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TUCoPS :: TV, Cable, Satellite :: paytvati.txt

Pay-TV Scrambling by ATI

                    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 
 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 
    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
t  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
troy 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 puls
 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 si
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)

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