[ARC5] What is a "radio range filter"?

[Jim Haynes]

I agree.  The old Adcock or "four arm beam" radio range modulated the
carrier with 1020 Hz.  Sending Morse N in two quadrants and A in the
other two, such that the N and the A overlapped into a steady tone when
you were "on the beam" at the boundary between two quadrants.

Then they also transmitted voice communication on the same transmitter,
so that the airplane needed only one receiver and could receive voice
communication while using the beam to navigate.  There were also weather
broadcasts every half hour.  The range filter, FL8 or FL5 allowed the 
pilot to either suppress the beam signal so he could hear the voice
messages or suppress the voice so he could hear the beam more clearly.
Hence "range"  or "voice" or "both".