The exhibit is a mock-up of one-third of the entire SIGSALY system, which weighed 55 tons and consisted of 40 racks of equipment. SIGSALY was the first secure voice encryption system for telephones. It was invented and built by Bell Telephone Laboratories in 1943. It had several technological "firsts" including pulse code modulation for speech transmission, multilevel frequency shift keying, and bandwidth compression. It took 13 people to operate and required 15 minutes to set up a phone call.