wdr at world.std.com wrote:
: In article <3640DBF3.65587DF8 at ll.mit.edu>,
: Mike MacDonald <mmacdon at ll.mit.edu> replied to Anna:
:: > > was in my physics lab at tertiary school. there was a piece on radar, it
: > > said "some people can hear radar waves directly in their ears, but it is
: > > not thought to be harmful."
:: Interesting if true. I'd like to know what was actually observed.
:: > > 2 I can hear these noises in one of my ears, 2 tones only, very artificial
: > > sounding, like sine waves. They switch very fast, and randomly. To me it
: > > seems like some kind of data transmission,
:: That discription does _sound_ like typical radio data modulations.
It accurately describes how relatively low data rate frequency shift
keying (FSK) sounds when played through a speaker. This is commonly
used for radio-teletype transmissions (TTY), FAX, and I believe some
navigations systems (Loran?).
You can generally hear examples of this on some of the lower frequency
amateur radio bands (RTTY). The Navy has a very powerful FSK RTTY
system with the call letters NSS that operates (if I recall correctly)
between 2.5 and 3.0 MHz) almost continuously transmitting to ships
at sea. (RTTY hams once frequently monitored its signals as a check
of their receiving equipment operations.)
Still, one problem in hearing these signals as audio tones is that
the most common FSK methodology shifts the carrier frequency by
800 cycles originally and now frequently less. Unless you have a
local oscillator beating with the carrier (a BFO), the frequencies
involved are above the audible spectrum.
Audio Frequency Shift Keying (AFSK) would be less problematic to
hear (because it involves the frequency shifting of audable tones),
however, to the best of my knowledge its use is largely limited to
amateur radio and relatively low power commercial applications.
Harry C.