http://elecraft.85.s1.nabble.com/here-is-how-to-get-a-feel-for-a-roofing-filter-tp446699p446700.html
REALLY nicely done, Doug. An excellent analogy. And I would wager heavily on
the correctness of your prediction. ;-)
> Hi all:
>
> I knew this would be a tough one for many (most). Try this on for
> size....
> Imagine that you walk into a very large office space (the band),
> filled with desks and a hundred people working at those desks. You
> sit down at a desk in the middle of the room.
>
> At first, nobody is talking (everyone is tuning the band). You sit
> down at a desk in the middle of the room and you strike up a
> conversation with the person next to you (one qso on the band). You
> can easily hear them.
>
> Now assume that five other "deskmates" (hams) within the room (band)
> also start talking and that they are randomly distributed about the
> room (the band). Your conversation will likely continue without
> issue. But maybe, ONE of the other conversations if taking place only
> 5 desks (kc) away form yours. It's a bit annoying. What to do?
> Well, you would ask your conversation-mate to talk louder (run more
> power), move to another desk (qsy), or you could put up some walls
> around the four desks adjacent to you (make a big cubicle). This
> helps to shut out some of the adjacent conversation (you narrow a
> conventional IF filter).
>
> If another conversation strikes up only 3 desks away from you, you
> might compensate by making the cubicle even smaller - say around just
> your two desks (narrow the conventional IF filter even more).
>
> Now assume that everyone in the room is communicating verbally with
> another person. A few people are whispering to each other (qrpers),
> some are at normal conversation levels, many are shouting (KW), and
> some are screaming at the top of their lungs (you know who they are).
> You have already build the smallest cubicle possible, but the other 99
> conversations are still audible. You can not actually understand the
> other conversations, but the "noise" (IMD) is clearly there and making
> it hard or impossible for you to hear the person you are talking with.
> What to do?
>
> One solution is to make a smaller room with the big room (add a
> roofing filter). You might build a sound-insulated, concrete room
> that is one-tenth of the original size, but housed within the big
> room. This must mean that there are fewer other conversations within
> ear-shot because there are fewer desks in the "new" sub-room.
>
> So...lets suppose that you now have your smaller room built within the
> larger room. You now discover that there are NO other conversations
> that are so loud that you can hear them. In fact, it is so quiet now,
> that you can even take down your cubicle walls (open up - widen - the
> conventional IF filter) and still hear your own conversation without
> any other noise (IMD).
>
> [By the way, noise here = IMD squeaks, pops, bleeps, boops, etc., not
> static crashes or ignition noise, etc]
>
> Let's go back to the beginning. If you are having your solitary
> conversation in the original GIANT room and then build a walled-off
> section (sub-room), you WON'T notice any difference because it was
> quiet to start with and making your private room (smaller part of the
> big room) had no impact. It is still just as quiet after the new
> walls are put up.
>
> In other words, you can NOT hear the difference that a roofing filter
> makes unless there are lots of other LOUD signals within the first
> (typically broad) IF. I can guarantee you that Wayne and Eric will
> get "complaints" that the roofing doesn't work because folks just
> don't understand it's purpose. They will say (and I'll take bets on
> this), "I don't hear any difference whether I use the wide or narrow
> roofing filter." And they'll be correct.
>
> Hope this analogy makes some sense for you.
>
> de Doug KR2Q
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