Television system transmitting enciphered data signals during field blanking interval

A television system for broadcasting data simultaneously with but independently of television programs, in which digitally coded information carrying signals are superimposed or otherwise added to a video system in the field blanking interval. The local receiver is fitted with a decoder by means of which a viewer is able to select a particular page of supplementary information which is then superimposed over the broadcast program. Generally, this information is available to any member of the public who has a requisite decoder. In this system the digitally coded information carrying signals are enciphered prior to transmission in accordance with a predetermined key and the receiving terminal has a cipher circuit operable in accordance with the key to decipher the enciphered signals. In a preferred form each cipher circuit is a pseudo-random sequence generator for generating pulses in accordance with the predetermined key and the output of which is connected to one input of a gate, the other gate input being connected to the data signals. By enciphering the signals prior to transmission some pages can be dedicated for the private use of selected viewers to whom the cipher key is available.

This invention relates to television systems for broadcasting data 
simultaneously with, but independently of, television programs. 
Recently a number of such systems have been proposed for more efficiently 
utilizing broadcast television channels either by providing supplementary 
material relating to a program or transmitting additional unrelated 
information. Among the systems which have been proposed, the ORACLE and 
CEEFAX systems are best known. The ORACLE system is described in a paper 
by G. A. McKenzie and entitled "ORACLE--An Information Broadcasting 
Service Using Data Transmission in the Vertical Interval" and the CEEFAX 
system is described in a paper by S. M. Edwardson and A. Gee and entitled 
"CEEFAX: A Proposed New Broadcasting Service". Both papers were published 
in the Journal of the S.M.P.T.E., Vol. 83, January 1974. 
In view of the above mentioned publications, no detailed description of the 
ORACLE or CEEFAX system will be given. Briefly, digitally coded 
information carrying signals are superimposed or otherwise added to a 
video signal in the field blanking interval. The local receiver is fitted 
with a decoder by means of which a viewer is able to select a particular 
`page` of supplementary information and this is then superimposed over the 
broadcast program. The information is, therefore, available to any member 
of the public who has the requisite decoder. 
According to this invention a television system comprises a transmitting 
terminal including a source of video signals, means operable to 
superimpose or otherwise add to the video signal without interfering with 
the picture signal, digitally coded information carrying signals, and a 
receiving terminal for displaying a picture corresponding to a picture 
signal and including means for extracting from the video signal and for 
storing the information carrying signals and a decoder operable to convert 
the stored signals into a repetitive vision signal for selective display 
of the information by the receiver, wherein the digitally coded 
information carrying signals are enciphered prior to transmission, in 
accordance with a predetermined key, and wherein the receiving terminal 
comprises a cipher circuit operable in accordance with the said key to 
decipher the enciphered signals. 
By enciphering the digitally coded signals prior to transmission, some 
`pages` can be dedicated for the private use of selected viewers, to whom 
the cipher key is available. Applying the enciphered signals to a receiver 
having only a conventional decoder, would produce meaningless colors, 
graphic symbols etc. 
Preferably, the transmitting terminal comprises a cipher circuit identical 
to the receiving terminal cipher circuit, for enciphering the digitally 
coded information carrying signals prior to transmission. 
The invention also includes a television receiving terminal for use in a 
television system wherein digitally coded information carrying signals are 
superimposed or otherwise added to the video signal without interfering 
with the picture signal and wherein the digitally coded information 
carrying signals are enciphered prior to transmission in accordance with a 
predetermined key, the receiving terminal being adapted for displaying a 
picture corresponding to the picture signal and comprising means for 
extracting from the video signal and for storing the information carrying 
signals, a decoder operable to convert the stored signals into a 
repetitive vision signal for selective display of the information by the 
receiver, and a cipher circuit operable in accordance with the said key to 
decipher the enciphered signals. 
In a preferred embodiment, each cipher circuit comprises a generator for 
generating a sequence of pulses in accordance with a predetermined key, 
and the output of which is connected to one input of a gate, the other 
gate input being connected to the data signals. Preferably, the generator 
is a pseudo-random sequence generator.

In the current ORACLE and CEEFAX systems data pulses are transmitted on 
otherwise unused television lines during the field blanking interval using 
a bit rate of 6.9375 M bit/s, each television data line carrying 
information for a row of forty 8-bit (including one parity bit) 
characters. One television frame is referred to as `page` and consists of 
twenty four, forty character rows including a page header row which 
contains information for control and display purposes. The first few 
characters in each row are clock run-in, framing code and control and row 
address group characters. 
Up to 800 pages of data in the form of digitally coded signals are stored 
in a central processor unit 10, from which data is passed via interface 
controller 12. The signals pass bit by bit through a cipher circuit 14 to 
a data transmitter 16 which actually constructs the signal for insertion 
equipment 18. The cipher circuit 14 is normally inhibited but is actuable 
by a control signal from the central processor 10, to encipher the 
digitally coded information carrying signals, in accordance with a 
predetermined cipher key set by a selector 19. 
Referring now to FIG. 2, the entire video signal is passed to the data 
receiver 20 which checks for the presence of a unique start code in the 
page header and then searches for a selected page number. The extracted 
page of information is fed via a cipher circuit 22 in which the 
appropriate cipher key is set by selector 26, to the text signal generator 
input store 24 where the deciphered digitally coded signals are stored so 
that a complete page can be fed through a data signal video converter 29 
and displayed continuously, even after the input to the generator store 24 
has been removed. 
Timing for the positioning of the data signals is controlled by a display 
controller 25 which is driven by a standard synch separator 23. As good 
video frequency response is needed for ORACLE and CEEFAX, a filter 27 is 
provided to eliminate traces of sound carrier on the video signal. 
A video switch 28 enables the stored page of data to be substituted for the 
normal television picture. 
The transmitting cipher circuit 14 and the receiving cipher circuit 22 are 
identical and can be set to produce an identical sequence of pulses in 
accordance with a predetermined cipher key. They operate by gating the 
sequence of pulses respectively with the digitally coded signals from the 
interface controller 12 in the transmitting terminals, and the video 
signal at output from the data receiver 20 in the receiving terminal. 
If an n-bit shift register has its input controlled by certain of its 
stages, it can be made to pass through (2.sup.n -1) different states. This 
device is known as a maximum length sequence generator or a pseudo-random 
sequence generator. The cipher circuit shown in FIG. 3 includes a 
pseudo-random sequence generator 30 having a 16-stage shift register 32 
made up of two serially connected 8-bit shift registers 34 and 38. 
Outputs A and C of the register 34 and outputs D.sub.1 and H.sub.1 of the 
register 38 are connected respectively to exclusive OR gates 40 and 42, 
the outputs of which are connected to a further exclusive OR gate 44 
supplying the serial input of the first register 34. The generator 30 has 
a pseudo-random sequence of (2.sup.16 -1)=65,535 different states. 
In use, a preselected 16-bit number is applied to the inputs a-h and 
a.sub.1 -h.sub.1 and upon receipt of a load signal L at 46, the number is 
set in the register. The generator 30 is driven by a clock (not shown) 
providing a clock signal C connected to inputs 48 and supplying pulses in 
synchronism with the data signal. Each clock pulse shifts the register to 
a new state. 
Output H.sub.1 is applied to one input of an AND gate 50, the output of 
which is gated with the serial input data signal in an exclusive OR gate 
52. The other input of the AND gate 50 is a digital control signal and 
with this control signal set to binary 1, the exclusive OR gate 52 changes 
a data signal binary 0 to a binary 1 and vice versa, when the output 
H.sub.1 is binary 1 and leaves the bit unchanged when output H.sub.1 is 
binary 0. The cipher circuit is effectively inoperative when the control 
signal is binary 0, the data signal being passed through the exclusive OR 
gate 52 without modification. 
If required, the digital control signal of input to AND gate 50 may be 
manually switched or supplied automatically at the transmission terminal, 
by the central processor 10 or, at the receiver, by the data receiver 20. 
The load signal L is generated also by the processor unit 10 or data 
receiver 20, at the end of the control and row address group of the first 
row transmitted after row 0 which is the page header row, and each 
subsequent row, so that only the message need be enciphered or deciphered. 
Because the encipherment changes throughout the message, unauthorized 
decipherment is made even more difficult and the system is applicable both 
to decoders where the receive clock runs continually, and to those where 
it restarts each line. 
The preselected number which constitutes the cipher key, as described 
above, is set in the register 38 by the selector 26, the number being 
entered therein by means of, for example, thumb wheels or push buttons. 
This latter method produces a more flexible system which is secure against 
all but computer analysis. If the cipher key (i.e. the preselected number) 
is changed frequently and is known only to the message originators PG,9 
and the intended viewers, the system can be completely secure. One or more 
of the bits a to h and a.sub.1 to h.sub.1 could be made to change with the 
time of transmission and this would make the information even more 
difficult to decipher. Similarly, the page or row number may form part of 
the selecting code. 
If high security is desired, messages may be enciphered, using a cipher 
circuit such as described above, before being passed to the broadcasting 
agency for transmission. 
The above makes reference only to television broadcasting, but it will be 
understood that the invention is equally useful in closed circuit systems.