ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI Pay Tv-Scrambling ATI ATI ATI ATI By ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI Advanced Telecommunications Inc.. How Scrambling Works -_-_-_-_-_-_-_-_-_-_ First, lets review the characeristics of a standard T al. In order to produce a picture, the entire face of the icture tube is scanned line-by-line, starting at the top lef and continuing to the right and down, in what is called rastr scan. Each complete scan is called a frame, and takes 1/30th f econd to complete. During that 1/30th 525 lines are ransmitted. Therefore, in one second there are 15,750 lines transmitted. Inorder to produce a readable image on the screen, necessary to transmit sinchronizig (or sync) pulses. Those ulses are used to assure the scan begins at the correct time Both vertical- and horizonal-sync pulses are used for ths purpose. A vertical-sync pulse defines the beginning of each frame field(half a frame) and a horizontal-sync pulse defines the eginning of each line. In addition to sync pulses, there is a additional signal called a blanking pulse. To understand te purpose of the blanking pulses, rember how the raster scan goes by-line from left to right down the screen. In order for it o get from the end of one line to the boginning of the next, it ust sweep back or retrace it's path. However, during the brief eriod when it is doing that, it is nessary to turn off the beam blank it. That is nessary to turn off the beam, or blank it. hat is the function of the blanking pulse. The sync pulses are uperimoposed on the blanking pulses. What happens is that every time the beam sweeps to the end a line, or frame, it is extinguished or blanked by the orizontal or vertical blanking pulse before it resets to the eginning of the next line, or frame, by the sync pulse. Now, suppose we alter the charactristics of the sync pulses, r even remove them entirely. What happens? You guessed it-a on the screen instead of a picture. The waveforme of a crambled signal have there characteristics:Non-standard sync an blanking pulses. That Standard signal is compared to a standad al, Which represents about 1 1/2 loines of video information. With anormal signal, the set's circuitry expects to see the olor-burst signal during a specified interval. If it dosn't ecognize the blanking pulse, it can't reognize the color-burst l eather. So the set's circuitry is eather unsyncronized or osn't work at all. Just by changing one part of the signal-the lanking pulse-it is possible to destroy not only the pictur sync but also the color sync. All that has been done to the scrambled video signal has to reduce the horizontal blanking-signal level below the ideo-signal level. To reconstitute the picture, all that is equired to restore everything to it's original levels. That can e done verry simply by momentarily increasing the gain of the Tv ing the Horizontal-blanking interval. There is a verry important correlation between the sound and the restoration of the sync and blanking pulses. Program audo is transmitted via a 31.5-kHz sub-carrier, the information n sub-carrier is double-sideband, supressed carrier signal. n order to demodulate it, a refrence signal, or pilot carrier, s required. The frequency of that pilot carrier is one-half the odulating-carrier frequency. That number-15.75-is the zontal-line frequency we talked about earlier, and represents he number of horizontal-sync pulses generated in one second. bviously there is some correlation here. A pilot-carrier signal can be used as a timing reference to eate the sync and blanking pulses. The pilot carrier nerates a 15.75-kHz square wave signal in a decoder IC such as an LM1800. That signal in turn is used totrigger two cascaded ne-shot multivibrators that produce a gate pulse of exactly th width and phase as the horizontal-blanking pulse. That gate pulse is used to increase the IF gain of the TV reciver durrig the horizontal-blanking interval, restoring the sync- ad blanking-signal strengths to normal. The method is the same as the one used to transmit the rence information that produces an FM-stereo signal. And, ince the same process is involved, the same hardware can be used to retrieve the signal. The only difference between the stero em and this system is the way the signals are retrived on the utput. In a stereo, the main-channel signal is added to th sub-carrier signal to produce two separate channels. In te unscrambling application, the main channel signal is subtracte, only the sub-carrier signal is used. C) 1986 Advanced Telecommunications Inc.. Master Blaster (313) ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI ATI DOWNLOADED FROM P-80 SYSTEMS..... ������������������������������������������������������������������������������������������������������������������