RY (test signal)
RYRYRYRY... is a character string that was widely used to test a five-level teleprinter or RTTY channel. The characters R and Y are "01010" and "10101" in 5-bit ITA2 code, also known as Baudot. Thus they are Boolean complements of each other.
Switching between the two characters is a stressful test for electromechanical teleprinters. Repeated over and over, RYRYRYRY... outputs a carrier wave that regularly and rapidly shifts back and forth in frequency.
This signal pattern also provided a test for signal polarity; if polarity was reversed, the test signal would print as "SG".
The corresponding string of complementary characters in 7-bit ASCII is U*U*U*U*...