A receiver/decoder for use in a digital satellite radio or television system includes a decoder and means to accommodate a credit or bank card carrying a microprocessor, and means to interact with said microprocessor when the credit or bank card is inserted into an operative position in said receiver/decoder in order to enable data carried by said credit or bank card to be read and data to be input to the microprocessor carried by said credit or bank card. A PIN number may be transmitted to the receiver/decoder in a secure fashion by means of a remote controller, which is also described. Applications of the invention include Pay-Per-View television, teleshopping and telebanking.