1. A method for processing a coded video information signal, which coded information signal is transmitted from a signal source to a signal receiver, in order to mark the information content of the information signal and thereby to permit tracing of unauthorized copying of said information signal, the method comprising: decoding the video information signal at a signal receiver during reception; obtaining an identification information and unique modification information from a data carrier, said identification information comprising personal identification information in the form of a personal code comprising a bit sequence of a given number of bits; modifying the decoded information signal at said signal receiver by introducing said obtained identification information into the decoded information signal by a repeated bit-by-bit introduction of said personal identification bit sequence into said decoded information signal; said identification information being a characteristic of at least one of said signal receiver and its possessor; and said identification information being introduced into at least one of time and frequency spaces of the decoded information signal such that the information content of said information signal is essentially unchanged; said modifying step comprising introducing at least one bit of said bit-by-bit personal identification bit sequence within one line interval of each field of said video signal, and wherein said interval is randomly selected for each field as a function of said modification information obtained from said data carrier.