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15.5 Telegraphy and Keying
To produce telegraph signals such as morse, a simple but extreme
form of modulation is used: its depth is always either 0 or 100%. It
used to be done by a morse key—a form of switch that makes a
contact when pressed and breaks it when let go—in such a way that it
turned the output of the sender on and off. Nowadays most of the
remaining radiotelegraph communication is done at high speed by
punched tape controlling contactors, which are electrically operated
switches. There are a great many alternative positions in the sender
circuits where a key or contactor would start and stop the output, but
in practice quit ...