How to modify this loop?

var scrollSpeed = 50; 		// Speed in milliseconds
var step = 1; 			// How many pixels to move per step		    
var current = 0;		// The current pixel row
var imageHeight = 393;		// Background image height
var headerHeight = 300;		// How tall the header is.
			
//The pixel row where to start a new loop
var restartPosition = -(imageHeight - headerHeight);
			
function scrollBg(){
				
	//Go to next pixel row.
	current -= step;
				
	//If at the end of the image, then go to the top.
	if (current == restartPosition){
			current = 0;
		}
			
		//Set the CSS of the header.
		$('#header').css("background-position","0 "+current+"px");
				
		}
                      //Calls the scrolling function repeatedly
			var init = setInterval("scrollBg()", scrollSpeed);