Hi all:
I got a nice email from Bill (ZV) noting that my description was
rather lacking in helping to define IMD. I fully agree. In fact,
what I described was about as far from what IMD is as one can get.
My intention was to describe how to envision (via analogy) what a
roofing is and does. I think I did a reasonable job at that. But I
did a horrible job of providing an analogy for what IMD is.
So here is here is the briefest of descriptions, trying to fit it into
my original "room" analogy.
When two people talk at the same time (not part of the same
conversation (qso), but 1/2 each of two different conversations
(qso's), ie, different desk to desk conversations), imagine that their
voices interact with each other in such a way that "new" voices
appears in the aisles between the desks. Sitting at your desk, trying
to carry on your conversation, you hear these "voices" (which you
cannot understand because they are a mixture of two others) that cover
up the voice of the person that you are trying to talk with.
The louder the voices coming other desks, the higher the probability
that a "phantom voice" coming from the place between the desks will be
so loud that it will cover up the voice you want to hear.
So....when the roofing filter "blocks" the voice of a really loud
talker in a desk 5 desks away, it also "blocks" (removes the source
of) that phantom voice, so you can now hear the voice you want to.
You only need to block ONE of the interfering conversations, because
then there is no mixing of voices and no phantoms can voices result.
That's it. Not great and perhaps not that easy to follow either, but
at least I gave it a shot. The important thing is that IMD is not
either original signal by itself. It is the result of 2 signals
combining and yielding a new signal on a frequency that defined by the
original two frequencies (somewhere in between...also beyond).
Hope this helps and did not actually muddy the waters.
de Doug KR2Q
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