Path: utzoo!attcan!uunet!maverick.ksu.ksu.edu!unmvax!bbx!yenta!dt From: dt@yenta.alb.nm.us (David B. Thomas) Newsgroups: sci.electronics Subject: Re: Descramble Cable Using Filter? Message-ID: <1560@yenta.alb.nm.us> Date: 28 Jun 90 05:36:16 GMT References: <3525@umbc3.UMBC.EDU> Organization: yenta unix pc, Rio Rancho, NM Lines: 29 ra1d+@andrew.cmu.edu (Robert L. Armitage) writes: >I thought they just added a "noise" signal that a simple >band-pass filter could eliminate. >I'm sure if I'm wrong, some one will "enlighten" us ..... :-) You're right. Some cable companies use a narrow band noise signal to jam the 'premium' service, and a band-reject filter will eliminate it. (Not that I would ever actually BUILD one of these, mind you, but if I had I would have used a 100 ohm pot, a 150 pf variable cap, two coils consisting of four turns of #20 wire wound on a 1/4 inch form, loosely spaced... and I would have made a "pi" filter with the two coils and one capacitor, tying the bottoms of the coils together to the wiper of the pot and grounding another of the pot's terminals. This would have been to adjust the bandwidth of the notch thus hypothetically created.) There are other schemes, which I have never been sophisticated enough to understand or defeat, but which are nonetheless beatable. About the slickest system I heard of uses a decoder box capable of decrypting the signal in some ridiculously large number of ways. (This will sound familiar to most unix buffs!) Every so often a 'newcode' control message comes in, causing the box to automatically adjust to a new code. Followups to alt.cable-tv.tech.myths :-) Okay, someone else pick this up! David