Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SU~I~IARY OF THE INVENTIC)N
The present invention relates to pay television systems
and in particular to means for coding the video and/or audio
signals.
A primary purpose is a subscription television system
in which the video signals are coded by suppressing the
horizontal and vertical synchronizing information.
Another purpose is a subscription television system
of the type described in which the sync pulses during the
horizontal and vertical blanking intervals are suppressed and
clock and address signals are inserted therein which signals
are utilized at the receiver to control a sync pulse
generator.
Another purpose is a subscription television system
of the type described in which digital sound may be inserted
in the suppressed horizontal blanking intervals and utilized
at the receiver to provide the audio portion of the television
signal.
Another purpose is a subscription television system
in which digital sound may be inserted in the suppressed
horizontal blanking intervals to provide either the primary
audio signal or an alternate sound channel.
Other purposes will appear in the ensuing specifica-
tions, drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated diagrammatically in the
following drawings, wherein;
Figure 1 is a block diagram of the transmitting portion
of the described subscription television system, and
Figure 2 is a block diagram of the receiver utilized
with the transmitting arrangement of Figure 1.
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1~50~318
` DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention relates to subscription
television systems in which over the air television signals
are coded at the transmitter and include, with the video
signal, signals for use in enabling and authorizing decoding
circuitry at individual receivers. The synchronization pulses
in the horizontal and vertical blanking intervals of the video
signal are suppressed to approximately 25~ of the original
amplitude. Thus the picture will be distorted in both
horizontal and vertical directions and will be completely
unusable unless the required decoding equipment is operable.
Clock signals for operating the sync pulse generator in each
receiver are inserted into the vertical blanking intervals.
In addition authorization or address signals for enabling
the decoding equipment are also inserted in the vertical
blanking intervals. The audio portion of the television signal
may be clear or not coded. In the alternative, the audio
portion can be digitized and the digital sound information
inserted in the horizontal blanking intervals. In the later
instance, the sound is reconstituted at each receiver and the
program audio is then recombined, at the receiver, with the
decoded video. As a further alternative, audio information
can be included in the described horizontal blanking intervals
to provide an alternate sound channel.
In Figure l the program video signal is provided on
line 10 and the audio signal on line 12. The video signal
provides one input for a sync processor 14 and for a gate 16.
The sync processor will provide several independent signals
for use as described. The first output, along line 18 to gate
16, is effective to operate gate 16 in a manner to suppress
the sync pulses during the vertical interval of the video
signal. Thus the signal at the output of gate 16 is the normal
video signal with the vertical interval suppressed, preferably
to a level of approximately 25 percent of its original amplitude.
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The second output from sync processor 14, along line
20, is connected to gate 22. The other input for gate 22 is
the video signal from gate 16. The signal from sync processor
14 provided at gate 22 is effective to suppress the horizontal
blanking interval to approximately 25 percent of its original
amplitude. Thus, the output from gate 22 is the original video
signal with both the horizontal and vertical blanking intervals
suppressed to approximately 25 percent of their original
amplitude.
A clock generator is indicated at 24 and may provide
a clock signal of 3.58 mhz. One output from clock generator
24 goes to divide circuit 26 which provides an output signal
at 31.5 khz which is the sound sampling frequency. To digitize
sound, with a maximum frequency of 12 khz, it is necessary to
have a sampling frequency ~f at least 24 khz. For a aynamic
range of 48 dB there should be 8 bits so that the minimum bit
rate will be 192 K bit/second. Because the synchronization
frequency is approximately 15 khz and thè sampling frequency
should be at least 24 khz it has been determined to combine
two sound samples into each horizontal blanking interval.
Delay circuit 28 is effective to delay every other or alternate
sound samples as provided at the sampling frequency of 31.5 khz
from divide circuit 26. Thus, each horizontal blanking interval
will have two sound samples. In order to properly reconstitute
the audio signal it will be necessary~that at the decoder one-
half of the total number of bits inserted in each horizontal
blanking interval be delayed. In practice it has been deter-
mined that 24 bits per horizontal blanking interval time slot
may be utilized with 2 eight bit sound samples and the remaining
8 bits utilized for synchronization and control. Thus, the
signal from delay circuit 28 will be as described and will
provide one input for gate 30. At the other input for gate 30
is a timing signal from sync processor 14 along line 32 which
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will cause insertion of the described digital sound information
into the horizontal blanking intervals.
Clock generator 24 is connected to a divide circuit
38 which provides clock signals for utilization at the receiver
at 4/7 of the clock frequency of 3.58 mhz. In this connection,
the frequencies provided herein are only illustrative and the
invention should not be limited thereto. The output from divide
circuit 38 is connected to line modulator 36 which has a further
input, along line 34, from sync processor 14 which is ~tilized
to control the insertion of the clock signals in the first nine
lines of the suppressed vertical blanking interval. Line mod-
ulator 36 receives, from gate 30, the video signal with suppressed
horizontal and vertical blanking intervals and with the inserted
~igital sound. The output from modulator 36 will be the described
signal with the addition of clock signals during the first nine
lines of the suppressed vertical blanking i~terval.
A line counter 40 is connected to sync processor 14
and has its output connected to a computer 42. The output from
computer 42 is connected to an address gate 44. Computer 42
contains the addresses of all subscribers authorized to receive
subscription programs. Not all addresses will be sent in every
vertical blanking inter~al, but rather they will be sequentially
fed by the computer to the address gate for insertion in the
vertical blanking interval after the clock signals in the first
nine lines. Assuming 48 bits per address, and one address per
line, with twelve unused lines, it is possible to insert twelve
addresses in each vertical blanking interval. In normal opera-
tion, however, prior to the actual transmission of the subscription
program, all receivers will be addressed as it will be possible
to utilize the entire video transmission. The addressing during
each vertical blanking interval can be utilized to continually
enable the decoders in each receiver once they have been initially-
enabled by a transmission prior to the start of the subscription
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program. Thus, the output from address gate 44 will be the
above-described video signal, with the digital sound inserted
in the horizontal blanking interval and with address or enabling
information in the vertical blanking interval after the above-
described clock signals. This signal is sent to transmitter 46
which will then transmit the described signal over the air for
reception by those subscribers having the necessary decoding
equipment.
- Figure 2 illustrates the decoding apparatus at the
receiver. A tuner 50 which may be either a UHF or VHF tuner
is connected to an IF filter 52 providing an approximate 45 mhz
output signal which is in turn passea to a video detector 54.
The output from detector 54 is the above-described video signal
and is connected to a decoder 56 and a combining circuit 58.
Decoder 56 will provide one output of the above-describea clock
signal along line 60 to sync generator 62., The output from sync
generator 62 which will be the vertical and horizontal sync
pulses suppressed at the-transmitter is sent to combining
circuit 58 whose output is thus the reconstituted video with
appropriate sync signals. Sync generator 62 will be continuously
providing normal sync pulses, but the clock signals from decoder
56 are necessary to properly coordinate the timing of the sync
generator and the video signal.
- A second output from decoder 56 is the digital sound
information which is connected to a digital to analog converter
64. The output from converter 64 will be the reconstituted
audio signal. A 4.5 mhz oscillator 66 is connected to an FM
modulator 68 which receives the audio signal from converter 64
with the output from modulator 68 being the audio portion of
the original television signal at a 4.5 mhz frequency. This
signal is passed to a combining circuit 70 which receives the
combined video signal from combining circuit 58. Thus, the
output from combining circuit 70 is the video and audio portions,
properly decoded, of the original television signal.
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The third output from decoder 56, the address or
enabling information which had been previously inserted in the
vertical blanking interval is passed to an address authorization
decoder 72 which compares the decoded address with the address
of the receiver. If there is comparison, switch t4 is enabled
to permit modulator 76 to function in the appropriate manner.
Modulator 76 changes the frequency of the video and audio signals
from combining circuit 70 to an RF signal at the appropriate
frequency for use in a television set. Normally this will be
channel three or four, unused channels in many areas. The
output from modulator 76, a channel three or channel four
television signal will be passed through filter 78 and then to
the television receiver.
The subscription television system described herein
not only provides adequate safeguards to preventing piracy of
the signal, but does so in a reliable and relatively uncomplex
manner. The vertical and horizontal synchroni,zation is removed
from the video signal and is provided at the receiver by an
independent sync generator,the timing of which is controlled
by clock signals forming a part of the transmitted video.
Substantially complete picture di'stortion is effected by removal
of both horizontal and vertical synchronization.
The audio may be coded as described, in the form of
digital sound. In the alternative, the audio may be transmitted
clear, without coding. In such event the horizontal blanking
interval can be used to provide an alternate sound channel, or
it can have no coding information superimposed thereon~ As
another alternative, the clock and address information described
as being inserted in the vertical interval can be inserted in the
horizontal blanking interval. Thus it is possible to utilize the
horizontal blanking interval for an alternate sound channel, for
coded audio information, for no signal at all or for additional
code data supplementing that superimposed on the vertical blanking
interval.
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Whereas the preferred form of the invention has been
shown and described herein, it should be realized that there may
be many modifications, substitutions, and alterations thereto.
