>Imagine trying to hear a whisper across a crowded room. Nearby louder
voices drowned out the one familiar voice you are trying to hear. All
radio receivers face the same issues. (Jon Stone)
This is where the magic of the newer receivers comes into play. Like any
other radio they also have a hard time listening for the whisper across
the crowded room but they use their knowledge of what the whisperer is
saying to build an adaptive filter that picks out the whisper from the
junk. Current techniques are not perfect because the radios are all
saying the same thing -- its like everyone's repeating the same phrase
-- but they try to pick up on slight differences in inflexion to
distinguish between the talkers.
Conventional radio, the sort we've grown up with, distinguishes between
information by sending carrying in different parts of the R/F spectrum
-- that is, we 'tune' the receiver to pick up the transmitted
information stream that we want to receive. This kind of selection is
technically straightforward, its the only sort that could be envisaged
using the technology available to us when radio was first invented, but
its got the problem that the filters that select one stream from an
adjacent one are always going to be imperfect -- no matter how well we
design them some unwanted signal's going to get into the receiver if the
interference is strong enough or close enough in frequency. The trick
has been to design the receiver so that the filtering's adequate for the
intended use, and to do within a size and price point that's realistic.
There are other ways of putting streams of information out, possibly the
earliest common example is how television got adapted to first carry the
sound signal (it started as essentially two distinct transmissions,
video and sound) and then to put color information into the same signal.
Things gradually got more exotic but we were unaware of it (I was,
certainly) because the applications were things like receiving
information from spacecraft or satellites (there's a limit to how
sensitive you can make a receiver even if you cool it) but gradually
they started turning up in everyday stuff such as cellphones and digital
radio and TV. Our R/C radios are very crude by comparison but they're
still adequate for what they do (anyway, modern technology doesn't
render old stuff useless per se -- you can still use a CB radio to talk
to someone instead of a cellphone). Conventional radio is essentially
obsolete, it'll eventually go the same way as spark gap or R/F
alternator technologies, but it will take some time before it fades away.
Martin Usher
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