Anachronistically, Jamie's voice is recorded on tape. While compact magnetic tape recorders were developed in Germany in the 1930s, the technology did not make its way to the rest of the world until after World War II. Wire recording was used by the BBC during this period, but recording gramophones, using wax discs as a medium, were more common. Steven Moffat acknowledges this mistake in the DVD commentary for "The Doctor Dances", but jokingly suggests that an ancestor of Brigadier Lethbridge-Stewart stole the machine from Germany to help with the war effort.