1. Field of the Invention:
The invention relates to CATV systems, and more particularly to a method and apparatus for selectively jamming selected channels to selected subscribers in a CATV system.
2. Description of Related Art:
CATV systems commonly provide a multiple of channels to their subscribers; such systems are well known in the art and have been for many years. Many CATV systems, however, do not require each of their subscribers to subscribe to every one of the channels they offer. The result is that an effective method, and apparatus to carry out that method, is needed to allow a CATV system to provide different numbers of channels to different subscribers, and at the same time retain the ability to provide up to all of their channels to up to all of their subscribers. It is even more desirable for a CATV system to be able to easily change the number of channels it is providing to any, of its subscribers, at any time, and from time to time. That is generally desirable because most CATV systems charge different amounts to their various subscribers, depending on how many of the supplied channels the particular subscriber wants to receive, and those features allow a subscriber to easily change the number of channels he receives. The result is that with those features a CATV system can easily sell additional channels to its subscribers, or stop sending channels to subscribers who do not want them or are not paying for them.
Scrambling and de-scrambling systems have been developed and are used by some CATV systems to control which channels a particular subscriber receives. Certain channels are scrambled prior to transmission from the supplier's head end, and subsequently de-scrambled at the home ends of those subscribers who have paid for those scrambled channels. A device commonly referred to as a de-scrambler is installed in the paying subscribers homes, which restores the scrambled television signal. Those systems require access to the subscriber's home, and man power to do the installation, or to remove the de-scrambler in the event the subscriber no longer wants it, or has stopped paying the fee. Those systems also run the risk of channels being pirated by subscribers who have built, or purchased, their own illegal de-scrambler.
Another known means of controlling subscriber access to the total channels in a CATV system is to selectively jam the channels to be denied to the subscribers who have not paid for them. The jamming of channels is usually accomplished by sending a separate jamming signal for each of the channels to be jammed, together with the television signals, to the subscriber(s) for whom those channels are to be jammed. Each jamming signal can be a signal having a repetition rate close to the repetition rate of the vertical synchronization signal of the channel to be jammed, and of a greater amplitude. The television will pick up the jamming signal as the synchronization signal and will therefore produce a vertically unstable picture. Similarily, the jamming signal can be a signal having a repetition rate close to the repetition rate of the horizontal synchronization signal of the channel to be jammed, and of a greater amplitude. The television will then pick up the jamming signal as the horizontal synchronization signal and will therefore produce a horizontally unstable picture.
A device is required that will not only produce the required jamming signals, but also, will only combine the jamming signals with the television channel signals going to the subscriber(s) who have not paid for those channel.
Some of the earlier prior art has used one separate line per subscriber, and one RF switch per subscriber, per channel that might be jammed. The separate subscriber lines are each coupled to a different RF switch for each channel that might be jammed, thereby creating a switch and subscriber line matrix in which each subscriber line has a unique RF switch for each channel that might be jammed, controlling that subscriber's access to said channel, and in which all RF switches controlling subscriber access to a particular channel are connected to each other. For example only, if the situation is that channel "C" is to be jammed to subscribers "101", "222", and "223", each of the channel "C" RF switches that couple the jamming signal to those subscriber lines must be closed. Accordingly, if there are 2500 subscribers and 4 channels to be jammed, 2500 separate subscriber lines are needed, and (4.times.2500)=10,000 RF switches are required. Accordingly, a large number of RF switches are required. In such a system it would be difficult to add further subscriber lines and switches to the switch and subscriber line matrix.
Some of the more recent prior art use apparatuses that are capable of generating up to a fixed number of different jamming signals, (for example the maximum might be 6) sometimes continuously, and sometimes on a time divisional basis. In some of the more sophisticated present art systems the jamming signals are added on a time shared bases to the subscribers lines who are not permitted to view the particular channel being jammed. In many of the more sophisticated prior art systems that add jamming signals to subscriber lines on a time divisional basis the maximum number of jamming signals that can be generated is eight or less. Therefore, the maximum number of channels that can be jammed is eight or less. That is obviously a problem for a CATV system that wants to offer nine or more alternative channels.
A number of difficulties have to be overcome when jamming signals are not continuously added to the appropriate subscriber lines. However, the benefits of adding jamming signals to the appropriate subscriber lines on a time divisional basis are great. Therefore it is worth the effort of trying to overcome the difficulties involved in time divisional jamming. A major benefit to time divisional jamming is that only one line can be used to carry all of the different jamming signals to one subscriber. Only one line has to be used, because when the jamming signal being transmitted is to jam a channel the subscriber is not entitled to receive, then the RF switch to that subscriber's single jamming signal line is closed, hence the subscriber receives the jamming signal, and that channel is jammed on the subscriber's television set. However, when the jamming signal being transmitted is to jam a channel the subscriber is entitled to receive, then the RF switch to that subscriber's single jamming signal line is left open, hence the subscriber does not receives the jamming signal, and that channel is not jammed on the subscriber's television set. The result is that the channel is jammed during the time its jamming signal is being transmitted, but not during the time it's jamming signal is not being transmitted. Therefore, if the time between transmissions of the same jamming signal is too great, the channel will only be jammed sometimes, hence it would be partly viewable. It should therefore be a goal of time divisional jamming systems to reduce the time between transmissions of the same jamming signal so that the channel is humanly unviewable.
It is more desirable to produce a horizontally unstable picture than a vertically unstable picture, as the former is more difficult to watch. To produce a horizontally unstable picture the jamming signal must be sent to each subscriber who is to be jammed by it many thousands of times per second, thereby effectively always jamming the television signal. To accomplish transmitting different jamming signals on the same line many thousands of times per second, rapid switching is required; rapid or continuous generation of the jamming signals is also required, and finally, the rapid switching and the jamming signals must be combined correctly to the appropriate subscribers. Those skilled in the art will perceive many problems in rapidly switching between frequencies and maintaining accuracy of frequency within a small range.
Some of the more sophisticated prior art systems have used fixed inductive circuits which are alternately coupled with an oscillator to generate different frequencies through that oscillator. Some of the other more sophisticated prior art systems have used a voltage controlled oscillator to generate the different jamming frequencies on a time divisional basis. The goals in the prior art have generally been to effectively jam the largest number of channels with the least amount of expensive components, while at the same time allowing for control of what channels are jammed to which subscribers, from the head end, with a minimum of equipment malfunction. The goals of the present invention are the same as the stated generally accepted goals in the prior art. The present invention accomplishes those goals in a new and effective way not contemplated by the prior art, and for a greater number of channels than the prior art inventions of which the Inventors of the present invention are aware.