In facsimile, a frequency modulationsystem where one frequency represents picture black and another frequency represents picture white[?]. Frequencies between these two limits may represent shades of gray
An intentional frequency change used for modulation purposes
... from the travels of Martin Gerritz de Vries[?] in the 17th century, and still better from those of La Pérouse (1787) and Krusenstern (1805). Both, however, regarded ...