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Proc-Type: 2001,MIC-CLEAR
Originator-Name: keymaster@town.hall.org
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 pKb9/DClgTKIm08lCfoilvi9Wl4SODbR1+1waHhiGmeZO8OdgLUCAwEAAQ==
MIC-Info: RSA-MD5,RSA,
 XUgw/azXDCKYCfmBdFjSMKzC6axd9dnMO8WjcWO4hbWGQhtQk5PW5bjrwfWpSP2j
 ygTsAu5aDLRMhuByJa5LJw==


PATN  Patent Bibliographic Information
WKU     Patent Number:				05319708
SRC     Series Code:				7
APN     Application Number:			9535187
APT     Application Type:			1
ART     Art Unit:				222
APD     Application Filing Date:		19920929
TTL     Title of Invention:			System for the simultaneous coding of a number of television signals and
      for decoding thereof in radiofrequency
ISD     Issue Date:				19940607
NCL     Number of Claims:			13
ECL     Exemplary Claim Number:			1
EXP     Primary Examiner:			Buczinski; Stephen C.
NDR     Number of Drawings Sheets:		4
NFG     Number of Figures:			12

INVT  Inventor Information
NAM     Inventor Name:				Reolid-Lopez; Ricardo
STR     Inventor Street:			Rio Vinuela, 44
CTY     Inventor City:				29650 Mijas-Costa (Malaga)
CNT     Inventor Country:			ESX

INVT  Inventor Information
NAM     Inventor Name:				Diez-Follente; Emilio
STR     Inventor Street:			Rio Vinuela, 44
CTY     Inventor City:				29650 Mijas-Costa (Malaga)
CNT     Inventor Country:			ESX

INVT  Inventor Information
NAM     Inventor Name:				Carmonia-Garcia; Jose L.
STR     Inventor Street:			Rio Vinuela, 44
CTY     Inventor City:				29650 Mijas-Costa (Malaga)
CNT     Inventor Country:			ESX

INVT  Inventor Information
NAM     Inventor Name:				Fernandez-Vinuesa; Jesus J.
STR     Inventor Street:			Rio Vinuela, 44
CTY     Inventor City:				29650 Mijas-Costa (Malaga)
CNT     Inventor Country:			ESX

PRIR  Foreign Priority
CNT     Priority Country:			ESX
APD     Priority Application Date:		19911002
APN     Priority Application Number:		P-9102165

CLAS  Classification
OCL     Original U.S. Classification:			380 15
XCL     Cross Reference Classification:			380 20
EDF     International Classification Edition Field:	5
ICL     International Classification:			H04N  7167
FSC     Field of Search Class:				380
FSS     Field of Search Subclass:			10;14;15

UREF  U.S. Patent Reference
PNO     Patent Number:					4527195
ISD     Issue Date:					19850700
NAM     Patentee Name:					Cheung

UREF  U.S. Patent Reference
PNO     Patent Number:					4598313
ISD     Issue Date:					19860700
NAM     Patentee Name:					Hendrickson

UREF  U.S. Patent Reference
PNO     Patent Number:					5034981
ISD     Issue Date:					19910700
NAM     Patentee Name:					Leonard et al.
OCL     Original U.S. Classification:			380  5

UREF  U.S. Patent Reference
PNO     Patent Number:					5144663
ISD     Issue Date:					19920900
NAM     Patentee Name:					Kudelski et al.
OCL     Original U.S. Classification:			380 16

LREP  Legal Information
FRM     Legal Firm:				Lucas & Just

ABST  Abstract

A system for the simultaneous coding of a number of television signals and
for decoding thereof in radiofrequency.

The radiofrequency TV signal or signals to be distributed undergo
remodulation on at least one channel by a signal which does not alter the
synchronism pedestal and which varies in accordance with an established
sequence resident in the memory of the coder, offering differentiated
application to each individual channel or to the whole range thereof. A
data channel is also installed to include a channel identification code,
an algorithmic code and a synchronisation code, these being the
determinants for interpretation of the codes by a decoder at the
subscriber's terminal and for application of a process of remodulation
which is identical, synchronous and inverse in form to that of the coder
in order to restore the original signal.

It is of application in the protection of subscriber TV channels.

BSUM  Brief Summary

     This invention relates to a system for the coding and decoding of
television signals modulated in radiofrequency independently of the
carrier frequency in accordance with which the aforesaid signals are
modulated.

     This invention combines a number of essential distinguishing features which
together represent a significant improvement over other systems in use,
these features, the object of this invention, being as follows:

 Both the coding and decoding processes are applied to television signals
  modulated in RF on any channel assigned, rendering unnecessary the use of
  RF demodulators in the decoding process.

 Coding may be carried out on one or more channels simultaneously.

 A single decoder is sufficient to decode a number of channels.

 Neither teletext nor any other information inserted in the unused lines of
  the image undergo significant modification during the coding and decoding
  process.

 These features make this invention applicable to cable television systems,
  in which free access channels are normally assigned alongside pay
  channels, use of which is restricted to specific subscribers.

 The field of application of this invention is fundamentally the cable
  television networks, owing to the large number of channels which it can
  encode simultaneously, without this, however, limiting its use in other
  systems of signal transmission.

     Currently, owing to the growing number of television channels and the
situation as regards distribution, it is becoming necessary for cable
operators to arrange the services offered to their customers in a manner
sufficiently flexible to adapt to the different tastes and requirements of
these latter as well as achieving the lowest possible costs both for
installation and for the customer, while at the same time guaranteeing
sufficient protection against unauthorised use of the services.

     The use of systems which differentiate between a certain number of channels
is normal on cable television distribution networks.

     One of the systems employed filters the band segments where the restricted
access channels are located.

     Another system codes the signals in video frequency and decodes them by
means of a decoder supplied to the subscriber.

     The first system mentioned involves the connection of selective band
segment filters to the subscriber feed line of the distribution network.
This system is economical to install but expensive to use, lacking
flexibility, since the subscriber has to subscribe to a fixed package of a
certain number of channels. Similarly, since the signals are carried by
the cable without any coding, the system is very vulnerable to
unauthorised manipulation, making it necessary to restrict subscriber
access to the place where the filters are located.

     The second system mentioned offers greater security against unauthorised
manipulation, a feature of these networks being high installation costs,
since in the decoding process it is necessary to demodulate the RF signal,
process it and subsequently deliver it to the television receiver, in many
cases involving its conversion to another frequency.

     The need is clear, then, for a system which embodies to the maximum extent
possible the features of protection against unauthorised use, versatility
and economy.

     The system offered by this invention gives a high degree of protection
against unauthorised use, as the description will hereinbelow demonstrate.
It furthermore offers great versatility in that it allows authorisation of
any configuration of channels to each subscriber on an individual basis.

     Economy is also a feature of this invention, in that demodulation of RF
before decoding is not necessary, this implying a consequent reduction in
the costs of installation and use.

DRWD  Drawing Description

     Other objects and advantages of this invention may be more clearly
understood by making reference to the description of the system given
hereinbelow and to the following Figures, in which:

 FIG. 1 represents the amplitude against time graph for horizontal scanning
  elements of a television picture or scanning lines.

 FIG. 2 represents the amplitude against time graph for the lines previously
  represented, negatively modulated in RF.

 FIG. 3 represents the graphs for variable amplitude against time based on
  routines laid down in the coding system.

 FIG. 4 represents the amplitude against time graph for the modulated signal
  represented in FIG. 2, remodulated by the signal represented in FIG. 3 and
  corresponding to the signal coded in RF leaving the coder.

 FIG. 5 represents the signal coded in RF on entry to the decoder.

 FIG. 6 represents the graphs for amplitude varying with time based on
  routines laid down in the decoding system.

 FIG. 7 represents the remodulated signal leaving the decoder system.

 FIG. 8 represents the amplitude against time graph for horizontal scanning
  elements of the television picture or scanning lines demodulated by the
  television receiver.

 FIG. 9 represents a block diagram of a possible coder.

 FIG. 10 represents a block diagram of a possible decoder.

 FIG. 11 represents a block diagram of a possible system of sequential
  control of initiation of coding cycles for the system of simultaneous
  coding of a number of TV channels.

 FIG. 12 represents the graphs for amplitude against time for triggering
  pulses for the sequential system in relation to the synchronism graphs for
  a number of channels and the data sequences generated over the time in
  question.

DETD  Detail Description

                         DESCRIPTION OF THE SYSTEM

     Every television signal coding system involves the processing of the signal
so that on a standard television receiver the image appearing is falsified
with respect to the original image which provided the source of the
television signal in question, making the use of a decoder an absolute
requirement in order to exactly reproduce the original signal.

     In order to describe the coding process in the system which is the object
of this invention, we shall analyse in principle its operation on a
horizontal scanning element or scanning line.

     FIG. 1 represents some video signal scanning lines, showing the segment
corresponding to the synchronism pedestal X, which has a duration of 12
.mu.sec, and the line information segment Y, with a duration of 52
.mu.sec.

     FIG. 2 represents the negative RF modulation signal corresponding to the
video signals represented in FIG. 1.

     This RF signal passes to a coding modulator which remodulates it with a
signal synchronised with the vertical and horizontal synchronisms
extracted from the video signal which modulates the first modulator. The
modulating signal passed to the coding modulator is generated in
accordance with a pre-defined routine resident in the coder. Some examples
of remodulating signals are represented in FIG. 3, where it can be seen
that, during the 12 .mu.sec of the synchronisation segment, the signal
adopts a reference value which coincides with the synchronism value. In
this way, the levels of synchronism are not modified during remodulation.
The 52 .mu.sec duration line information segment is remodulated with an
amplitude-modulated signal generated by means of pre-set digital routines.
FIG. 3 shows possible shapes for this amplitude-modulated signal, which is
capable of adopting a large variety of shapes depending on the routines
utilised from among all those defined in the coding system and resident in
its memory.

     On leaving the coding modulator, the remodulated signal presents a graph
similar to that represented in FIG. 4. This remodulated signal is, in
itself, a coded signal, since were we to supply it to a standard
television receiver, the image appearing on the screen would be very
different from the original, given that the luminance values at each point
would differ according to the changes in amplitude of the signal with
which the RF signal has been remodulated.

     This remodulation takes place on all the lines comprising a raster. If the
system automatically varies the order of the routines giving rise to the
different remodulating signal shapes every "x" lines, "x" being a variable
number within a specific segment of lines of a raster and also varying its
sequences in each field in accordance with algorithms resident in the
coder, a total falsification of the image on the television screen is
achieved.

     When the coder activates coding in synchronisation with the signal
modulated in RF, remodulating it with the signals of varying amplitude
with time generated through the predefined routines and following set
algorithms, the microprocessor has, during the time taken for the previous
line, and via a data channel, sent the codes corresponding to the
algorithms which it is using, followed by a synchronisation code for the
signal encoded by it.

     The signal modulated in RF and thus encoded is carried alongside the data
channel to the subscriber's receiver and passed to the decoder.

     The coded RF signal is passed to a decoding modulator, while the codes from
the data channel are recognised by the decoder and, according to the
algorithm codes received, the appropriate routines resident in the
decoder's memory are generated. The routines generated give rise to
amplitude-modulated signals which are the inverse of those employed in the
coding modulator and which synchronously modulate the decoding modulator.
In this way, the RF signal originating from the first coding modulator is
obtained on leaving this latter modulator.

     This decoding process is represented in the graphs in FIGS. 5 to 8. FIG. 5
shows the signal coded in RF by the coder system in the manner hereinabove
described and which is passed to the decoding modulator after being
carried to the subscriber's receiver.

     FIG. 6 represents the amplitude-modulated signals occasioned by the
routines generated by the decoder in accordance with the algorithm
received via the data channel. It can be seen here that the inverses of
the routines used by the coder are correct.

     FIG. 7 represents the outgoing signal from the decoding modulator, being
clearly identical to the outgoing signal from the first modulator before
remodulation in the coding modulator.

     FIG. 8 represents the graph for the video frequency signal demodulated by
the television receiver after decoding and which clearly constitutes a
true reproduction on the screen of the original image.

     It will be understood that in describing the system, and in order to
facilitate understanding thereof, the figures and concepts adduced herein
relate to a particular system. It is however clear that it may be applied
to any system. Nevertheless, the descriptions provided are illustrative
and do not limit the scope or object of this invention. Any change or
modification within the spirit and scope of this invention which is
evident as such to an expert in the subject shall be understood to be
included in this description as one more form which may be adopted by this
invention.

     Having provided a sufficient description of what the coding and decoding
system consists of, a possible model for application is described and,
subsequently, a description will be given of the way in which this system
makes possible simultaneous coding of a number of TV channels using a
sequential control system for the respective coders.

     FIG. 9 represents, by way of example, a block diagram of a coder. The audio
A and video V signals are passed to a line amplitude modulator M which
modulates the signals to a specific TV channel. This signal modulated in
RF is passed to a coding modulator MC which remodulates the signal in RF.
A sample of the unmodulated video signal is passed to a pulse separator S.
The horizontal and vertical synchronising pulses are sent to the
microprocessor P. When a horizontal synchronising pulse arrives, the
microprocessor P prepares an algorithm which determines which routines it
will use, for how many lines it will use each of them, and in what order
it will apply them for the "n+a" lines following the next synchronising
pulse, where "n" is the number of TV channels which the system has to code
simultaneously and "a" is a specific number of lines offset.

     At the same time, the microprocessor generates three codes, which it sends
to the data modulator DM. The first is a channel identification code, the
second is a code for the algorithm it has selected for the next "n+a"
lines, and the third is a synchronisation code. When the microprocessor P
detects the next horizontal synchronising pulse, it activates the
addresses of the memory containing the routines R in the order established
by the algorithm, these being converted via a digital-analogue converter D
to amplitude-modulated signals. These signals are then passed to the
coding modulator MC, which effects synchronised remodulation of the RF
signal from the first modulator M.

     From this moment on, the microprocessor P starts to count "n+a" lines, in
order to admit another horizontal synchronising pulse and to repeat the
cycle. The signal thus coded on exit from the coding modulator MC is
passed to the TV signal distribution system via a mixer X, where it is
combined with the data channel.

     As will be described hereinbelow, the system of simultaneous coding of a
number of channels necessitates the use of a coder for each channel to be
coded. The microprocessor in each of them has a channel selector SC which
assigns a number to the latter. The codes corresponding to each channel,
and which are sent to the decoder via the data channel, are generated over
the time taken to scan one line and it is for this reason that the
algorithms created by each microprocessor of each of the coders in use
indicate the task to be carried out over a specific number of lines, the
data channel being occupied during this time by the codes sent
sequentially by the coders. The number of lines is determined, then, by
the number of channels coded simultaneously by the system and by one or
more offset lines, which are reserved, and for which the need will be
understood when the operation of the sequential control CS as of the
system is described hereinbelow.

     It will be understood that in this way, even though the decoding codes are
sent via the data channel sequentially, encoding of all the channels coded
occurs simultaneously.

     The algorithms created by the microprocessor in the coding process may
either be generated randomly or following pre-established sequences.

     FIG. 10 represents, by way of an example, the block diagram of a system
decoder.

     The signals picked up by the subscriber's receiver from all the TV channels
(coded and uncoded) carried by the distribution system RFC are sent to a
decoding modulator MD after separation of the data channel by means of a
filter F. The data channel is demodulated and the data extracted are sent
to a microprocessor P.

     If the subscriber selects an uncoded channel SC, the decoder sends no
signal to the decoding modulator and the signals in question are sent
directly to the television receiver for viewing.

     Where the subscriber selects a coded channel, the microprocessor will
recognise from among the pieces of information it receives from the data
demodulator DD the one corresponding to the channel requested, assuming
that the channel in question is one of those which have been authorised.

     Once the channel has been identified, the microprocessor prepares the
corresponding algorithm in accordance with the information contained in
the second code it receives from the data DE modulator DD. On receipt of
the third code, which corresponds to synchronism, the microprocessor
triggers a phase detector CF through which the decoder's oscillator is
phase synchronised, and indicates the exact moment of activation of the
memory addresses R of the decoder, which stores the relevant routines and
which, using a digital-analogue converter D, forms an amplitude-modulated
signal which is passed to the decoding modulator MD. The memory addresses
so activated obey an algorithm which is generated in the microprocessor P
and which is a function of the code received from the data channel and
sent by the coder. These routines are such that the amplitude-modulated
signal which they generate is the inverse of that used as a remodulating
signal during coding.

     Once the algorithm code tells the microprocessor to which, in what form,
and for how many lines it should apply the routines, this latter remains
inhibited until it receives a new channel code, after which a new
algorithm code and synchronisation reaches it, initiating the cycle once
more.

     The other channels on the network will be ignored by the decoder, their
codes not being recognised, either because they were not selected by the
subscriber or because their use has not been authorised.

     The remodulated signal RFD leaving the decoding modulator is passed to the
subscriber's television receiver for viewing.

     FIG. 11 provides, by way of example, a schematic representation in block
form of the coding sequential control system. This consists essentially of
a microprocessor P controlled by a X which receives the synchronisms S
from each coder, these being obtained from their respective pulse
separators. This microprocessor measures the amounts of time between the
synchronisms of the different video signals as they continue to appear and
assigns them an order starting with one which it takes as a reference.

     In this way, the microprocessor sends a number of instructions H to each of
the microprocessors in the coders, triggering them individually with
respect to the pulse on which each of them should begin its coding cycle
in line with the order of arrival of the horizontal synchronising pulses
of the various video signals from different programmes, one of which is
taken as a reference for this process. This triggering sequence occurs
with a separation of 64 .mu.sec, plus the difference offset from one
synchronism to another. This offset is always less than the scanning time
for a line, which explains why the sequential coding recurrence time on
each channel is equal to the scanning time for as many lines as there are
coded channels in the system, plus at least one offset line. In the light
of the foregoing, it will now be clearly understood why the algorithms
generated in the microprocessors of the coders contain all the information
relating to the routines to be used for "n" lines, plus an offset of "a"
lines, "a" normally being one line. This latter number of lines is fixed
as desired according to the features which one wishes to add to the
system. If one wishes to change the configuration of authorised channels
individually for each subscriber from the system's central unit, it will
be necessary to send a series of individually recognisable codes to each
decoder with the desired configuration of channels. We will use the offset
lines to send the said codes. So the number of lines offset will depend on
the amount of information we need to send in addition to that relating to
the coding-decoding process.

     The system offers an additional possibility in respect of protection
against unauthorised use, since each time the signal taken as a time of
arrival reference for the others varies, the sequence in which the codes
corresponding to each channel are sent by the data line changes, making it
more difficult for anyone trying, without authorisation, to decode the
coded signals by observing the recurrence frequencies of the codes on the
data channel.

     FIG. 12 gives a schematic view of the synchronisation graphs for a number
of TV signals. The first three graphs 1, 2, n, refer to the triggering
pulses from the sequential control system, which it sends to the coder for
each channel in an order which it determines. It may be observed that when
a coder is triggered on receipt of a horizontal synchronisation pulse 1,
2, n, during the time taken by the following line it transmits, via the
data channel, the three codes corresponding to channel identification I,
the algorithm code A prepared during that time and the synchronisation
code S. It should be noted that these codes are transmitted on the data
channel in the same order as that of arrival of the television signals
coded and with a separation between them equal to the time taken for one
line plus the period of delay between a channel synchronising pulse and
the one which precedes it.

     Up to now, what has been described is a system which codes and decodes a
number of television signals simultaneously and which uses a data channel.
It is held to be evident that the system is also applicable to a single
channel, the possibility of sending the relevant codes as insertions in
reserved picture lines providing a substitute for the data channel.
Therefore, the coding and decoding of a single television signal modulated
in RF in the manner described in this instrument shall be deemed to be an
obvious application of this invention and to be thus included in the said
instrument.

     Having provided sufficient description of the nature of this invention, it
should be placed on record that the items and arrangements hereinabove
described are offered by way of illustration and are therefore liable to
modifications or variations of detail, insofar as these latter do not
alter the effects and fundamental principles on which the invention rests.

CLMS  Claims
STM     Claim Statement:			We claim:
NUM     Claim Number:				1.

     1. A system for the simultaneous coding of a number of television signals
and for decoding thereof in radiofrequency, characterised in that the
coding consists of a first remodulation of all the horizontal scanning
lines of a television signal modulated in RF on any channel by an
amplitude-modulated signal in perfect synchronisation with a first signal,
the first remodulation obeying preset functions over the period of
duration of each line and wherein synchronization is controlled so that
over time corresponding to the synchronism pedestal of each line, the
amplitude of the signal is constant and equal to a reference value
extracted at random from a memory, and further characterised in that the
decoding consists of a second remodulation, at the point of use, of all
the horizontal scanning lines of the signal coded in RF by an
amplitude-modulated signal and in perfect synchronisation with the coded
signal, the second remodulation obeying sets functions which are the
inverse of those used in the coding modulator whereby there is obtained
from the decoder the same television signal modulated in RF as that which
entered the system uncoded.
NUM     Claim Number:				2.

     2. A coding system for TV signals, as in claim 1, characterised in that
coding is carried out by remodulating the signal previously modulated in
RF on any channel on which it is to be carried by the distribution system.
NUM     Claim Number:				3.

     3. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that the RF signal passed to the coder is remodulated
line by line by amplitude-modulated signals which respond to preset
functions over the period of duration of a line, the different functions
set being unlimited in number, and all of them adopting a constant value
equal to the zero reference level over the period of duration of the
synchronism pedestal of each line.
NUM     Claim Number:				4.

     4. A coding system for TV signals modulated in RF, as in claim 3,
characterised in that the amplitude of the remodulating signal during
coding is constant and equal to the reference value for horizontal
synchronism throughout the 12 .mu.sec duration of the synchronism pedestal
of the TV signal, the amplitude of the RF signal corresponding to the
horizontal synchronism pulse not being changed.
NUM     Claim Number:				5.

     5. A coding system for TV signals modulated in RF, as in claim 3,
characterised in that the line information segments of each of the lines,
having a duration of 52 .mu.sec, are remodulated by amplitude-modulated
signals and are generated by memory-resident preset routines, offering a
host of variants.
NUM     Claim Number:				6.

     6. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that the routines preset in the coder and which give rise
to the amplitude-modulated signals which synchronously modulate each line,
obey algorithms generated in the control circuits which determine the
routines and number of lines on which each of them will repeat over the
next "n+a" lines, "n" being the number of TV channels coded and "a" a
specific number of offset lines.
NUM     Claim Number:				7.

     7. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that the system has a sequential control circuit, which
triggers the start of the coding cycles, following an order coinciding
with the order of arrival of the horizontal synchronisation pulses of the
different TV signals, in relation to one of them which it takes as a
reference and wherein there is sequential recurrence of "n+a" lines for
all the signals coded, "n" being the number of channels coded by the
system and "a" being the number of lines defined for a predetermined
offset, the offset in no case being less than the time for one line.
NUM     Claim Number:				8.

     8. A coding system for TV signals modulated in RF, as in claim 7,
characterised in that when the control circuit receives a horizontal
synchronising pulse, together with a triggering pulse from the sequential
control system, it generates, during the time taken for that line, a
channel code, a code for the algorithm it is to use and a synchronisation
code, sending them, during the aforesaid time, to the data channel
modulator and activating the cycle of "n+a" lines in accordance with the
algorithm chosen, just as the next horizontal synchronising pulse appears.
NUM     Claim Number:				9.

     9. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that decoding is carried out by remodulating the signal
which is received modulated in RF and remodulated in the coder, on any TV
channel on which it has been carried to the point of use.
NUM     Claim Number:				10.

     10. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that the RF signal sent to the decoder is remodulated by
an amplitude-modulated signal which obeys preset, memory-resident
routines, thereby giving rise to a function which is the inverse of that
employed in the coding system and which is synchronised with the signal
received from the channel selected by the coding system via a data
channel.
NUM     Claim Number:				11.

     11. A coding system for TV signals modulated in RF, as in claim 9,
characterised in that during decoding, the preset routines which give rise
to the amplitude-modulated signals which are the inverse of those
generated in the coding system and which remodulate each line of the
RF-modulated signal, obey algorithms generated in the control circuits of
the decoding system according to a code received from the coding system
via a data channel and wherein the algorithms generated identical to those
which are used by the coder and which determine the routines and number of
lines on which each of them will repeat over the next "n+a" lines, "n"
being the number of TV channels coded and "a" being a specific number of
offset lines.
NUM     Claim Number:				12.

     12. A coding system for TV signals modulated in RF, as in claim 1,
characterised in that the coder sends three codes along a data channel,
these being a channel identification code, an algorithm code and a
synchronisation code.
NUM     Claim Number:				13.

     13. A coding system for TV signals modulated in RF, as in claim 9,
characterised in that both the routines used in the coding system, which
give rise to the amplitude-modulated signal which remodulates the signal
during coding, and also the routines which give rise to the
amplitude-modulated signal which is the inverse of the former, and which
remodulates the signal in the decoding system, are resident in the
memories of both of the coding and decoding systems and are not carried
from one system to the other.
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