Cable TV Scrambling Techniques
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.