yhu@umbc5.umbc.edu (Mr. Ying Hu; CMSC (GRAD)) (06/27/90)
I have seen different type of descrambler of cable signal, but have no idea how the filter works. Anyone knows about connecting a filter to the cable and descramble the signal? Any article talking about that. It seems the filter knows not to filter when the signal is not scrambled. yhu@umbc5.umbc.edu
ra1d+@andrew.cmu.edu (Robert L. Armitage) (06/28/90)
yhu@umbc5.umbc.edu.UUCP (Mr. Ying Hu) asks: >I have seen different type of descrambler of cable signal, but have no >idea how the filter works. Anyone knows about connecting a filter to >the cable and descramble the signal? Any article talking about that. It >seems the filter knows not to filter when the signal is not scrambled. 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 ..... :-) Buz.. Rob Armitage Carnegie Mellon University Pittsburgh, PA Internet: ra1d+@andrew.cmu.edu BITnet: ra1d+@andrew UUCP: ...!{ucbvax,harvard}!andrew.cmu.edu!ra1d+
dt@yenta.alb.nm.us (David B. Thomas) (06/28/90)
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
downin@dtoa1.dt.navy.mil (Downin) (06/28/90)
I was under the impression that all that was really done was the video sync signal was somehow reduced in signal strength thus causing the wavy picture and good sound. If that technique is used all you would need would be a simple signal amplifier. Anyone know for sure? Dave Downin\ downin@dtoa1.dt.navy.mil
naiming@eng.umd.edu (Naiming Shen) (06/29/90)
>wavy picture and good sound
I don't think the filter type scrambler is only mess up with sync signal,
it mess up with video and audio too.rdw2030@venus.tamu.edu (06/29/90)
In article <2417@nems.dt.navy.mil>, downin@dtoa1.dt.navy.mil (Downin) writes... >I was under the impression that all that was really done was the video >sync signal was somehow reduced in signal strength thus causing the >wavy picture and good sound. If that technique is used all you would >need would be a simple signal amplifier. Anyone know for sure? > >Dave Downin\ Check out the May 1990 issue of Radio-Electronics for a neat project that will restore the sync signals (horizontal AND vertical) to a wide variety of sources. There is also some discussion of methods of scrambling video. This project will allow MacroVision to be defeated, as well as the scrambling technique used on direct satellite transmissions, and that from the local cable company (I speak of the non-sound-distorting method!). I haven't built it yet, but I will. It should be interesting! It's called the Universal Descrambler! Hehehe... Mark C. Lowe - KB5III
markz@ssc.UUCP (Mark Zenier) (06/30/90)
In article <3525@umbc3.UMBC.EDU>, yhu@umbc5.umbc.edu (Mr. Ying Hu; CMSC (GRAD)) writes: > > I have seen different type of descrambler of cable signal, but have no > idea how the filter works. Anyone knows about connecting a filter to > the cable and descramble the signal? Any article talking about that. It > seems the filter knows not to filter when the signal is not scrambled. One of the early methods of "Scrambling" a cable channel was to add a signal. Judging from the effects on the signal, where the sound was overridden and ripples in the video from a signal about 1 Mhz away, I guess it was an FM sound signal added 4.5 Mhz below the color subcarrier. (or 4.5 - 3.579 Mhz below the video carrier). When you paid the premium, the cable company would put filter in line to take this jamming out. A couple of folks built filters from a photocopy of an article from an Austin Texas newspaper with varying degrees of success. With adressable decoders, Videochipher, Orion, and other stuff, I assume this is ancient history. markz@ssc.uucp
paul@hpldola.HP.COM (Paul Bame) (06/30/90)
Go to your friendly SAMS book outlet and find their book on TV scrambling. It's also been in magazines for several years - with a not too bad series in Radio-Electronics - check the periodical guide at your local library. In short: There are many scrambling methods in use today but they fall into about 4 (could be more) categories for cable TV - and another for pay satellite. One scrambling method does involve narrow-band jamming and it is old and easy to defeat with the notch filter as mentioned. You can purchase probably-suitable notch filters as "interference filters" from companies advertising in the back of popular elecronics magazines. Most recent methods mess with the video sync in one or more ways - sometimes (varisync) to the detriment of your properly unscrambled picture. The earliest of these was probably gated sync supression. Moving the sound to another frequency is also done. You can buy your own descrambling boxes of any kind from these magazine advertisers too - ostensibly to save the monthly box rental fee - not to dodge the cable services fee. -Paul "Spice is the Variety of Life" paul@hpldola.hp.com N0KCL
cgordon@vpnet.chi.il.us (Gordon Hlavenka) (07/04/90)
ON-TV used a system (back in the goodle days...) where the sync pulses were attenuated. The sync information was then AM'd as a sine wave onto the FM audio subcarrier, which in turn was in a non-standard location. You had to retune the audio tank, recover the sync, phase shift it by some amount, integrate it, then mix the spikes back into the video. The easier-to-build boxes had their own audio and speaker; you just turned the volume down on the TV... Nowadays, of course, most of this scrambling stuff is done with digital encryption, or at least digital access to the descrambler box. ---------------------------------------------------------- Gordon S. Hlavenka cgordon@vpnet.chi.il.us Disclaimer: He's lying
tschoole@blackbird.afit.af.mil (Tony L. Schooler) (07/10/90)
rdw2030@venus.tamu.edu writes: >Check out the May 1990 issue of Radio-Electronics for a neat project that will >restore the sync signals (horizontal AND vertical) to a wide variety of sources. >I haven't built it yet, but I will. It should be interesting! It's called the >Universal Descrambler! Hehehe... >Mark C. Lowe - KB5III Would this project also take care of the flicker from rented tapes or improve the quality of the video (sisters weding tapes look real bad)? How do I back order May 1990 issue of Radio-Electronics? -- ----------------------------------------------------------------------- Anthony Lamar Schooler tschoole@blackbird.afit.af.mil
freeman@eola.UCF.EDU (07/10/90)
About 10 years ago, our local cable Co. jammed HBO by putting a narrow interference signal in the middle of the Channel 4 video. Picture was wavy, audio was clean. They installed band-reject filters in your line to remove (about 95% of) the noise if you paid for the service. Anyway, we made some cheap "HBO Boxes" out of 300-ohm twin lead cable. We'd simply cut the twin-lead to about 1/4 wavelength and hang it off the VHF antenna input screws. If that got us close, we'd wrap aluminum foil (a strip maybe 2 inches wide) around the twin-lead, and slide it up and down to tune out the noise. For what we paid, it produced a watchable picture. I think that was before spectrum analyzers were invented. |^) MarkJ freeman@eola.ucf.edu
depolo@eniac.seas.upenn.edu (Jeff DePolo) (07/19/90)
On some of the older systems, there are filtering methods that will work. In fact, when you decide to "buy" one of the pay channels, they just come to your house and put a filter in line. Probably the most common scrambling system nowadays is SSAVI, which stands for suppressed-sync audio-video inversion. It's basically works just like it says. The sync signal is supressed below what a normal TV is capable of locking on and the audio and video are "swapped" positions inside the normal TV channel bandwidth. So, to get a usable signal, you first have to swap the audio and video back to where they are supposed to be, and they boost the sync signal back up to a usable level. Obviously filtering the signal doesn't get you anywhere. I'm no expert on the subject, so maybe somebody else can give you more and better info. --- Jeff +-----------------------+-----------------------------------------------------+ | Jeff DePolo N3HBZ/AA | Internet: depolo@eniac.seas.upenn.edu | | Univ. of Pennsylvania | RF: 146.685- 224.40- 442.70+ 144.455s (Philadelpia) | | Computer Science Eng. | Twisted pair: (215) 386-7199 home | | Class of 1991 | Carrier pigeon: 420 South 42nd St. Philly, PA 19104 | +-----------------------+-----------------------------------------------------+