The SSAVI Cable Scrambling System           
by Mad Phone-man

-------------------------------------------------------------------------------
Short of the D.E.S. based Video Cypher system, one of the most sophisticated
and versitile video/audio scrambling systems is the S.S.A.V.I. system. The 
acronym is the "Suppressed Sync Active Video Inversion". Zenith has exploited
this system for years and later enhanced versions, known as Z-Tac and A-Tac
have SSAVI at their roots.
SSAVI was sucessfully marketed to numerous over-the-air subscription services,
most of which are defunct now. In the wake of these services, however are 
thousands of SSAVI decoder units being sold by a variety of companies and
individuals for use on CATV systems.
 
There is an inherent problem in this re-marketing of STV units in that the
Zenith tuner has been stripped of its VHF  capability. The STV services were
UHF systems. The STV SSAVI units, therefore, had no need for VHF tuning 
capabilitys.
There are, on the other hand, SSAVI units whose initial purpose was CATV based
and which do have VHF tuning capability. The average consumer, however is hard
pressed to know just what he might receive when ordering a SSAVI unit. 
companies employ a variety of techniques to modify the STV (UHF) units for VHF
reception. There are also numerous revisions of the SSAVI units, all from
Zenith, that date back to the pre-VLSI era. Most units which the author has
dealt with do employ VLSI technology and therefore are minus an entire PCB
which earlier models had mounted in their top shells and accomidated discreet
circutry, later replaced by a single VLSI device (GATEARRAY).
 
The old discreet versions are the most versitile in terms of modifying, but
least available in numbers. Schematic diagrams for the discreet SSAVI devices
are available from Shojiki Electronics, (716) 284-2163 
This article, therefore will deal with the SSAVI units at a more superficial
level. There are 4-modes of operation obtained from 3 variables available to
the SSAVI operator. These variables are:
 
       1) Normal/ Suppressed sync
       2) Normal/ Inverted video
       3) Normal/ Suppressed audio
 
The 4 video modes of operation thus yield:
       1) Normal video/ Normal sync
       2) Normal video/ suppressed sync
       3) Inverted video/ Normal sync
       4) Inverted video/ suppressed sync
 
The first of these modes is "clear" transmission or "non-scrambled". The 
remaining three are designed to foil reception by standard TV receivers. In the
case of mode 2, thw sync pulses are offset from their normal "Blacker-than
black" position such that the front and back porch of the sync pedistal are at
+80 IRE units. This action prevents the sync-seperator in a standard TV from
stripping off the sync pulses. The result is that horizontal sync is lost and
the picture tends to "tear" or roll horizontaly. In addition, the AGC circutry
is confused and tends to DC clamp the blackest portion of the video to the
sync level.
 
The level used in maximum security is mode 4. In this mode, the video is 
inverted between each horizontal sync pulse from line 25 to line 260 of the
active scan lines. The sync pulses are suppressed as described earlier, but
NOT inverted. This is a clever technique to foil pirate decoders. This is 
because if one simply inverts the composite video, one also inverts the sync
pedestal, thus inverting the chroma burst on the sync back porch. Thus the 
video chroma (color) will be incorrect. The successful decoder must, therefore
invert the video ONLY between horizontal sync pulses, and provide an offset
pulse gated to shift the sync pulses back to their normal level.
 
To further compicate matters, modes 1-4 may be switched between at random,
under command of the head end, to foil simple static decoders which cannot 
automaticly track these mode switches.
 
Audio in the SSAVI system may also be displaced, preventing reception on a
standard TV receiver. It, when desired can be shifted, SCA style, to a
subcarrier. One can see that the SSAVI system provides a fairly high degree
of security.
 
The availability of SSAVI units and their employment by unauthorized persons
caused some inital grief for CATV operators. To render the SSAVI units non-
usable, Zenith changed the video inversion key employed by the CATV-SSAVI units
to differ from the STV SSAVI units.
 
The SSAVI units key on the binary level transmitted during the second half of
line 20 during the vertical blanking interval. When this level is high, the 
comming frame is to be inverted. When this level is low, the comming frame is
to be normal (non-inverted).
 
SSAVI cable systems, therefore, employ a couple of techniques to foil STV units
which are keying on line 20. One technique involvs maintaining the video in the
inverted state, but transmitting a "bogus" line 20 ke to cause the STV SSAVI
units to switch states at a random, frequent rate. This results in "flashing".
The picture switches between normal and inverted at a high rate producing an
annoying FLASH syndrome.
 
Another technique used by CATV-SSAVI systems is to transmit the bogus line 20
signal as described, but to transmit the key on line 21 which then allows the
video to become dynamically switched from normal to inverted once again.
 
Shojiki sells a manual on a circut called Z-trap. This circut foils the first 
of these techniques by returning control of the line 20 key to the user. The
circut provides the user with a switch which selects between high and low for
insertion during line 20. The circut stops the flashing.
The more state-of-the-art systems like Z-tac use a still different inversion 
key. The sync suppression technique is never-the-less, identical to the SSAVI
system.
It is an easy mater enough to use a STV-SSAVI unit for CATV reception. One
need only to employ a "block converter" ahead of the SSAVI unit. The STV-SSAVI
units can be tuned through the upper 2/3 of the UHF spectrum by means of a
multi-turn pot inside the unit. The block converter will up-convert CATV
frequencys into this same band of freqs. The block converters are available 
from Radio Shack and the likes.
 
The limitation in the block converter technique is that hyper-band and a large
portion of the super-band signals fall above UHF channel 83 and above the 
SSAVI's tuning range. For systems where all premium channels are in the 
mid-band, however, this technique works well.
 
If one has need of access to super/hyper band channels a converter-to-block
converter to SSAVI hook-up works equaly well with an important caveat. The
converter must NOT re-modulate the video. Converters which provide mute/volume
control capability are therefore not acceptable. The reason for this follows.
The hook-up then, looks like this:
 
 
 
CATV-> Converter -->ch 3 --> Block conv --> ch 34-36 -->SSVI -->ch 3 --> TV
         down        out       up           tunable
 
The SSAVI decoder relies on a 504khz syncronizing signal derived from the 
carrier itself. Therefore, down converters which re-modulate destroy this 
reference and cause the SSAVI to malfunction. Simple hetrodyning down-
converters allow the SSAVI unit access to the actual carrier of the 
transmitted video.
To circumvent all these frequency conversions, many resellers install small
VHF tuners into the SSAVI units. Depending on the quality of the tuner, the
reception may be better or worse than the multi-conversion system.
 
SSAVI units may also be modified to "skew" their internal timing so as to key
off of line 21 so as to be compatable with systems whose real inversion key 
resides on line 21 as described earlier. The older discreet IC SSAVI units lend
themselves to this most readily. The VLSI equipped units, never the less can 
also be modified to be one scan line shifted, by interuption of the 504khz
reference for 32 cycles.
