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with the coded information, as the premium services were also added The control computer sent converter activation refresh signals along with premium channel selections to the addressable converters on a rotating basis The more premium subscribers a system had, the longer the refresh time cycle If a converter was unplugged or disconnected from the cable system, it would automatically shut down and become dormant This prevented passing the addressable converter around or, as they say, kept the box from walking Still, actual signal security depends on the level of signal scrambling or encoding methodology 6242 The more complex scrambling is, the more secure the signal becomes Also, the more complicated the scrambling technique becomes, the more dif cult the descrambling method Of course, the added complexity of the whole process results in a higher cost The two basic categories for signal scrambling are analog and digital Analog methods are often regarded as soft and digital methods as hard, referring to the signalsecurity level As discussed previously, signal-trapping methods using either positive or negative traps are regarded as soft signal security This signal could be defeated simply by removing the negative trap (usually at the tap) or replacing it with a look-alike dummy trap The positive trap, which was needed to remove the interfering carrier, could be placed at the subscriber s TV set Such traps are easy to obtain and can often be found in the back merchandising pages of electronic hobby magazines A more secure analog method is the sync suppression method, which can be done at RF or on the base-band video signal The horizontal sync pulses are essentially suppressed a suf cient amount, causing the TV set to lose its horizontal hold Both methods used a 1573-KHz sine-wave signal to suppress the sync pulse, which is shown in Figure 6-16 In order to unscramble the signal, the reverse process is used Essentially, a sine wave at the same frequency is used to restore the synchronizing pulse and the brightness level This sine wave is transmitted on the cable system either just above the FM broadcast band or on any FM carrier and is transmitted as part of the FM audio signal on the scrambled channel The unscrambling process is performed in the set-top converter usually just after the input converter circuit The restored or unscrambled picture is then converted to the set-top output frequency, usually channel 3 or channel 4 Another analog horizontal sync suppression method is the gated sync suppression method This method can be used for RF, IF, or base-band video In the RF/IF methods, pulses with the same timing as the horizon-
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Waveforms for RF sine-wave scrambling Sync pulse
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Figure 6-16 Sync-suppression methods using sine waves
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tal sync for the channel to be scrambled are applied to the RF or IF signal suppressing the sync pulse, usually at 6 to 10 dB Some signal scramblers have the amount of suppression selectable at 0 (no suppression), 3, 6, or 10 dB Suppressing the horizontal synchronizing signal fools the television receiver s sync separator circuits, thus preventing the set from locking the picture Other methods provide increased signal security because they further changed the video signal A form of video inversion can be successfully used in conjunction with the gated sync method Inverting the video signal (between horizontal sync pulses) interchanges the white and dark portion of the picture, resulting in an image like a lm negative Unscrambling is more complicated and hence more expensive to cable operators who think the higher level of security is worth it Zenith Electronics Corporation used this type of signal scrambling on its Z-TAC system Because digital television is upon us, digital encoding with highly complex encrypting methods will offer cable operators extremely high levels of signal security Two methods of digital encoding are line shuf ing and line dicing Of course, by mentioning lines we are referring to NTSC
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