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 which says if the number of set bits (bits equal to one) 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 re-send that byte (or word, or even chunk of data). We’re pretty sure that if our machines are more or less functioning, we won’t get the same mistake twice. Simple, but effective!