1. Apparatus for tracking the flow of television programs within a program distribution system, each program including a video signal which contains information to generate a television picture, the distribution system including a plurality of units through which the television programs flow, said apparatus comprising: first signal processing means connected within the system for adding signal elements to the video signal of a program in order to add patterns of nonbinary graphic symbols to the television picture such that the nonbinary graphic symbols are disposed at predetermined locations of the television picture and a plurality of successive patterns constitute a code associated with the program; second signal processing means connected within the system for detecting signal elements previously added to a video signal and for producing, in response to such detection, information relating to the program which included the video signal; and control means, connected to said first and second signal processing means, for receiving information produced by said second signal processing means and for supplying to said first signal processing means information associated with a program which is flowing through said first signal processing means, in response to information received from said second signal processing means, in order to control generation of associated signal elements by said first signal processing means.