A method and apparatus for encoding a serial number into a program event. The encoder embeds the serial number into the program event so that it is imperceptible to the user when the program event is played to the user. The embedded serial number is unique to the user so that any storage medium storing the program event can be traced back to the user. This encoding technique is especially useful in interactive systems where a server at the head end receives subscriber identification numbers along with subscriber requests for program events. Thus, in response to a subscriber request for a program event, the server can generate a serial number unique to the user. The serial number can include a time and date stamp, and a cyclic redundancy check code. In an interactive system, the encoder may be located in a set-top terminal, which receives the serial number from the server in the head end. The encoder can embed each bit of the serial number into at least one field of the program event, where the program event comprises multiple video images, and each image consists of two interlaced television fields. The encoder can differentially encode each serial number bit into alternating fields of the program event by adjusting the luminance level of the fields.