“O children of Benjamin.” Literally, “sons of Benjamin,” referring to the people of Benjamin. Jerusalem was in the tribal territory of Benjamin, so many of the inhabitants of Jerusalem were Benjamites. But it is likely that because people of other of the tribes of Israel had lived in Jerusalem for generations, even from the time of David hundreds of years earlier, that they were being considered as Benjamites also.
“shofar.” The ram’s horn trumpet, not the metal trumpet.
“raise a signal on Beth-haccherem.” The location of Beth-haccherem (“house of the vineyard”) is not known, but it would have been on a hilltop. The signals were fires that were lit and could be seen for miles.