This must be done efficiently and with a low level of signal distortion. For each application an appropriate method must be selected, or a new method developed, so that the system performs at the desired level. Digital data, for example the output of a computer, being already in digital form does not need any conversion. However, it may be possible to compress the data into a smaller number of bits and later recover the original data. Similarly, in a communications system more messages can be sent or stored when data compression is used.
Errors always occur.
It is possible to reduce, and in theory eliminate, errors in a transmission or storage system. Error correction codes performs this task. These codes can also be applied to humans. The ISBN number that identifies a book has a simple error detection code which is designed to detect the most common errors and prevent an incorrect book from being delivered. Transmission systems can use wires, fiber-optic cables, or radio waves.