error!

i get an error when i reach the cam1.Start();

private Capture cam1;
private void button1_Click(object sender, EventArgs e)
{
Filters filters = new Filters();

for(int i=0; i<filters.VideoCompressors.Count;i++)
listBox_classes.Items.Add(filters.VideoCompressors.Name);
cam1 = new Capture(filters.VideoInputDevices[1], filters.AudioInputDevices[0]);
cam1.VideoCompressor = filters.VideoCompressors[14];
cam1.Filename = @"f:\test";
cam1.FrameRate = 3;


cam1.PreviewWindow = panel1;
}

private void button2_Click(object sender, EventArgs e)
{
cam1.VideoSource = null;
cam1.Start(); <==== the error says "Error HRESULT E_FAIL has been returned from a call to a COM component."
}


dunno what it means or how to handle it!!