The original signal to be transmitted is the top line. The transmitted signal is the bottom line. One can see the obvious noise added to the signal in the transmission. But by the same token, the oscilloscope operator has no trouble at all identifying the noise and differentiating the signal with complete success and purity. He can mentally invert the received signal, straighten out the D.C. drift causing the slowly waving amplitude, and eliminate the small jittering noise component. With a pencil and paper he can draw the original signal in exactitude with 100% reliability.
Even a sophisticated filter network can't do what the human operator can do without even thinking, just using his eyes and common sense. The same effect can be seen in any printed book. We can have a 3rd generation photostat of a poorly printed book on cheap recycled newsprint, even cutting off a few letters, and blanching out the text, and yet an ordinary person can read the message completely accurately without any effort at all.
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Every time you sign up for membership in a forum, or try to post comment on many a blog, you'll be confronted with a simple test, which usually defeats the most sophisticated Optical Character Recognition (OCR) software, but which any below-average human can pass without undue effort:
Imagine the most critically important message one can think of, transmitted during a heated war and received by key operators in a crucial moment. Even the simplest strategy can practically guarantee the message gets through without error, even with 10% of the transmission dropping out randomly:
"XXXon't launch the ICBMs: repeat: XXXXX launch the ICBMs. repeat: Don't launch xxxxxxxBMs."
The receiver of this message need not worry. Just repeating the message three times has made reconstructing the original perfectly clear and certain, even with random dropouts at the most dangerous and significant places.
- Answer for Poorly Reproduced Text Above:
"...Dr. Temples doctrine, as above stated. It was a favourite article in the creed of the men who set up the guillotine in Paris, and who kept that instrument of progress in such constant action. The ultimate perfectability of the race was a prominent doctrine in the new Gospel then preached. since then, it has fascinated the speculative genius of our German neighbours in an extraordinary degree. And now the time has come, it seems, in which we Englishmen are to be treated to some grave discoursing on this theme."