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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
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Subject: Re: How does a mixer work?
Date: Tue, 22 Oct 2002 12:10:10 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 05:10:10 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> "Kevin Aylward" email@example.com wrote:
>>"Fred Bloggs" wrote in message
>>>Chuck Simmons wrote:
>>>>At this point I take a stand against the hidebound conventionalism
>>>>is the application of mathematics to engineering. I am very
>>>>this because I have no engineering education whatever and ALL
>>>>mathematical statements engineers make can be considered suspect
>>>>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
>>>>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
>>>>of the equation are correct, the right hand side is more correct.
>>>>mathematical hogwash. There can be no justification for that sort of
>>>If the early developers adopted your perspective, there would be no
>>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.
> Just to throw a spanner in the works......
> One side says "the carrier alone carries no information".
> That's not strictly true. If you didn't know you needed to re-insert the
> carrier at the receiving end you'd never be able to receive the transmitted
> information. Now some people will say "but we already do know that we need to
> re-insert the carrier". Well... that's like saying "we already know what's in
> the sidebands, so we know what information is in them, so we don't need them,
> so they don't carry any information either"
> The carrier does carry some information. It carries the information of "this is
> the carrier that you need to insert in order to demodulate the sidebands"
> If you insist in filtering the carrier with an almost infinitely narrow filter
> to prove that it is useless then try the same thing with one of the sidebands
> and see how much information you can get out of it.
> No flames please :)
This is not quite true. The carrier conveys no information but it was
necessary for the demodulation. Note that in the case of a tone
transmission, you have sin((Wc+Wm)t) in the time-domain. This is a
frequency-shift and not an amplitude modulation, the power level is
constant. The challenge for the receiver circuits is then to convert
this into something in the time-domain with an amplitude fluctuation
corresponding to the modulation. This necessitates carrier restoration
and another AM modulator.
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