Digital television signals can be scrambled in such a way that they cannot be understood by a conventional decoder. Only when unscrambled by a special system can the original pictures be seen by the viewer. By controlling the operation of the de-scrambling system through the use of a pre-paid access card, or by a transmitted code, the broadcaster can control access to a particular channel or service. Conditional access can be used to control many things from pay-per-view subscription through to target viewing areas. The ATSC specification, at press time, was not complete.