72 lines
4.4 KiB
Plaintext
72 lines
4.4 KiB
Plaintext
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CABLE TV SCRAMBLING TECHNIQUES by The Mad Phone-man
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There are 4 major methods of pay-channel security and each has different
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consequences for cable ready receivers. The 4 systems are jamming, trapping
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out-of-band scrambling and in-band scrambling.
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Jamming:
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A jamming signal is placed between the picture carrier and and the aural
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carrier of the secured channels. The cable operator supplies a filter for
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each customer for each paid channel. This type of security is easily defeated
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by homemade notch filters.
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Trapping:
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In these systems frequency filters are installed in line with the cable
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drops on telephone poles. The traps are removed for customers paying for the
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premium channels. Cable-ready TV's work fine in these systems.
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Scrambling - The gated Sync Methods:
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Scrambling in the cable TV business still generaly means pulsed sync
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suppression. In its simplist form, amplitude of the picture carrier is reduced
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by 6 db during the horizontal blanking intervals and sometimes during the
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vertical blanking intervals. The resulting video signal has sync tips between
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the black and white levels. Sync seperators in the set cannot operate properly
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with this signal, nor can AGC and color circuts, so the picture is scrambled.
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The decoder compensates by antennuating the signal during the time in which
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the transmited signal was not antennuated. In order to accomplish this, the
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logic controlled gain switch must get timing information. In-band systems
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transmit pulses as amplitude modulation of aural carrier or a seperate carrier
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in out of band systems.
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Out of band scrambling:
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The usual setup is that the decoder is connected directly to the cable
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ahead of the channel converter. Decoding is done at the pay channel frequency.
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The decoder is likely to be in a seperate box, added to an old system to
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provide pay channels. The box consists of a simple receiver (90-120mhz) for the
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out-of-band data carrier and a broad band 6db gain switch. There is provision
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for several scrambled channels, each which has a different data carrier.
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This system is directly compatable with cable ready receivers. Without the
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cable converter, the decoder is connected to the TV. Tuning and remote features
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of the TV are preserved with the only inconvience being the need to operate the
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switch on the decoder when changing to and from any scrambled channel. Out-of-
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band systems tend to last until the operators using them rebuild to provide for
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a large increase in the number of channels.
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In Band Scrambling:
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In this system any number of the available channels can be scrambled.
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Because the data carrier for each scrambled channel is its own aural carrier,
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only one data receiver, at the aural carrier frequency (eg. ch 3) is required.
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The decoder detects the presence or absense of data automaticly switching
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itself in or out. The converter-decoder box can be hardwired to decode just the
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channels ordered, using a prom like device. Alternatively, the transmitted
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channels can be "tagged" by time division multiplexing binary tag (program
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identification) data with the sync data on the aural carrier. The decoder
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boxes can be wired for "tiers" (groups of programs the cable operator sells
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togeather) rather than fixed channels, giving the operator more flexibility.
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The decoder boxes can be "addressable". These boxes have a seperate out of band
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data channel for data from the head end. Each box has a serial number burned
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into its logic or otherwise available to its logic circutry, and its channel
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or tier authorization stored in volatile ram. A computer at the headend
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periodicaly addresses all decoders in the system individualy and loads each
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with the channel or tier capacity ordered by the customer. The need for house
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calls is reduced, PPV (Pay per view) is possible, and missing boxes cam be
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turned off, rendering them useless for premium channel viewing. Some but not
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all of these features can be programmed into out-of-band systems.
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Aside form their ability to generate sync pulses, thus foiling the scrambling
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system, cable ready TV's have presented another dificult problem for in-band
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systems. Because the decoder operates at the converted channel, a channel
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converter is required ahead of it. Wheather the TV receiver is cable-ready or
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not, it operates only at the converted channel, wasting the tuning and remote
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control features.
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