Hermit
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There are a total of 16 possible message variations.
The first 4 bits represent the message itself.
The 5th bit is 0 if the number of 1s in the message is even (i.e., zero 1s, or two 1s), and 1 if it's odd (one 1 or three 1s).
Therefore, if there's an error in the message itself or in the 5th bit, it's immediately apparent. And there's no situation where there's no error, and it's not obvious.
For example, let's say we receive the message 11100**.
How do we know if there were an even number of 1s, and the first bit was corrupted from 0 to 1? Or the second? Or the third? Or maybe the fourth was changed from 1 to 0? Or maybe the fifth bit was corrupted, but the information part is correct?
Всё не так плохо как Вы думаете. Всё намного хуже!