The cable TV (CATV) industry uses traps and filters installed at a subscriber drop to decode or recover a scrambled pay channel. A problem occurs when these filters and traps are stolen and later sold to persons trying to circumvent having to pay for services.
Conventional tunable filters or cable traps of the kind described are typically referred to as “tuned notch filters” which are used for removing frequency scrambling signals provided within, for example, a TV channel band to eliminate reception of that channel. This is referred to as positive trapping in the art. These filters may also be used for negative trapping, which involves removing specific frequencies within the TV channel band, such as the video carrier, in order to prevent reception of the channel information.
Such tunable notch filters should be capable of approximately 80 dB of attenuation at the center frequency of the notch, and in all cases should be capable of 60 dB at the scramble signal frequency, the notch being sufficiently narrow to prevent serious degradation of the video information. In order to achieve such levels of notch attenuation, conventional filters are designed with one or more filter sections which are employed in cascade to achieve the high attenuation required. One such example of a conventional tunable filter with multiple sections is described in U.S. Pat. No. 5,278,525 issued to Palinkas, incorporated herein by reference.
What is needed is a filter that can be installed without a special tool but which breaks the connection between multiple sections when removed, thus rendering the filter useless for unauthorized reinstallation.