function pb_prev () {
  var base, current, last;
  
  if (document.getElementById) {
    base = document.getElementById ('frame').src;
	current = 1 * base.substr (base.length - 9, 2);
	last = base.substr (base.length - 6, 2);
	(current > 1) ? current = "" + (current - 1) : current = last;
	if (current.length == 1)
	  current = "0" + current;
	document.getElementById ('frame').src = base.substr (0, base.length - 9) + current + base.substr (base.length - 7, 7);
  }
}

function pb_next () {
  var base, current, last;
  
  if (document.getElementById) {
    base = document.getElementById ('frame').src;
	current = 1 * base.substr (base.length - 9, 2);
	last = 1 * base.substr (base.length - 6, 2);
	(current < last) ? current = "" + (current + 1) : current = "01";
	if (current.length == 1)
	  current = "0" + current;
	document.getElementById ('frame').src = base.substr (0, base.length - 9) + current + base.substr (base.length - 7, 7);
  }
}