In step 512, it is determined whether {TAIL ~ the first receiving unit} has {HEAD ~ an unused tuner} 
In step 512 , it is determined whether { TAIL ~ the first receiving unit } has { HEAD ~ an unused tuner }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD