Notes regarding use of multi-level signaling in telegraph systems in 1872 taken from "Edison: A Life of Invention", by Paul Israel, John Wiley & Sons, 1998 ISBN 0-471-52942-7. This excerpt describes part of the development of Edison's duplex system which independently modulated the amplitude and polarity of the telegraph signal to double the transmitted bit rate on what was essentially a bandwidth-limited communication channel.

[p. 79] [in 1872] "Edison took a significantly different approach from the one most commonly employed by those working on duplex telegraphy. Most other inventors sought to balance the [receiving] relay electrically at the transmitting station to prevent it from responding to [outgoing] signals. Edison instead used a neutral relay at one end to respond to variations in current strength and the familiar polarized relay at the other end to respond to changes in the polarity of the current."