function speed()
{
   while(1) 
   {
     vec_set(Old_Rot, my.pan); 
     wait(1);
     vec_set(Now_Rot, my.pan);   
     vec_diff(Rot_Speed, Old_Rot, Now_Rot);
     vec_scale(Rot_Speed, time_frame);   //JCL-inspired frame-rate correction
     wait(1);
   }
}