CABLE TV SCRAMBLING TECHNIQUES by The Mad Phone-man
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There are 4 major methods of pay-channel security and each has different
consequences for
cable ready receivers. The 4 systems are jamming, trapping
out-of-band scrambling and
in-band scrambling.
Jamming:
A jamming signal is placed between the picture carrier
and and the aural
carrier of the secured channels. The cable operator supplies a filter
for
each customer for each paid channel. This type of security is easily defeated
by
homemade notch filters.
Trapping:
In these systems frequency filters are installed
in line with the cable
drops on telephone poles. The traps are removed for customers
paying for the
premium channels. Cable-ready TV's work fine in these systems.
Scrambling
- The gated Sync Methods:
Scrambling in the cable TV business still generaly means
pulsed sync
suppression. In its simplist form, amplitude of the picture carrier is
reduced
by 6 db during the horizontal blanking intervals and sometimes during the
vertical
blanking intervals. The resulting video signal has sync tips between
the black and white
levels. Sync seperators in the set cannot operate properly
with this signal, nor can AGC
and color circuts, so the picture is scrambled.
The decoder compensates by antennuating
the signal during the time in which
the transmited signal was not antennuated. In order to
accomplish this, the
logic controlled gain switch must get timing information. In-band
systems
transmit pulses as amplitude modulation of aural carrier or a seperate carrier
in
out of band systems.
Out of band scrambling:
The usual setup is that the decoder
is connected directly to the cable
ahead of the channel converter. Decoding is done at the
pay channel frequency.
The decoder is likely to be in a seperate box, added to an old
system to
provide pay channels. The box consists of a simple receiver (90-120mhz) for
the
out-of-band data carrier and a broad band 6db gain switch. There is provision
for
several scrambled channels, each which has a different data carrier.
This system is
directly compatable with cable ready receivers. Without the
cable converter, the decoder is
connected to the TV. Tuning and remote features
of the TV are preserved with the only
inconvience being the need to operate the
switch on the decoder when changing to and from
any scrambled channel. Out-of-
band systems tend to last until the operators using them
rebuild to provide for
a large increase in the number of channels.
In Band
Scrambling:
In this system any number of the available channels can be scrambled.
Because the data carrier for each scrambled channel is its own aural carrier,
only one
data receiver, at the aural carrier frequency (eg. ch 3) is required.
The decoder detects
the presence or absense of data automaticly switching
itself in or out.
The converter-decoder box can be hardwired to decode just the
channels ordered,
using a prom like device. Alternatively, the transmitted
channels can be "tagged"
by time division multiplexing binary tag (program
identification) data with the sync data
on the aural carrier. The decoder
boxes can be wired for "tiers" (groups of programs the
cable operator sells
togeather) rather than fixed channels, giving the operator more
flexibility.
The decoder boxes can be "addressable". These boxes have a seperate out of
band
data channel for data from the head end. Each box has a serial number burned
into
its logic or otherwise available to its logic circutry, and its channel
or tier
authorization stored in volatile ram. A computer at the headend
periodicaly addresses
all decoders in the system individualy and loads each
with the channel or tier capacity
ordered by the customer. The need for house
calls is reduced, PPV (Pay per view) is
possible, and missing boxes cam be
turned off, rendering them useless for premium channel
viewing. Some but not
all of these features can be programmed into out-of-band systems.
Aside form their ability to generate sync pulses, thus foiling the scrambling
system,
cable ready TV's have presented another dificult problem for in-band
systems. Because the
decoder operates at the converted channel, a channel
converter is required ahead of it.
Wheather the TV receiver is cable-ready or
not, it operates only at the converted channel,
wasting the tuning and remote
control features.
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