We really do expect errors in digital copying, and all digital media have some way to check for and correct transmission errors.
One of the simplest schemes is called parity checking. For every chunk of data, we also include one bit that says if the number of set bits (bits equal to 1) was odd or even. This is called the parity of the chunk.
If the parity of the received byte is different from the parity of the sent byte, we know that we have a mistake, and we should resend that byte (or word, or even chunk of data). Were pretty sure that if our machines are more or less functioning, we wont get the same mistake twice. Simple, but effective!
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