| United States Patent |
4,616,261
|
|
Crawford
, et al.
|
October 7, 1986
|
Method and apparatus for generating subliminal visual messages
Abstract
A system for generating a subliminal message during the display of a normal
television program on a television receiver utilizes a personal computer
to generate an RF carrier modulated with video signals encoding the
subliminal message. The computer runs under the control of an application
program which stores the subliminal message and also controls the computer
to cause it to generate timing signals that are provided to a single pole
double-throw switch. The source of the normal television program and the
video output of the computer are connected to the two switch inputs and
the switch output is connected to the television receiver antenna system.
The timing signals cause the switch to normally display the conventional
television program and to periodically switch to the computer output to
generate the subliminal message. The video output of the computer includes
horizontal and vertical synchronizing signals which are of substantially
the same frequency as the synchronizing signals incorporated within the
normal program source but of an arbitrary phase.
| Inventors:
|
Crawford; James R. (Lainsburg, MI);
Winegeart; Jerald L. (Niles, MI);
Erb; Michael H. (DeWitt, MI)
|
| Assignee:
|
Stimutech, Inc. (Ann Arbor, MI)
|
| Appl. No.:
|
542467 |
| Filed:
|
October 17, 1983 |
| Current U.S. Class: |
348/563; 348/584; 348/589; 348/706 |
| Intern'l Class: |
H04N 005/262; H04N 013/02 |
| Field of Search: |
358/181,183,92,142,236
|
References Cited
U.S. Patent Documents
| 4006291 | Feb., 1977 | Imsand | 358/92.
|
| 4151557 | Apr., 1979 | Iida | 358/181.
|
| 4424591 | Jan., 1984 | Boardman | 358/181.
|
Primary Examiner: Chin; Tommy P.
Attorney, Agent or Firm: Krass & Young
Parent Case Text
This application is a continuation-in-part of application Ser. No. 491,612,
filed May 4, 1983 abnd.
Claims
We claim:
1. The method of generating subliminal visual messages on a television
receiver, having horizontal and vertical scan generators, during the
display of a supraliminal television program derived from a program source
comprising:
generating a signal representative of one of said subliminal messages with
independent horizontal and vertical synchronizing signals independent of
synchronizing signals of the program source and switching the television
receiver from the program source to said subliminal signal for intervals
sufficiently short to prevent the presentation of said subliminal signal
as a supraliminal display, whereby the horizontal and vertical scan
generators of the television receiver are controlled by said synchronizing
signals of the program source at such time as the receiver is connected to
the program source and by said independent horizontal and vertical
synchronizing signals of said signal representative of one of said
subliminal messages at such time as the television receiver is switched to
the last said signal to produce display of a program containing one of
said subliminal messages.
2. The method of claim 1 in which said signal representative of one of said
subliminal messages constitutes a radio frequency signal modulated with
one of said subliminal messages and with said independent horizontal and
vertical synchronizing signals and wherein the television receiver
includes an antenna circuit which is switched between said modulated radio
frequency signal and the program source.
3. Apparatus for generating a subliminal message on a conventional
supraliminal television display, the display having horizontal and
vertical scan generators, comprising:
a source of a supraliminal television signal encoded with video signals and
horizontal and vertical synchronizing signals;
a source of a subliminal television signal encoded with a subliminal video
signal and independent horizontal and vertical synchronizing signals
generated at the same rate as the horizontal and vertical synchronizing
signals of the supraliminal signal but with independent phase;
switching means having inputs from the supraliminal signal source and the
subliminal signal source and having output to a television receiver; and
means for controlling the switching means to normally provide the
supraliminal signal to the television receiver in order to generate a
display of the supraliminal television signal and to switch the television
receiver to the subliminal source for a period sufficient to generate the
subliminal message and for intervals sufficiently short to prevent the
subliminal signal from being supraliminally observable, whereby the
horizontal and vertical scan generators of the television receiver will be
controlled by either the subliminal signal source or the supraliminal
signal source to produce appropriate display thereof without need of
synchronizing the independent horizontal and vertical synchronizing
signals from the subliminal signal to the respective signals of the
supraliminal signal.
4. The apparatus of claim 3 wherein said supraliminal television signal and
said subliminal television signal both constitute radio frequency signals
modulated with their respective video signals and synchronizing signals
and the output of said switching means is connectable to an antenna
circuit of a television receiver.
5. The apparatus of claim 3 wherein said source of a subliminal television
signal encoded with a subliminal video signal and horizontal and vertical
synchronizing signals constitutes a general purpose computer running an
application program strong signals representative of the subliminal video
signal, the computer including a video display generator and having its
video output connected to said switching means.
6. The apparatus of claim 5 wherein said means for controlling the
switching means operates under control of said application program.
7. The apparatus of claim 6 wherein said application program is a readonly
memory adapted to interconnect to said computer.
8. The apparatus of claim 7 wherein said read-only memory is a module which
includes a bistable multivibrator operating under control of the general
purpose computer and having its output connected to said switching means
and constituting the means for controlling said switching means.
9. An apparatus for connection to a general purpose computer and a
television receiver adapted to receive a program from a supraliminal
signal source operative to cause the receiver to generate a subliminal
message during supraliminal generation of the program from the
supraliminal signal source comprising:
a single pole double-throw switch having a common terminal adapted to be
connected to the television receiver and having pole terminals adapted to
be connected to said supraliminal signal source and the video output of
the general purpose computer;
an application program for the general purpose computer adapted to cause it
to generate a subliminal signal comprising video signals representative of
said subliminal message and horizontal and vertical synchronizing signals
independent of horizontal and vertical synchronizing signals of the
program through its video output;
and means for causing the switch to normally connect the supraliminal
signal source to the television receiver and to periodically disconnect
the supraliminal signal source from the television receiver and to
simultaneously connect the subliminal signal to the television receiver
for periods not exceeding the time required to generate a single picture
field on the television receiver and at intervals sufficiently long to
generate a supraliminal display of the program and a subliminal display of
the subliminal message.
10. The apparatus of claim 9 wherein said means for causing the switch to
normally connect the supraliminal signal source to the television receiver
and to periodically disconnect the supraliminal source from the television
receiver and to simultaneously connect the subliminal signal to the
television receiver operates under control of the general purpose
computer.
11. The apparatus of claim 10 wherein the general purpose computer includes
input means and programmable memory means and the application program
includes means for generation of the subliminal message which is inputted
through the input means and stored in the memory means.
12. The apparatus of claim 9 wherein said application program for the
general purpose computer is a module adapted to be connected to said
general purpose computer and said module includes a bistable device having
its output connected to the switching and operative to be controlled by
the application program in timed relation to the generation of display
signals by the video output of the computer.
13. A switch system operative to be connected to a television receiver, the
television receiver having horizontal and vertical scan generators, a
source of television programs having horizontal and vertical synchronizing
signals, and a source of a subliminal message having independent
horizontal and vertical synchronizing signals in order to generate a
supraliminal display of a television program on the receiver with a
superimposed subliminally perceptible generation of a subliminal message,
comprising:
a single pole double-throw switch adapted to have its common terminal
connected to the television receiver and its two pole terminals connected
to the television program source and the subliminal message source;
and means for causing the switch to normally be positioned to connect the
television program source to the television receiver and to intermittently
disconnect the television program source from the television receiver and
to connect the subliminal message source to the television receiver for
periods not exceeding the time required to generate a single television
frame display and at intervals sufficiently large to generate a
supraliminally perceptible image of the television program and
subliminally perceptible image of the subliminal message whereby said
horizontal and vertical scan generators of the television receiver are
controlled by either the television program source or the subliminal
message source to produce appropriate display thereof without need of
synchronizing said independent horizontal and vertical synchronizing
signals of the subliminal message to said respective signals of the
television program.
14. The switch system of claim 13 in which the television program source
and the subliminal program source constitute radio frequency signals of a
common frequency adapted to be tuned by the television receiver each
encoded with horizontal and vertical synchronization signals of
substantially common frequency and independent phase.
15. An application program adapted to be connected to and executed by a
general purpose computer having a video output signal in the form of a
radio frequency signal having synchronizing signals for use with a switch
adapted to receive the video output signal of the computer and a
television picture signal having independent synchronizing signals and to
selectively connect one of the video output signal or the television
picture signal to a television receiver in order to generate a
supraliminal display of the television picture signal superimposed with a
subliminally perceptible display of the computer generated video output
signal, said program comprising:
means for causing the computer to modulate said radio frequency signal with
a subliminal message signal and to cause the generation of two-level
switch control signal operative to cause the switch to periodically
connect the television receiver to the video output signal for periods not
exceeding the time required to achieve synchronization of the television
receiver to the video output signal and display a single video frame at
intervals sufficiently large to cause the generation of a supraliminally
perceptible display of the television picture with a superimposed
subliminally perceptible display of the subliminal message whereby the
synchronizing signals of the video output signal need not be in
synchronization with the independent synchronizing signals of the
television picture signal.
16. The application program of claim 15 wherein the application program is
stored in a read-only memory and includes said subliminal message.
17. The application program of claim 16 wherein the read-only memory is
adapted to be connected to an input port of the computer and further
includes a cable adapted to be connected to the switching means, whereby
the application program causes the computer to generate timing signals
which cause the generation of a switch controlling output on the cable.
18. A method of generating a subliminal visual message using a television
receiver, comprising the steps of:
displaying a supraliminal image on a screen of said television using a
first set of signals, including horizontal and vertical synchronizing
signals, from a convention television program source;
generating a second set of signals representing the subliminal message
using a subliminal message program source, said second signals including
independent horizontal and vertical synchronizing signals for
synchronizing raster scanning of said screen
interrupting the display of said supraliminal image on said screen;
displaying said subliminal message on said screen using said second set of
signals during the interruption of the supraliminal message display for
intervals sufficiently short to prevent the subliminal message from being
supraliminally observable, without need of synchronizing the independent
horizontal and vertical synchronizing signals of the subliminal message to
respective signals of the television program source; and
resuming the display of said supraliminal message on said screen after said
subliminal message has been display on said screen.
19. The method of claim 18, wherein the display of said supraliminal
message is interrupted for a period of time sufficient to terminate
control of said television receiver by said independent horizontal and
vertical synchronizing signals associated with said television program
source.
20. The method of claim 19, wherein said subliminal message is displayed on
said screen within a single picture field of raster scanning.
21. The method of claim 18, wherein the step of generating the second set
of signals is performed using a computer provided with a set of programmed
instructions.
22. The method of claim 18, wherein said resuming step is commenced within
150 milliseconds following the interrupting step.
Description
FIELD OF THE INVENTION
This invention relates to a method and apparatus for causing the generation
of a subliminal message superimposed on a supraliminal program being
displayed on a television receiver.
BACKGROUND OF THE INVENTION
A substantial body of scientific evidence exists to support the proposition
that a human subject may be influenced by visual messages generated at
intensity and duration levels sufficiently low that they are not
consciously perceived by the subject. U.S. Pat. Nos. 3,060,795 and
3,278,676 disclose a variety of systems adapted to display a
supraliminally perceptible visual image having a subliminally perceptible
message superimposed thereon. These patents disclose motion picture and
television systems for displaying conventional programs along with
secondary visual signals that have such low levels of intensity and/or
duration that they are not consciously perceptible by a human observer but
are capable of impressing themselves upon the subconscious mind of the
observer to influence behavior.
The systems disclosed in these patents as well as the subsequent subliminal
visual message system disclosed in U.S. Pat. No. 4,006,291 all involve
arrangements wherein both the source of the supraliminal program signal
and subliminal message signal are under control of the system's operator.
In the motion picture version disclosed in U.S. Pat. No. 3,060,795, a pair
of separate motion picture projectors employ mechanism connecting them so
that they operate in synchronism to generate the supraliminal program
signal and the superimposed subliminal mesasge. In the television versions
disclosed in the above-noted patents, pairs of television cameras are used
to generate the two signals and their outputs are synchronized to provide
a combined signal suitable for use directly by a television receiver or
for radio transmission to remote receivers.
The highly specialized apparatus required for these prior art systems has
severely limited their application. If a subliminal mesasge system could
be formed with readily available equipment, such as a home television
receiver, which could use conventionally broadcast programs as the
supraliminal program source, it would have great utility for impressing
the observer with subliminal message that might educate the observer or
direct him or her toward desirable action.
SUMMARY OF THE INVENTION
The present invention is accordingly directed toward a method and apparatus
for superimposing subliminally perceptible messages on a conventional
television receiver which is displaying normal program sources derived
from broadcast or cable TV, a video record or the like. The system is
intended to be attached to an entertainment television receiver without
the necessity of any mechanical or electrical modification of the
television set. It utilizes as its primary components apparatus often
available in the home for other purposes.
Broadly, the present invention provides means for generating a video signal
encoded with a subliminal message and horizontal and vertical
synchronizing signals which are of substantially the same frequency as
those forming part of the supraliminal signal source connected to the
receiver but have an artibrary phase with respect to the supraliminal
synchronizing signal. The present invention further contemplates a single
pole double-throw RF switching device having inputs from the supraliminal
program source and the subliminal message source and having an output to
the television receiver. The switch is controlled so that it normally is
connected to the supraliminal program source causing the receiver to
display that program under control of the horizontal and vertical
synchronizing signals forming part of the program composite signal. At
regular intervals the switch is controlled to disconnect the program
source from the television receiver and supply the subliminal signal with
its independent synchronizing signals. The subliminal signal may be
applied to the receiver for as little as 500 microseconds, but, in the
preferred embodiment, it is generated for the time required to display one
full raster field on the television screen, normally about 1/60th of a
second. During this time, the synchronization generators of the television
receiver are under the control of the synchronizing signals forming part
of the subliminal program source and will switch the phase of the
generated raster scan to match these new synchronizing signals. The
subliminal message will, therefore, be generated on the TV screen during
one raster scan or field.
In order to assure synchronization of the vertical and horizontal synch
signals as well as synchronization of color signals, AFG and AGC, display
of the subliminal message is delayed from one to 24 picture fields
following switching off from the supraliminal program source. During this
delay, a completely black or blue field is displayed so that the
synchronization is not perceived by the viewer. The delay is not visually
objectionable and does not negatively affect the device as a subliminal
generator.
In the preferred embodiment of the present invention a subliminal video
signal is generated by a general purpose computer, preferably of the
"home" or "personal" type, running an application program which causes
repeated generation of the subliminal video message, once for each raster
field, at the video output of the computer. The application program also
causes the generation of two-level switching signals for the antenna
switch and also synchronizes display of the subliminal program material
with vertical retrace signals so that the program material or "message" is
displayed at a predetermined position in each picture field. In one
embodiment of the invention, which will subsequently be disclosed in
detail, the application program for the computer takes the form of a
module, including a read-only memory (ROM), adapted to be connected to the
computer. The module may also contain a bistable switching device
(flip-flop) for generating the timing signals. Alternatively, the
application program may make use of a flip-flop and an output port
available on the computer to generate the timing signals. The application
program stored in the ROM causes the computer to generate logic level
signals which are applied to the flip-flop and cause it to provide a
higher power output switching signal. This switching signal is coupled to
the RF switch, along with the subliminal RF signal, and controls its
operation.
In an alternative embodiment of the invention, the subliminal message is
not encoded in the application program but the application program allows
the computer operator to input to the computer the particular subliminal
message that is to be displayed.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, applications and advantages of the invention will be made
apparent by the following detailed description of a preferred embodiment
of the invention. The description makes reference to the accompanying
drawings in which:
FIG. 1 is a schematic diagram illustrating the interconnection of a
television receiver, a computer incorporating an application program
module formed in accordance with the present invention, and an antenna
switch for the television receiver;
FIG. 2 is a schematic diagram illustrating the antenna switch and the
read-only memory module construction in greater detail; and
FIG. 3 are illustrations of television program and computer video output
signals and timing signals illustrating their interrelationship to one
another.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, the present invention preferably employs a
conventional entertainment television receiver 10. The receiver has an
antenna input line 12 connected to the common terminal 14 of an
electronic, single pole, double-throw switch 16. One of the input
terminals 18 of the switch 16 is connected to a conventional television
signal source 20, such as an antenna, cable TV outlet, video cassette
player, video disc player or the like. The signal source 20 provides a
conventional television signal in the form of radio frequency carrier
modulated with horizontal and vertical synchronizing signals and
appropriate video signals. They may constitute black and white signals or
color signals.
The second input terminal 22 of the switch 16 is connected to the video
output of a subliminal signal source which preferably takes the form of a
"personal" or "home" general purpose computer 24 of the conventional type
embodying a microprocessor and a keyboard 26, or alternatively, a video
game or the like.
The computer incorporates a plug-in program module 28 formed in accordance
with the present invention. The program module has an outpute cable 30
which connects to the antenna switch 16 and controls the position of the
switch in a manner which will be subsequently described. In alternative
embodiments of the invention the program could be provided in other forms
such as a disc or cassette and alternative methods could be used to
provide the signals to the switch 16.
The RF output of the computer 24 which is provided to the switch 16
constitutes a radio frequency signal modulated with conventional
horizontal and vertical synchronizing signals and a video signal
determined by the nature of the application program module 28. In the
preferred embodiment of the invention the radio frequency of the video
signal provided by the computer 24 must be the same as the freeuency of
the channel to which the television receiver 10 is tuned. This may be
achieved in a number of ways. In the case of cable TV in which a converter
is provided which tunes any of a number of incoming channels to a common
channel of the television receiver, the frequency of the video source can
be matched to that channel frequency. Alternatively, with a conventional
broadcast reception television receiver the radio frequency of the video
output of the computer may be chosen to match one of the popular channels
to which the television receiver is tuned.
In an alternative embodiment of the invention wherein the television
receiver is of the type equipped with inputs directly to its decoded video
circuits, the output of the switch could be provided directly to these
video inputs, by bypassing the antenna circuits. In this case, means would
have to be provided for demodulating the broadcast signal source.
In still another alternative embodiment of the invention a channel
converter could be provided to match the frequency of the subliminal
source to that of the television program source 20.
The application program module 28 preferably takes the form of a read-only
memory module that plugs into the personl computer 24. The program will
preferably incorporate a subliminal message that is to be displayed on the
television receiver, such as the message "I am a non-smoker" which is
illustrated in FIG. 1 as a typical message. Other messages could be
directed toward altering the viewer's dietary habits, such as "I see
myself as thin"; the viewers personal habits such as "I am neat", or the
like.
Alernatively, the program stored in the module 28 may allow the operator to
generate any desired subliminal message, using the keyboard 26 as an input
source.
In either event, the video output of the computer, applied to terminal 22
of the switch 16, constitutes an RF signal modulated with conventional
horizontal and vertical synchronizing signals and a video message that is
the same from frame to frame; i.e., the message such as "I am a
non-smoker" is generated during one full raster scan (picture field) and
is repeated at the raster scan rate. The scan lines encoding the actual
message may occur at a delay relative to the vertical retrace signal
forming part of the composite video output of the personal computer or may
commence concurrent with such signal. As will be subsequently described,
the video synchronizing signals of the computer are allowed sufficient
time in which to gain control over the horizontal and vertical oscillators
of the television receiver 10 when the receiver is switched to that video
output. In some types of receivers, it may be necessary to utilize a
longer stability period, up to 150 milliseconds, before the message is
generated. During the delay period, a blank screen is displayed on the
television receiver. This longer delay period may be simply accomplished
using instructions in the application program and has the advantage of
assuring that synchronization of AFG, AGC, and color is properly achieved.
A 150 millisecond delay period is not believed to reduce the affect of the
subliminal message on a human subject and does not pose an objectionable
distraction from the supraliminal programming.
The switching signal on line 30 is such as to normally cause the television
receiver to be connected to the conventional television pogram source 20.
At regular intervals the input 30 causes the switch 16 to throw to connect
the television receiver to the video output of the computer for a minimum
of 500 microseconds, but preferably, for at least one field of the video
ouptut; i.e., approximately 1/60th of a second. When a delay period is
used, the television receiver will remain connected to the computer for up
to 150 milliseconds, depending on the length of the delay period.
The synchronizing signals for the computer video output are generated
independently of the synchronizing signals that form part of the signal
from the program source 20. They will be of substantially the same
frequency since they are generated using a quartz clock source of 3.759545
megahertz as a base, the same base as used with the NTSC signal standard
utilized in most of the western world. When other signal standards are
used, the computer video signal source would have a related frequency.
However, the phases of the video signals generated by the computer 24 and
the program source 20 will be arbitrary with respect to one another.
Assuming that the horizontal and vertical scan generators within a
television receiver lock on the horizontal and vertical synchronizing
signals accompanying the video signal from the computer 24 within a few
raster scan lines after switch 16 connects to the computer, the subliminal
message encoded in the computer video signal will have the proper
horizontal displacement relative to the TV receiver screen but may have an
arbitrary vertical position. Since the vertical retrace operation of the
television receiver will still be controlled by the previous signal from
the source 20, the possibility exists that the subliminal message will be
generated during vertical retrace or will be split by vertical retrace. If
the subliminal message is maintained small enough the possibility of this
occurrence is minimized and its occasional occurrence will not affect
provision of the subliminal message to an appreciable degree. Accordingly,
the source of the subliminal signal and the source of the program signal
may be unsynchronized with respect to one another and the system may
operate properly.
This problem can be eliminated however by providing a delay period
following switching to the subliminal program source. The delay may be
from 1 to 24 picture fields, i.e. up to about 150 milliseconds, during
which time the picture screen is blanked out.
In the preferred embodiment, the delay lasts for 11 picture fields and is
followed immediately by a single picture field containing the subliminal
message. During the delay period, the television receiver 10 is given an
opportunity to lock on to the horizontal and vertical synch signals from
the computer 24 and the possibility of spliting the message due to a
retrace signal from the source 20 is eliminated. This delay period also
permits the television receiver 10 to lock onto the color, AFG and AGC
signals of the computer 24; in this manner, the appearance and intensity
of the colors displayed during the message are maintained, thus preserving
the sublimality of the message.
The relationship of the computer 24, its application program module 28, and
the antenna switching circuit 16, are illustrated in more detail in FIG.
2. The module 28 stores the application program 32 in read-only memory,
and also may include a flip-flop 34 which connects to the computer and
provides an output on line 30 to the switch 16. Alternatively, the
computer itself may contain a flip-flop which can be used by the
application program 32 to provide the output to the switch 16. In either
case, the application program 32 stores alphanumeric characters
representative of the subliminal message to be generated. The application
program may also allow the operator to generate his own texts for
subliminal programs from the computer keyboard 36 which is illustrated in
phantom lines.
The program 32 causes the computer to generate a repetitive radio frequency
video signal on line 36, each signal consisting of one full screen of
raster scans and also includes instructions to cause a delay in the
display of the message in order to achieve signal synchronization as
previously discussed.
As will be subsequently described in connection with the description of
FIG. 3, the full raster begins with a horizontal synchronizing pulse and
the program 32 controls the computer 24 to cause it to provide signals to
the flip-flop 34 which generate output timing signals on line 30 and
synchronize their initiation with the generation of the horizontal
synchronizing pulses on line 36. The cable 30 consists of two wires 30a
and 30b. The flip-flop 34 provides a high signal on one of the lines at
any given instant and a low signal on the other. When the line 30a is
high, a signal from a program source or antenna 20 is switched to the TV
antenna terminal 12. When the output on line 30b is high, the video signal
in the computer on line 36 is switched to the antenna terminal 12.
Normally, the line 30a is high and line 30b is low, connecting the regular
program source 20 to the TV receiver.
Once each predetermined interval, such as once each 60 seconds, the
computer provides signals to the flip-flop 34 causing the output on line
30a to go low and the output on line 30b to go high, switching the
subliminal signal source into the computer. This occurs at the beginning
of the first horizontal scan of the full raster scan and lasts from about
one-sixteenth of a second, if no synchronizing delay is used, up to about
150 milliseconds when the display is synchronized. The interval between
raster scans of the subliminal message must be sufficiently long to
prevent supraliminal observation of the generated display.
Considering the detailed construction of the switch 16, the program source
20 is coupled to the antenna terminal 12 by a pair of isolating capacitors
42 and 44, and a pair of series connected diodes 38 and 40. Similarly, the
video input 36 of the computer is coupled to antenna terminal 12 through
an isolating diode 46 and a pair of series connected diodes 48 and 70. The
line 30a connects to the midpoint between the capacitor 42 and the diode
38 through a choke coil 72 and connects to the midpoint between the diodes
48 and 70 through the midpoint of a series connected capacitor 74 and a
diode 76. Similarly, line 30b connects to the midpoint between the
capacitor 46 and the diode 70 through a coil 78 and connects to the
midpoint between the diodes 38 and 40 through the midpoint between a
series combination of a grounded diode 60 and a capacitor 62.
When line 30a is high and the line 30b is low, the diodes 38 and 40 are
forward biased to provide a low resistance path to the antenna terminal
from the program source 30, and a low impedance path to ground for the
video signal 36. At the same time, diodes 48 and 70 are back biased to
present a high impendance to the RF signal 36. This relationship is
reversed when the line 30b goes high and line 30a goes low, shorting out
the program source and switching the video source to the antenna terminal.
FIG. 3 illustrates the relationship between the composite television signal
from the program source 20 generally indicated at 50, the composite
television signal from the computer and occurring on line 36, generally
indicated at 52, and the switching signal on line 30, generally indicated
at 54. The composite signals 50 and 52 are illustrated in demodulated
form. They include video signal sections 56, which may include chrominence
and luminence signals if the transmission is in color, interspersed with
periodic horizontal synch signals 58. The phase of the horizontal synch
signals forming part of the composite signal 52 is illustrated as being
out of phase with the synch signals forming part of the signal 50, which
will normally be the case.
The switching signals 54 are generated by the computer in timed
relationship with the horizontal synch signals forming part of the
subliminal signal 52. The output on line 30 is normally low but goes high
for normally more than one full raster scan (field), consisting of a
plurality of segments between the horizontal synchronizing pulse. The
duration of a high pulse is designated in FIG. 3 as R, will constitute
1/60th of a second plus the delay period required to achieve
synchronization. The interval I between successive pulses that cause the
generation of the subliminal message should be larger than several frames
to avoid integration of successive subliminal frames by the eye of the
observer, and may be as long as a minute or two apart, without diminishing
the effectiveness of the system.