[AMRadio] AM Broadcast Quality


Donald Chester k4kyv at charter.net
Thu Dec 19 04:03:01 EST 2013


>>From: <manualman at juno.com>

It might be teaser picture for an upcoming future article. That's been
done a number of times over the years.

Pete, wa2cwa>>

The most notorious case has to be the cover back in the late 70s with
eye-catching splashing full-color graphics and text introducing a new
super-narrow bandwidth, analogue voice mode that was supposed to
revolutionise amateur radio. Inside, it said the topic would appear in a
"future" issue. The article finally appeared a few months later. It
consisted of filtering out the mid-range frequencies of an already severely
restricted slopbucket signal, and then using frequency conversion to "fold
back" one of the  remaining frequency ranges of the voice into the gap left
by what was deleted. The receiving unit re-folded the range back to its
normal position, supposedly resulting in a voice that could be understood
even though the total transmitted bandwidth would occupy less than 1.5 kHz.
The concept was a flop; it never caught on, and I never ever heard of anyone
actually trying it over the air. I'm sure the voices transmitted from  the
space shuttle would have sounded high-fidelity by comparison.

But the worst and most misleading QST cover ever, was published sometime
back in the late 40s or early 50s, to promote SSB. It showed the envelope
pattern of a carrier modulated 100% by a sine wave, slit down the middle at
the base line, with the bottom half of the pattern still visible but greyed
out. This would have been a good graphic for demonstrating the rectifying
action of a diode detector, but of course had nothing to do with removing
one sideband to generate SSB. This photo generated a misconception of what
SSB was amongst many amateurs of that era for a long time to follow, leading
many to believe that one sideband was removed by the diode detector when it
recovered the audio. I can recall a couple of times talking over the air to
hams who would have  argued with me till hell froze over and there was no
convincing him otherwise, that an AM signal  was converted to SSB by the
diode detector in a receiver.


Don k4kyv



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