A signal interceptor was a device that allowed its user to eavesdrop on encrypted communications. Stormtrooper X had such a device installed in his stormtrooper helmet.
Sources
"Bounties to Die For" — Star Wars Gamer 6 (First mentioned)
A signal interceptor was a device that allowed its user to eavesdrop on encrypted communications. Stormtrooper X had such a device installed in his stormtrooper helmet.