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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: How does a mixer work?
References: <email@example.com> <3DB2E3CE.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <3DB34C15.5203CF0@webaccess.net> <3DB37C55.6F63@Spam.Bots> <3DB36A58.4E49CEAC@webaccess.net> <3DB5324F.email@example.com> <3DB53DD8.firstname.lastname@example.org>
Date: Tue, 22 Oct 2002 13:27:13 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 06:27:13 PDT
Fred Bloggs wrote:
> Kevin Aylward wrote:
> > "Fred Bloggs" wrote in message
> > news:3DB5324F.email@example.com...
> >>Chuck Simmons wrote:
> >>>At this point I take a stand against the hidebound conventionalism
> > that
> >>>is the application of mathematics to engineering. I am very
> > sensitive to
> >>>this because I have no engineering education whatever and ALL
> >>>mathematical statements engineers make can be considered suspect
> > because
> >>>of the hidebound conventionalism.
> >>>Consider the equation:
> >>>This is really an identity. The left hand side says that the carrier
> >>>varies in amplitude according the the operations on the field of
> > real
> >>>numbers. The second says that, fully equivalently, there are three
> >>>constant frequencies and amplitudes.
> >>>It is pure hidebound conventionalism that says that though the two
> > sides
> >>>of the equation are correct, the right hand side is more correct.
> > Pure
> >>>mathematical hogwash. There can be no justification for that sort of
> >>>narrow thinking.
> >>If the early developers adopted your perspective, there would be no
> >>sideband communication.
> > I do actually remember some stories like this, vaguely. Apparently,
> > people were sceptical that sidebands were real, and that if you had a
> > really sharp filter you could still recover the modulation. This was, I
> > understand even experimentally "proven". In reality, they just did not
> > use a sharp enough filter.
> They were actually dealing with entities with too sharp a characteristic
> such as the tuned stages and the antenna. It was recognized early on
> that the modulation spectral spread had to be a very small percentage of
> the carrier for distortion-free communication. They also discovered that
> a really sharp filter could not recover the sideband by peak detection.
> The carrier had to be "restored" first. The product detector was then
> developed which does the restoration and detection in one step-including
> a simple low pass. You have the sideband sin((Wc+Wm)t) for example, and
> the product with restored carrier is sin(Wct)*sin((Wc+Wm)t) which is
> actually 0.5*( cos(Wmt)-cos( (2*Wc+Wm)t)), an easy job for a lowpass to
> extract cos(Wmt). The tradeoff was the added complexity of automatic
> frequency control, AFC, for carrier restoration, but this was easily
> handled, and a small price to pay for the benefit of SSB such as better
> S/N, greater range, and less transmission power to do the same job.
AFC is not possible in SSB or independent sideband communication unless
you put something in a sideband that can be recognized without a
carrier. The FM stereo solution was to use a 38kHz carrier for AM
modulation, suppress the carrier and then send a 19kHz "pilot tone" for
carrier restoration. In HF independent sideband communication, at least
in the 1960s when I was in that for a while, the trick was to agree with
the distant end on what the carrier was. Both ends would set the
carriers to the same frequency using frequency synthesizers. One type of
synthesizer I used allowed 500Hz steps from 2 to 32 MHz (Collins) and
another used 100Hz steps in the same HF range (TMC I think). It was
rather funny when one end or the other had synthesizer trouble because
it was sometimes hard to agree about who was wrong. The sidebands
themselves do not even give a tiny clue as to what the carrier should
be. I recall that particularly bitchy sites would complain if I had an
error of only 10Hz in the transmission.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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