Frequency-change signaling

In telecommunications, frequency-change signaling is a telegraph signaling method in which one or more particular frequencies correspond to each desired signaling condition of a telegraph code. The transition from one set of frequencies to the other may be a continuous or a discontinuous change in the frequency or phase.{{cite book | title = Modern Dictionary of Electronics | author = Rudolf F. Graf | publisher = Newnes | year = 1999 | isbn = 0-7506-9866-7 | url = https://books.google.com/books?id=uah1PkxWeKYC&q=%22Frequency-change+signaling%22&pg=PA304 }}