Advertising is marked with a code in a way which makes it difficult to fool the system. The advertising is marked with a code at the time the advertising is produced. Then, when the advertising is broadcast, the code on the advertising is analyzed. Different security measures can be used, including producing the code in the closed captioning so that many different people can see the code, or comparing codes in one part of the signal with a code in another part of the signal. Measures are taken to prevent the code from being used to detect commercials. According to another part of this system, a paradigm for a clearinghouse is disclosed in which the user signs up with the clearinghouse, obtains a line of credit, and the advertiser, the agency, and the ad producer also subscribe to the service. When the ad is actually aired, the payment can be automatically transferred.