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
... Lane (Metropolitan Line) tube station[?] (aka White City; on what is now the Hammersmith & City Line)
Uxbridge Road tube station[?]
York Road tube station[?]
As are ...