Morse code
From Tardis Wiki, the free Doctor Who reference
Luke's Morse message, visually rendered by Mr Smith as a series of dots and dashes. (TV: The Man Who Never Was)
Morse code was a symbolic representation of the alphabet, in which letters were represented by a series of sonic bursts of varying lengths. These bursts were visually represented by dots and dashes. The code allowed for messages to be compiled, one letter at a time, and sent between any two parties who could understand them.
Luke Smith was proficient in the code. He once used his K9 dog whistle to send a Morse message to Clani, via Mr Smith. (TV: The Man Who Never Was)
Harry Sullivan once said he felt "a bit like a Morse message" after a broken teleportation via transmat from the space station Nerva to a future Earth. (TV: The Sontaran Experiment)
