The system relates to a system for preventing unauthorized viewing of CATV etc. wherein each terminal is provided with a terminal own address generator and an auxiliary address generator. The center station allows outputs of respective address generators to be sent thereto in conformity with a time map to compare a signal sent back from the center state with the own address and the auxiliary address of the terminal of concern. The center station confirms that there is no overlap of addresses of respective terminals to provide a receiving signal permission if an abnormal condition is not found. Such an address overlap takes place at the time when the system is initially powered or when the pirater enters. Thus, when an overlap of addresses occurs, the address of the terminal concerned is changed. For such an address, an initial address is used as the own address, and an address based on the pseudo number is used as the address offset and the auxiliary address. Since the overlap when the system is initially powered is mainly due to the addresses attached at the time of fabrication at a factory, an address offset is added, thereby to cope with this overlap. In addition, since the overlap after the system has been powered is due to the enlargement or the invasion of the pirator, an address offset is changed or an address based on the pseudo number used to cope with such an overlap.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
This invention relates to an anti-tapping system for cable televisions, 
etc., and more particularly to an anti-tapping system capable of securely 
performing protection of tapping in bidirectional cable television 
systems. 
2. Technical Background of the Invention and Problems Therewith 
Generally, in a pay broadcast which provides programs to only subscribers, 
reception terminals are given their own addresses. In order to prevent a 
nonsubscriber from tapping a program, only when the coincidence of the 
addresses of a terminal is detected, the terminal is authorized to view 
the desired program. That is, the reception terminal having an address 
corresponding to a polling from the center sends an answer signal to the 
center, and the center responds to the answer signal to authorize the 
subscriber to view the desired program and performs processing such as 
fee-charging. 
FIG. 11 is a block diagram showing a conventional anti-tapping system. The 
center 1 transmits a pay program signal through a trunk 2, and further 
bidirectional couplers 3 and 4 to the respective subscriber terminals 10 
and 20. As an example, subscriber terminal 10 will be taken. A program 
signal transmitted downstream from the center is separated by a 
bidirectional coupler 11 and applied through a signal path 12 to a 
converter tuner 13 where the carrier wave of the received program is 
converted to a predetermined frequency and then applied to a TV receiver 
14. 
The converter tuner 13 performs the frequency conversion for only the 
subscriber concerned. At this time, whether the frequency conversion is 
permitted or not is controlled due to the fact that a data processor 15 
transmits data to the center and receives it therefrom via a modem 16. 
Usually, in the case of a pay broadcasting system, the center 1 loads a 
contracted channel map indicative of a contracted channel corresponding to 
the address of that subscriber down on each terminal. Each subscriber 
terminal makes reference to the contracted channel using its own address 
to control its converter tuner 13 for authorization operation thereof. 
Namely, the contracted channel map loaded down by the center 1 using the 
address obtained by an address generator 17 of the terminal 10 is 
referenced. Unless the subscriber terminal 10 is authorized by the map 
thus referenced, the frequency conversion at the converter tuner 13 is 
stopped, or descrambling operation is stopped to prevent viewing of a pay 
program. 
Charging for each subscriber's viewing of a pay program is processed at the 
center 1 by forming charging maps in RAMs of the data processors 15 in the 
respective subscriber equipment to collect them by polling. 
As just described above, each contracted channel map showing the 
relationship between subscriber's address and contracted channel is stored 
and held by data processor 15 through a modem 16 using a downstream 18. 
The center 1 collects by polling the charging maps formed in the data 
processors 15 of the respective subscriber equipment through upstreams 19 
to process them as the pay program viewing data. 
The above conventional anti-tapping system prevents tapping using the 
respective terminal addresses alone, so that if a noncontractor steals the 
subscriber's address and causes an address generator 17 to produce that 
address, the following illegality will be performed. 
It is assumed that the subscriber terminal 10 is a legal subscriber, and 
that a person of nonsubscriber terminal 20 uses the address of the 
subscriber terminal 10 by stealth. At the illegal nonsubscriber terminal 
20, the converter tuner 13 is then authorized to view a pay program, so 
that the illegal nonsubscriber 20 can view the pay program illegally. 
If the upstream 19 is shut down at the illegal nonsubscriber terminal 20, 
the transmission of the charging data in response to the polling by the 
center 1 is stopped and illegality is also performed in the charging 
operation. 
Such illegal use of the subscriber's address is easily possible even if the 
address generator 17 is of a mechanical type such as a dip switch or of 
the type which is written into a ROM, so that the normal use of the system 
is hindered. 
SUMMARY OF THE INVENTION 
This invention has been made in view of the above problems, and its object 
is to provide an anti-tapping system which can prevent tapping even if the 
subscriber's address is used by stealth. 
In an anti-tapping system for cable television systems, etc., according to 
this invention, pseudo number data generated by a pseudo number generator 
is added as an auxiliary address data to the subscriber's own address. The 
viewing of a program is authorized using the subscriber's own address and 
pseudo number data. 
This prevents tapping due to stealth of the subscriber's own address.

DETAILED DESCRIPTION OF THE EMBODIMENT 
An embodiment of this invention will now be described with reference to the 
drawings. FIG. 1 is a circuit diagram showing an embodiment of an 
anti-tapping system according to this invention. In FIG. 1, reference 
numerals 100 and 200 denote the center and a subscriber terminal, 
respectively. The center 100 includes a program signal generator 110 which 
generates program signals indicative of pay programs, etc., and a data 
processor 120. The program signal generator 110 includes a video signal 
generator 111 and an audio signal generator 112. The signals from these 
generators 111 and 112 are subjected to a predetermined modulation by a 
modulator 113 and then transmitted to the respective terminals. 
On the other hand, the data processor 120 transmits data to the respective 
subscriber terminals and receives them therefrom, and also functions to 
control the respective terminals and to charge the respective terminals a 
fee for viewing, etc. These functions are performed based on the data 
processing by a CPU 123 through a modem 121 and a data transmit-receive 
controller 122. This CPU 123 executes data processing of a channel map 
buffer 124 and a pseudo number hold buffer 125. Formed in the channel map 
buffer 124 is a data map including data as to whether the pay programs are 
contracted in correspondence with respective subscriber terminal 
addresses. The pseudo number hold buffer 125 temporarily receives pseudo 
numbers generated at the respective subscriber terminals when the 
subscriber terminals are operated and then sends the pseudo numbers back 
to the corresponding subscribers. The sent-back pseudo numbers are used as 
data for preventing unfair tapping at the respective terminals. The 
operation for prevention of the unfair tapping using pseudo numbers at the 
side of the terminal subscriber will be described in more detail later. 
The program signals from the program signal generator 110 and a series of 
transmission data produced by the data processor 120 are transmitted 
through a bidirectional coupler 130 to the subscriber terminals. 
Then, the anti-tapping operation based on transmission and reception of 
data between the center and the subscriber terminals using pseudo numbers 
generated at the respective subscriber terminals will now be described. 
When a terminal control 210 of the subscriber terminal 200 is operated for 
viewing a pay program, a data transmission request signal is sent to the 
center 100. In response to this, the center 100 sends a transmission 
permission signal back to the subscriber terminal 200 concerned. The 
establishment of such a communication procedure is performed, for example, 
using the basic mode control procedure and the high level data link 
control procedure. When the communication procedure is established, the 
subscriber terminal 200 responds to a command from the center 100 to 
format at a data formatter 240 its own address produced by an address 
generator 220 and a pseudo number from a pseudo number generator 230, and 
then sends the formatted data through a modem 250 and bidirectional 
couplers 260 and 270 to a trunk 400. The center 100 temporarily stores at 
the pseudo number hold buffer 125 the pseudo number data sent from the 
subscriber terminal 200. The center 100 makes reference to whether or not 
the subscriber has contracted a pay program channel which is desired to be 
viewed formed in the channel buffer 124 according to need and then sends 
the address of the subscriber terminal 200 as well as the corresponding 
pseudo number held in the pseudo number buffer 125 through the 
transmit-receive controller 122 back to the subscriber terminal 200. 
At this time, the pseudo number sent back from the center 100 is extracted 
by a data extraction circuit 280 of the subscriber terminal 200. The 
pseudo number data extracted by the data extraction circuit 280 is 
compared by a pseudo number comparator 290 with the pseudo number sent by 
the pseudo number generator 230. When the pseudo number data sent from the 
subscriber terminal 200 and the pseudo number data sent back from the 
center 100 are coincident with each other, the comparison result from the 
pseudo number comparator 290 is output to a terminal P1. In addition, the 
address produced by the address generator 220 of the subscriber terminal 
200 and sent to the center 100 is compared by an address comparator 300 
with the address sent back by the center 100. The comparison result from 
the address comparator 300 is output to a terminal P2. The respective 
comparison results from the pseudo number comparator 290 and address 
comparator 300 are sent to a data processing circuit 310. If the 
coincidence data in respect to the pseudo number and the address is 
obtained at the data processing circuit 310, it is permitted to make 
reference to data necessary to view a pay program such as a contracted 
channel map loaded down by the center 100 and stored in a RAM 320. 
When viewing of the pay program is permitted, the data processing circuit 
310 performs a control operation necessary to obtain a descrambled signal 
at the output of the converter tuner 330 from the pay program signal 
separated by the bidirectional coupler 260. This allows for the viewing of 
the pay program on a television receiver 340. 
The scrambling processing applied to a pay program may be performed either 
based on the system of applying direct processing to a video signal e.g., 
applying an inversion processing to a video signal or based on a so-called 
jamming system to shift the carrier frequency of the video carrier wave. 
The processing for descrambling a scrambled pay program signal to permit 
the subscriber to view the pay program is executed at the data processing 
circuit 310 and the converter tuner 330 by making reference to a 
contracted channel map according to need. 
In this invention, the converter tuner 330 may be authorized in accordance 
with the contracted channel map loaded down onto the subscriber terminal 
by the center instead of scrambling the pay program signal. 
Namely, in this invention, the subscriber terminal 200 sends its own 
address and pseudo number temporarily to the center 100. The center 100 
sends them back to the subscriber terminal 200. The subscriber terminal 
checks whether or not the sent and sent-back data coincide in respect of 
address and pseudo number. When the coincidence in respect of address and 
pseudo number is detected at the subscriber terminal, the subscriber 
terminal concerned is permitted to view a pay program. Even in the case 
whether there is a terminal which uses the terminal address illegally, the 
coincidence between the address sent to the center and the illegally used 
address will be detected, however, no coincidence in respect of pseudo 
number is detected. Thus tapping of a pay program by the terminal which 
has used the address illegally occurs prevented. This is because the 
pseudo number data serves as a kind of password key by handling as seeming 
address data the combination of the terminals own address and the pseudo 
number produced by the terminal even if the terminals own address is used 
illegally. 
FIG. 2 is a response diagram showing the transmission and reception of data 
between the terminals and the center in order to prevent tapping of the 
programs. First, when the terminal 200 initiates an operation to view a 
pay program, a transmission request signal is sent to the center 100. In 
response to this, the center 100 sends to the terminals 200 a transmission 
permission signal and a first command to require to send the terminal 
address and pseudo number data back to the center. 
In response to this, the terminal having an address pertinent to the first 
command sends its own address and pseudo number data to the center 100. 
The center 100 receives the data sent back from the terminal and again 
sends the data by means of a second command back to the terminal. At this 
time, the pseudo number data serves as an auxiliary address. The terminal 
which has received the data from the center compares both data indicative 
of address and pseudo number data with its own data. In this case, if 
there is a terminal which uses the address illegally, the terminal can 
receive data from the center 100 by the illegal use of the address. When 
this terminal compares its own data with that from the center, however, 
the coincidence between the pseudo number data at the terminal and the 
data from the center cannot be obtained. Thus, illegal tapping of the pay 
program is prevented. 
While in the above embodiment the center 100 is described as sending back 
the data, which is transmitted from the terminal 200, as it is, if a 
pseudo number generating means of the pseudo number generator 230 is used 
by stealth, security against illegal tapping becomes a problem. 
In order to cope with this problem, the center 100 may refer to time maps 
corresponding to the respective terminals without sending back the pseudo 
number data, which is sent from the terminals, as it is, and perform clock 
control of the respective pseudo number generators 230 at the terminals 
using the times determined by these maps. Namely, for a time period 
designated by the center 100, a pseudo number obtained by controlling a 
clock generation time produced by a clock generator 360 using a clock 
controller 350 may be used as an auxilliary address for a particular 
terminal. The control of a terminal by the center using this auxiliary 
address would prevent tapping of a pay program even if the pseudo number 
generation sequence of the pseudo number generator 230 is used by stealth. 
FIG. 3 shows the format of downstream data trains sent from the data 
processing unit 600 of the center station to the terminal 800 wherein 
there are seven kinds of data trains labeled R01, R10, R11, R12, R13, R14 
and R15. 
These data trains are used in order to construct the system, respectively. 
At first, six data derived from the data train R01 are separately used in 
the case where the overlap of addresses is found out at the time when the 
system is initially powered, where the overlap of data is foundout at the 
time when the pirater enters, and where the overlap occurs during the use 
of the system, respectively. 
These data trains will be explained in turn from R01. 
The data train R01 consists of an address offset set command flag FD01, an 
initial address A01, an address offset set data D01 and a parity P01. The 
address offset set command flag FD01 comprises, for example, such as shown 
in FIG. 5, a command and data discriminator 11, a command content 12, and 
a kind discriminator 13 of transmission data. The discriminator 11 
indicates whether a signal which is to be sent is a command or data. 
Responding to this discriminator, the content of reception can be seen on 
the side of the terminal. In addition, the discriminator 13 shows the kind 
of data. 
Following the address offset set command flag FD01, the initial address A01 
is provided. The initial address includes numbers attached at the time of 
fabrication of the terminal, i.e., TV set. This address can be said as a 
physical address when viewed from its content. An address offset A11 
serves to provide a countermeasure when there are provided a plurality of 
terminals where initial addresses overlap with each other. At the time 
when the system is initially powered, the overlap is confirmed. If the 
overlap is confirmed, the center station produces a command to use such a 
address offset. 
Finally, the parity P01 for effecting error check follows. 
The data train R10 is used when pseudo numbers are caused to be produced at 
terminals. In this data train, a pseudo number transmit request command 
flag FC01 and an address offset A11 are substituted for the address offset 
set command flag FD01 and the address offset set data D01, respectively. 
This data train serves to change the address of the terminal using a 
pseudo number when the overlap of not only initial addresses but also 
address offsets is produced, for example, due to the invasion of the 
pirater, thereby to prevent overlap of addresses. When this data train is 
sent to a terminal, the terminal effects the operation for generating 
pseudo numbers. 
The data train R11 is used subsequently to the data train R10 in the case 
of causing the terminal to generate the pseudo number. This data train 
serves to change the address of a terminal using pseudo number when the 
overlap of not only initial addresses but also address offsets is 
produced, thereby to prevent the overlap of addresses. When such a data 
train is sent to a terminal, the terminal sends pseudo number data back to 
the center station. 
The data train R12 serves to send data including the pseudo number data 
received from a terminal back to the terminal concerned by the designation 
of timing to make a comparison of the pseudo numbers. To this end, there 
are included therein a pseudo number comparison request flag FD11 and a 
pseudo number data D11. 
The data train R13 is used in the case of providing an offset value to a 
terminal, for example, at the time of the enlargement of the system. This 
data train is characterized in that there are included therein an address 
offset change request flag FD12 and a new address offset value D12. This 
address offset value is used as a new offset value at the terminal. 
The data train R14 serves to send a channel map to a terminal at the stage 
where the terminal has been recognized as a true receiving terminal. This 
data train is characterized in that there are included therein a channel 
map set flag FD13 and a channel map data D13. 
Finally referring to the data train R15, this data train is used in the 
case of providing a descramble allowance or permission after a channel map 
has been sent to a true receiving terminal. This data train is 
characterized in that there are included therein a descramble allowance 
flag FD14 and a descramble key data D14. 
FIG. 4 shows three kinds of upstream data trains sent from terminals to the 
center station in accordance with the downstream data trains from the 
center station, wherein these data trains are labeled T01, T11 and T12. 
When viewed in respect of the address data, the data train T01 is deemed 
as the most primitive one indicating the initial address as its content. 
The data train T11 is obtained by adding an address offset thereto, and 
the data train T12 is obtained by further adding a pseudo number thereto. 
Namely, the data train T01 comprises an ACK command transmit flag FC03, an 
initial address A02, and a parity P03. The data train T11 comprises an ACK 
command transmit flag FC21, an initial address A02, an address offset A21, 
and a parity P21. The data train T12 comprises a pseudo number data 
transfer flag FD21, an initial address A02, an address offset A21, a 
pseudo number data D21, and a parity P22. 
FIGS. 6A to 6E show the segmented portions of the communication sequence 
between the center stations and the terminal using various kinds of data 
trains which has been explained with reference to FIGS. 3 and 4, 
respectively. 
Initially, FIG. 6A shows an address offset set sequence. When the data 
train R01 is sent from the center station to a terminal, the data train 
T01 is sent back from the terminal as the ACK command. The data train R01 
comprises an address offset set command flag FD01 and an address offset 
set data D01. Responding to them, the setting of the offset is made on the 
side of the terminal. 
FIG. 6 shows an address offset change sequence used when the terminal 
addresses overlap with each other. When the data train R13 is set from the 
center station toward a terminal as the REQ command, the data train T11 is 
sent back from the terminal as the ACK command. In the data train R13, 
there are included an address offset change request flag FD12 and a new 
address offset value D12. The new address offset value is set on the side 
of the terminal. 
FIG. 6C shows a channel map transmit sequence executed when a terminal is 
authorized. The data train R14 is sent from the center station toward a 
terminal as the REQ command, and the data train T1 is sent back from the 
terminal as the ACK command. In the data train R14, there are included a 
channel map set flag FD13 and a channel map data D13. The terminal can 
receive a pay program using the channel map data. 
FIG. 6D shows a pseudo number comparison sequence in the case where a data 
train to which the pseudo number data is added is used for the reason of 
respective overlaps of the initial addresses and the address offsets. In 
this case, the REQ command is sent three times from the center station 
toward the terminal and the ACK command is sent back three times from the 
terminal to the center station. The data train R10 is first sent from the 
center station as the REQ command. Responding to this, the data train T11 
is sent back from the terminal as the ACK command. By this procedure, the 
terminal is to be specified in principle. However, when the overlap of the 
terminal addresses is present, for example, due to the invasion of the 
pirator, such an overlap must be avoided using the pseudo number. Thus, 
the center station sends, to the terminal, the data train R11 which is the 
REQ command for sending the pseudo number. Responding to this, the 
terminal sends the data train T11 including the pseudo number data as the 
ACK command back to the center station. As a result, the center station is 
supplied with the pseudo number data and sends the data train R12 
including this pseudo number to the terminal as the REQ command. The 
terminal compares the address data possessed by the terminal itself with 
the address data from the center station. As a result, when they are 
coincident with each other, the terminal effects the receiving operation. 
When otherwise, the terminal does not effect the same. 
Responding to this receiving operation, the terminal sends the data train 
to the center station as the ACK command. 
FIG. 6E shows a transmit sequence of a key data for descramble required for 
reception by the terminal. The center station sends the data train R15 
including the descramble key data 14 to the terminal as the REQ command. 
Responding to this, the terminal sends back the data train T11 as the ACK 
command. 
FIG. 7 shows the detailed configuration of the center station which 
comprises a scramble processing unit 500 for applying the scramble 
processing to a program signal, and a data processing unit 600 for 
effecting the timing control of the scramble processing and the 
transmission control of data. The scramble processing unit 500 corresponds 
to the program signal processing unit 110 of FIG. 1, and the data 
processing unit 600 corresponds to the data processing unit 120 of FIG. 1. 
The scramble processing unit 500 mixes an image signal from an image signal 
generator unit 501 and an audio signal from an audio signal generator unit 
502 using a mixer 503 to deliver the output thus mixed to a TV signal 
transmit device 504 and to a scramble encoder 506. The scramble encoder 
effects the scramble processing in accordance with the Sync offset system 
or the Sync suppress system, or a technique such as image inversion, which 
has been well known in the art. The program signal thus scrambled is 
transmitted to a main line 700 via the TV signal transmit device 504. A 
scramble timing generator 505 determines the timing for effecting, for 
example, the Sync offset scramble to form a timing signal in response to 
the output of the data processing unit. 
The data processing unit 600 serves to perform bidirectional communication 
with a terminal shown in FIG. 8, and effects the processing of the 
downstream and upstream data and commands. An upstream data collision 
detection circuit 602 of the data processing unit 600 has an upstream data 
detection function to detect the collision of the upstream data from the 
terminal 800. The collision detection circuit 602 detects the overlap of 
addresses of the terminal 800 using the collision detection function. This 
upstream data collision detection circuit 602 will be described later in 
conjunction with FIG. 9. This circuit 602 detects whether a plurality of 
terminals are in the simultaneous responding states to detect the 
collision of the upstream data. If the collision has occurred, the address 
of the terminal is changed to avoid the collision of the upstream data. 
The collision of the upstream data occurs in the case (1) where initial 
addresses of respective terminals, i.e., addresses attached at the time of 
fabrication of respective decoders partially overlap at the time when the 
system is initially powered and in the case (2) where the pirater which 
has stolen the subscriber address at the time when the system is initially 
powered. 
When there is a collision in either case, its detection is made by the 
upstream data collision detection circuit 602. The output P1 of the 
collision detection circuit 602 indicates whether or not the length of the 
received data exceeds a predetermined value. The output P2 thereof 
indicates whether or not the parity detection is normally made. These 
outputs P1 and P2 are fed to an AND circuit 603. When there is a collision 
of the upstream data, the output S01 of the AND circuit becomes "0". The 
output S01 of the AND circuit 603 is fed to a receiving buffer 604 as well 
as a data computing and processing circuit 609. As a result, the receiving 
buffer 604 is supplied at the clear terminal CLR with a clear signal when 
the collision of the upstream data collision has been detected. At this 
time, upstream data T01, T11 and T12 which have been taken in the upstream 
data collision detection circuit 602 are cleared. The output S01 of the 
AND circuit is also fed to th data computing and processing circuit 609, 
whereby the operation for reception is blocked. 
On the other hand, in the case where the collision of the upstream data is 
not detected by the upstream data collision detection circuit 602, the 
data T12 is once taken in the receiving buffer 604, and is then to the 
flag discrimination circuit 605. Then, the dat T12 is fed from the flag 
discrimination circuit 605 to an address extraction circuit 606, an 
address offset extraction circuit 610, a pseudo number data extraction 
circuit 612, and an ACK command confirmation circuit 616. Thus, codes in 
the data trains transmitted are taken out separately. The initial address 
of the data train from the terminal 800, which has been transmitted as the 
upstream data, is extracted by the address extraction circuit 606. The 
output A02 of the address extraction circuit 606 is fed to the address 
comparison circuit 607. In addition, initial address, i.e., specific 
physical addresses according to another expression of respective terminals 
are held in the address map hold buffer 607A. 
On the other hand, the address offset data which has been extracted at the 
address offset extraction circuit 610 is fed to the above-mentioned 
address comparison circuit 607. Namely, to the address comparison circuit 
607, the initial address and the address offset which have been obtained 
via the upstream line are delivered. The initial address and the address 
offset which have been obtained via the upstream line are compared with 
data registered in advance in an initial address map hold buffer 607A and 
an address offset map hold buffer 611. The compared result is delivered to 
an AND circuit 608. As a result of comparison, when both inputs are not 
coincident, a "0" output is produced for the output S02, while when they 
are coincident, a "1" output is produced therefor. Thus, the data 
computing and processing circuit 609 performs the operation for reception 
or the operation for prevention of reception. 
The comparison of the address by the above-mentioned address comparison 
circuit 607 is made only to the initial address when the system is 
initially powered. When there is competition of the initial addresses, 
such a comparison is also made to the address offset data in addition to 
the initial address. 
The output D21 of the pseudo number data extraction circuit 612 is 
delivered to a pseudo number data buffer 613 for pseudo number comparison 
request, and to a pseudo number data map hold buffer 617. The pseudo 
number is transmitted via a downstream line where comparison is mae at the 
terminal 800. 
Various kinds of codes extracted are delivered to the address map held 
buffer 607A, the address offset map hold buffer 611 and the pseudo number 
data map hold buffer 617. These buffers are controlled respectively by 
outputs S03, S05 and S06 of the data computing and processing circuit 609 
to deliver their outputs to a transmit buffer A, 620 and a transmit B, 
622. In addition, the data computing and processing circuit 609 controls a 
descramble key data generation circuit 618 by the output S09 and a channel 
map hold buffer 619 by an output S10. Thus, the data computing and 
processing circuit 609 sends codes and key data of various data trains as 
well as a channel map as the service information to the terminal in 
accordance with the output S01 of the AND circuit 603 and the output S02 
of the AND circuit 608. In sending them, the data computing and processing 
circuit 609 controls a transmit flag set circuit 615 by the output S11 to 
read data which have been stored in advance in two transmit buffers 620 
and 622. Outputs of the address map hold buffer 607A, the address offset 
map hold buffer 611, the hold buffer 613 for the pseudo number comparison 
request, the address offset set and change buffer 614, and the channel map 
hold buffer 619 are delivered to these transmit buffers 620 and 622. In 
addition, an output of the descramble key data generation circuit 618 is 
delivered to the transmit buffer 622. 
By operating a transmit data selector 623 using an output S04 of the data 
computing and processing circuit 609, switching of the transmit buffer A 
having data of low secret stored therein and the transmit buffer B having 
secret data stored therein is carried out. The data thus taken out is 
delivered to the bidirectional coupler 601 via a data formatter 624 and a 
downstream data modulation circuit 625, and is then transmitted to the 
terminal via the main line 700. The buffer A holds data to be transmitted 
in common to respective subscriber terminals as open data. The other 
buffer B holds data to be transmitted to specified subscriber terminals. 
FIG. 8 shows an example of a terminal which communicates with the center 
station shown in FIG. 7, wherein this terminal communicates with the 
center station via the main line. Namely, the terminal 800 is connected to 
the main line 700 via a bidirectional coupler 702. A portion of the signal 
taken out from the bidirectional coupler 702 is delivered from the 
bidirectional coupler to a converter tuner 803 and a descramble control 
circuit 808C via another bidirectional coupler 802. 
The descramble control circuit performs the operation corresponding to the 
scramble encoder 506 of the center station in FIG. 7 on the basis of the 
control signal from a descramble timing signal generation circuit 808B 
which will be described later. It delivers a control signal to the 
converter tuner 803 to deliver a TV signal to a TV receiver 900. An output 
S20 of a terminal data computing and processing circuit 812 connected to a 
terminal input/output device 813 to which an input such as a channel 
selection by the viewer is fed is also delivered to the converter tuner 
803. The output S20 is used for the operation of control in conjunction 
with the control signal from the above-mentioned descramble timing circuit 
808B. 
On the other hand, another signal which has been taken out from the 
bidirectional coupler 801 is delivered to a parity calculation circuit 805 
and a parity comparison circuit 806. When parities are not coincident, a 
clear signal is delivered to the receiving buffer 807 to cancel the 
received signal. In contrast, when parities are coincident, the output of 
the receiving buffer 807 is delivered to a flag code discrimination 
circuit 808. The flag code discrimination circuit 808 delivers respective 
data outputs to the descramble decoder 808A, a channel map data reference 
enable circuit 809, an address offset extraction circuit 817, an initial 
address extraction circuit 819, a pseudo number generation permission 
confirmation circuit 822, and a pseudo number data extraction circuit 824. 
Initially, the data train R15 is delivered to the descramble decoder 808A. 
The key data D14 extracted therefrom is delivered to the descramble timing 
signal generation circuit 808B. 
Respecitve outputs of an address offset comparison circuit 818, an initial 
address comparison circuit 820, and a pseudo number data comparison 
circuit 825 are delivered to the descramble timing signal generation 
circuit 808B to effect the control operation. Thus, the descramble timing 
signal generator 808B generates a timing signal necessary for the 
descrambling operation. 
The data train R14 is delivered to the channel map reference enable circuit 
809. The output D13 extracted therefrom is delivered to a data write 
control circuit 810. The data write control circuit 810 responds to 
outputs of the address offset comparison circuit 818 and the initial 
address comparison circuit 820 to effect a control for writing the output 
D13 into the initial address comparison circuit 820. The contents of the 
channel map hold buffer 811 are processed by the output S21 of the 
terminal data computing and processing circuit 812. 
The data train R01 is delivered from the flag code discrimination circuit 
808 to the address offset set circuit 814A to produce an output D01. The 
output D01 thus produced is delivered to an address offset value hold 
buffer 815. To the address offset value hold buffer 815, an output D12 
which the address offset change circuit 814B has taken out from the data 
train R13 from the flag code discrimination circuit 808 is also delivered. 
When an output of a pseudo number data buffer 826 is supplied via a data 
transfer control circuit 816 controlled by a signal S24 from the terminal 
data computing and processing circuit 812, the address offset hold buffer 
815 stores it as a changed address offset value. Outputs D01, D12 and A21 
of the address offset value hold buffer 815 are delivered to an address 
offset comparison circuit 818. The address offset comparison circuit 818 
compares an output of the address offset value hold buffer 815 with an 
output ALL of an address offset extraction circuit 816, thereby to detect 
coincidence or anticoincidence. The compared result S25 is delivered to 
the terminal data computing and processing circuit 812, the descramble 
timing signal generator 808B, and the data write control circuit 810. In 
the case of the coincidence output, the descrambling and the channel map 
referencing are blocked. Thus, no receiving is conducted. 
Similarly, the receiving operation is blocked also by the output S26 of an 
initial address comparison circuit 820. The initial address comparison 
circuit 820 compares the output A01 of the initial address extraction 
circuit 819 with the output of the initial address generator 821 to detect 
coincidence or anticoincidence. All data trains are delivered from the 
flag code discrimination circuit 808 to the initial address extraction 
circuit 819. 
Further, blocking of the receiving operation is carried out also by the 
output S27 of a pseudo number data comparison circuit 825. The pseudo 
number data comparison circuit 825 compares the output of a pseudo number 
data extraction circuit 824 to which data trains R12 and R15 are delivered 
from the flag code discrimination circuit 808 with the output of a pseudo 
number data buffer 826. The pseudo number data buffer 826 responds to the 
output of a pseudo number generation control circuit 822 which generates a 
pseudo number generation command when data trains R10 and R11 are 
delivered from the flag code discrimination circuit 808 to hold pseudo 
numbers that a PN generator generates. 
In addition, blocking of the receiving operation is carried out also by the 
output S28 of the flag code discrimination circuit 808. This output is 
produced when the flag code discrimination circuit 808 has received the 
pseudo number transmit request command. The fact that the circuit 808 has 
received this command means that the overlaps of the initial addresses and 
the address offsets have occurred. In this case, it is required to avoid 
such overlaps using the pseudo number. To this end, the receiving 
operation is blocked. 
The output of the above-mentioned initial address generator 821 is 
delivered to transmit buffers 828, 829 and 830. The output S27 of the 
above-mentioned address offset hold buffer 815 is delivered to the 
transmit buffers 829 and 830. In addition, the output of the 
above-mentioned pseudo number data buffer 826 is delivered to the transmit 
buffer 830. 
Respective outputs of these buffers are taken out by the data formatter 832 
via the transmit data selector 831 and are sent to the main line 700 via a 
MODEM 804 and couplers 801 and 702. A data selector 831 is controlled by 
the terminal data computing and processing circuit 812, whereby one of 
three transmit buffers is selected. At this time, a transmit flag set 
circuit 827 responds to the control signal S23 by the terminal data 
computing and processing circuit 812 to set flags of respective buffers. 
Thus, one of three kinds of ACK commands shown in FIG. 4 is selected and 
sent. 
An apparatus shown in FIG. 8 effects the processing dependent upon whether 
the selected channel input from the terminal input/output unit 813 is the 
ordinary contract channel or the pay contract channel. Responding to the 
signal S20, when the ordinary contract channel is selected, a frequency 
control is conducted to inhibit the input from the descramble control 
circuit 808C to the converter tuner 803 to directly input the TV signal 
from the coupler 801 to output it to a TV receiver 900. In contrast, when 
the pay channel is selected, a frequency control is conducted, in a manner 
opposite to the above, to inhibit the direct input from the coupler 802 to 
output to the TV receiver 900 the input signal from the descramble control 
circuit 808C to the converter tuner 803. 
FIG. 9 shows a detailed circuit configuration of the upstream data 
collision detection circuit 602 provided in the center station. In the 
upstream data collision detection circuit 602, the format sent from the 
terminal via the bidirectional couplers 701 and 601 is amplified at a RF 
amplifier 602A. Then, the format thus amplified is subjected to the band 
selection at a broad band-pass filter 602B and is then converted into an 
intermediate frequency signal at an FM front end 602C. Only a necessary 
frequency portion of the intermediate frequency signal is taken out at a 
narrow band-pass filter 602D and is then delivered to a carrier level 
detection circuit 602E and an FSK data demodulation circuit 602J. 
The carrier level detection circuit 602E detects the carrier to deliver its 
output to a level slice circuit 602F to obtain a rectangular wave. The 
rectangular wave is delivered to a monostable multivibrator 602G and to an 
OR gate 602H. Since the monostable multivibrator 602G outputs a signal 
continuing for a predetermined time from the rise of the rectangular wave, 
the output of the monostable multivibrator always has a duration equal to 
or more than the output of the monostable multivibrator 602G. The outputs 
of the monostable multivibrator 602G and the OR gate 602H are delivered to 
an exclusive NOR 602I. The exclusive NOR 602I produces "0" output when the 
duration of the output of the level slice circuit 602F exceeds the 
duration of the output of the monostable multivibrator 602G. In contrast, 
in the case of the relationship opposite to the above, the exclusive NOR 
602I maintains "1" output. 
On the other hand, the output of an FSK data demodulation circuit 602J 
includes formats T01, T11 and T12. This output is delivered to the 
receiving buffer (FIG. 7) which is not shown here and is also delivered to 
a data buffer 602K for parity calculation. The data train is delivered 
from the data buffer 602K for parity calculation to a parity extraction 
circuit 602L. Thus, the parity data is taken out therefrom and the parity 
is calculated by a parity calculation circuit 602M. Outputs of the both 
circuits are delivered to an exclusive NOR 602N to check whether or not 
they are coincident. The exclusive NOR 602N produces "0" output in the 
case of anticoincidence in the same manner as the output of the exclusive 
NOR 602I. 
Outputs of the exclusive NOR 602I and the exclusive NOR 602N are both 
delivered to the AND gate (FIG. 7). 
FIG. 10 shows a detailed circuit configuration of the P.N. generator 823 in 
FIG. 8. This generator is called M-series pseudo number generator. The 
arrangement shown comprises five stages of flip-flops 823A to 823E, a 
shift register 823F, exclusive NORs 823G to 823H, a clock generator 823I, 
a system reset pulse generator circuit 823J, and a monostable 
multivibrator 823K. 
The P.N. generator 823 serves to place the system in reset state for a 
predetermined time at the starting time of the system, e.g., at the time 
of insertion of the AC plug by a starting circuit constituted with the 
system reset pulse generator circuit 823J and the monostable multivibrator 
823K which are operated in response to a command from the center station. 
The P.N. generator 823 delivers an output of the multistage flip-flop 
which is operated at a predetermined cycle determined in dependence upon 
which stages are connected to the exclusive NDRs 823G and 823H to the 
serial input Si of the shift register 823F, thereby to obtain a serial 
signal. A pseudo number output based on the serial signal appears on the 
P.O. terminal. 
A permission signal from the pseudo number generation permission 
confirmation circuit 822 (FIG. 8) is delivered to the LD terminal of the 
shift register 823F. Responding to this, the output D21 is delivered to a 
pseudo number data buffer 826. 
In accordance with the present invention, the center station is not 
provided with a special configuration for specifying respective terminals 
and each terminal itself is instead provided with such a configuration. 
Accordingly, this allows the system configuration to be simplified. 
As described above, in an anti-tapping system according to this invention, 
the use of pseudo number data produced by the respective terminals as an 
auxiliary address to the terminal's own addresses will prevent tapping of 
a pay program even if the terminal's own address is used by stealth.