1. A pay television broadcast system comprising a broadcast station including scrambling means for imposing a scrambling signal on the program signal, the scrambling signal being generated through use of the inverse of a signal generated from a pre-selected changeable system code, the broadcast station also broadcasting a broadcast code with the broadcast signal, the broadcast code identifying the program being broadcast; a plurality of receiving station decoders each including descrambling means for imposing a descrambling signal complementary to the scrambling signal on the received broadcast signal, the descrambling signal being generated from the same system code, each receiving station decoder including therein a fixed, station-unique internal code, the receiving station decoder further including means for demodulating the broadcast signal to recover the broadcast code; a removable, transportable digital memory module for each receiving station decoder, each module carrying thereon a fixed, station-unique external code which can be combined with the internal code of the proper receiving station to generate the system code in the receiving station, the module also having a portion onto which information as to programs descrambled by the decoder may be written, the memory module being periodically removed from the receiving station and replaced with a new memory module so that the system code can be periodically changed; the receiving station decoder also having means for writing on the memory module program information derived from the broadcast code indicating the programs actually descrambled; and means at a remote location, to which replaced memory modules are transported, for reading the memory modules to generate viewing usage charges based on the programs actually descrambled.