During the Vietnam war, the US army's Psychological Operations, or PSYOP, teams were deployed to battle communist Viet Cong guerillas and the North Vietnamese Army. Their goal was to try to weaken the enemy's willingness to fight. They used a variety of methods including playing spooky "Wandering Soul" tapes which preyed on local beliefs about the afterlife. Alex Last has been speaking to PSYOP veteran Rick Hofmann who was deployed to Vietnam in the late 1960s.