History
Polar modulation was originally developed by Thomas Edison in his 1874 quadruplex telegraph – this allowed 4 signals to be sent along a pair of lines, 2 in each direction. Sending a signal in each direction had already been accomplished earlier, and Edison found that by combining amplitude and phase modulation (i.e., by polar modulation), he could double this to 4 signals – hence, quadruplex.
Read more about this topic: Polar Modulation
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