In the past, two models, pay-per-channel (PPC) and pay-per-view (PPV), for content access control in pay-TV broadcasting systems have been proposed. In PPC, a subscriber subscribes to one or multiple group(s) of channels, prearranged by the service provider, for a fixed period of time. Thus, it is unfair for the subscriber because the subscriber can not subscribe an arbitrary combination of channels according to his/her preference, and can not dynamically unsubscribe/change his/her subscription during the subscription period. On the other hand, PPV specifies that a subscriber can pay for each single program; thus, it provides a fair service. However, compared with PPC, PPV makes the subscriber inconvenient because of the high subscription frequency and low flexibility of channel selection. Moreover, all the existing PPV systems are only suitable for a small-scale Pay-TV system with limited users because of the overhead for transmission and storage. In this paper, we define a new model, flexible-PPC (F-PPC), which combines the properties and the advantages of PPC and PPV. F-PPC is an improved model of PPC for its fairness. It allows a subscriber to subscribe freely to his/her favorite channels, and unsubscribe/change his/her subscription at any time. Furthermore, it eliminates the inconvenience in PPV, and is suitable for a large-scale Pay-TV system with numerous subscribers and channels. In order to fulfill the requirements of F-PPC, we propose a new conditional access system (CAS) with a four-level key hierarchy. The proposed CAS utilizes a structure of a binary tree to update keys rapidly; therefore, the service provider can promptly update necessary keys when the subscribers change their subscriptions or leave the system. Compared with the existing CAS systems, our CAS is more efficient and flexible to support F-PPC.