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.. 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