A "digital cable filter" is a very small device designed to be installed between a set-top-box (or "digital receiver") and a cable outlet. Marketed as "a convenience" or "for enthusiasts and hobbyists" as a means to "try out" Pay Per View programming, it has been roundly debunked as a "scam" and a "waste of money" by representatives of cable television and the cable industry since 2003.
A so-called "digital cable filter" is---in technical terms---a high-pass analog radio-frequency filter. Typically tuned anywhere between 45 MHz and 85MHz, these devices allow radio signals higher than the stated frequency to pass through, while blocking signals on lower frequencies. In theory, the digital receivers can tune-in the regular broadcast signals (higher frequencies) but can't send signals back to the cable company (using lower frequencies).
In high-end broadcasting and ham-radio industries, a high-pass filter is professionally installed to isolate one connection in a large-scale system. When used on digital cable receivers, vendors claim that it "blocks" the signals related to Pay Per View billing without any interference to the Pay Per View program. Like many "cable pirating" schemes, it implies the cable company is both unaware and unconcerned about the loss of the two-way signal.
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The device itself is not "digital" at all. As explained in the consumer warning (see Resources), the primary claim is how the "digital filter" blocks signals sent from a digital receiver to the cable company. Cable companies are aware of signal loss and may act in accordance with the subscriber contract, such as spontaneous service calls to inspect equipment as well as shutting-down or "de-authorizing" specific set-top boxes by remote.
A "filter" is about as small as a cable coupler and, unlike "positive trap" devices of the past, is no thicker than the connector. While these filters are very compact, they are often printed with the model and the frequency rating of the filter right on the casing. When applied as a "digital cable filter," it would be installed either on the set-top box "cable in" connector or on a nearby signal-splitter device.
Installing a "digital cable filter" for the purpose of blocking signals related to Pay Per View billing has been deemed a violation of both the Cable Communications Policy Act and the Digital Millennium Copyright Act. Both are federal laws with few limits on sentencing; known sentences have passed with up to 16 years in prison and up to $2.7 million in fines and restitution combined.
Warning and Disclaimer
The claims of "digital cable filter" vendors are misleading at best and criminal at worst. While purchasing such a device is not a crime, the use of a "filter" for the purpose of avoiding charges related to Pay Per View entertainment is as much a crime as any theft of cable TV services.