Stereoscopic video system and method with field synchronization and intensity control

Stereoscopic video system and method in which left and right images are displayed during alternate fields of a video picture frame. The equalization pulses occurring during the vertical blanking interval at the outset of each field are counted to identify the field. The field identification information is utilized to synchronize the presentation of the images to the proper eyes of a viewer. Flickering of the images seen by the viewer is reduced by limiting the intensity or brightness of the displayed images to which the eyes of the viewer are exposed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains generally to television and other video displays 
and, more particularly, to a system and method for viewing video images in 
three dimensions. 
2. Description of The Related Art 
Three dimensional realism or depth perception can be added to motion 
picture and television presentations by providing separate left and right 
images of an object or scene from slightly displaced points of view and 
presenting these images to the respective eyes of the viewers. To be 
effective, it is important that left eyes receive only the left images and 
the right eyes receive only the right images. 
In the past, one method of separating the two images involved the use of 
color filters in preparing the images and in viewing them. This method was 
used both for printed media and for film media such as the motion picture 
industry. The quality of images produced by this technique was limited by 
the quality of the filters, and only moderate image separation was 
possible. Another problem with this technique was that the filters 
attenuated all of the light passing through them to some extent, with the 
result that the viewed images did not have very high contrast and were not 
as bright as they should have been. Also, it was not possible to broadcast 
television images in black and white using the color filtering technique, 
thus limiting the technique to color transmissions only. 
For motion pictures, polarizing filters have been employed to separate the 
images for the respective eyes. The two images are projected onto the 
screen simultaneously, and the viewers are provided with eyeglasses having 
polarized lenses which are intended to pass the left and right images to 
the respective eyes. This technique provides less attenuation and 
distortion than the color filter technique, and it has met with moderate 
success. It is still used occasionally. 
U.S. Pat. No. 4,461,541 describes a technique in which polarizing lenses or 
filters are employed to provide stereoscopic images of pictures displayed 
on a television monitor. 
U.S. Pat. No. 3,621,127 describes a three dimensional system in which the 
two spatially displaced images are projected onto a screen sequentially, 
rather than simultaneously, and viewers are provided with spectacles 
having mechanical shutters for alternately exposing the left and right 
eyes to the images. In this system, there is a problem in providing 
synchronization between the image source and the spectacles to insure that 
the proper images are presented to the respective eyes. 
It is in general an object of the invention to provide a new and improved 
stereoscopic video system and method. 
Another object of the invention is to provide a system and method of the 
above character which overcome the limitations and disadvantages of 
stereoscopic systems heretofore provided. 
Another object of the invention is to provide a system and method of the 
above character in which presentation of the images to the proper eyes is 
accurately synchronized. 
Another object of the invention is to provide a system and method of the 
above character in which brightness or intensity of the images is limited 
to eliminate flicker in the stereoscopic image. 
SUMMARY OF THE INVENTION 
These and other objects are achieved in accordance with the invention by 
displaying left and right images during alternate fields of a video 
picture frame and by counting the equalization pulses occurring during the 
vertical blanking interval at the outset of each field to identify the 
field. The field identification information is utilized to synchronize the 
presentation of the images to the proper eyes of a viewer. Flickering of 
the images seen by the viewer is reduced by limiting the intensity or 
brightness of the displayed images to which the eyes of the viewer are 
exposed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, the stereoscopic video system of the invention is illustrated in 
connection with a television receiver or monitor 11 having a CRT display 
screen 12 on which a picture is scanned in raster fashion. The monitor has 
conventional circuitry for processing video signals from a source such as 
a television broadcast or a video recorder. 
The system includes a pair of glasses 13 having liquid crystal lenses 14, 
16 which selectively pass or block light in accordance with control 
voltages applied thereto. As discussed more fully hereinafter, left and 
right images for a stereoscopic or three dimensional presentation are 
displayed during alternate fields of the picture, and the glasses pass the 
appropriate images to the eyes of a viewer wearing them. The liquid 
crystal lenses are of a known design, and each includes a liquid crystal 
emulsion between a pair of light transmissive electrically conductive 
plates which are sometimes referred to as front and back planes. If 
out-of-phase driving signals are applied to the front and back planes, the 
liquid crystals are arrayed in such manner that they diffract or scatter 
incoming light to create an opaque condition. If the driving signals 
applied to the plates are in phase, the electric field aligns the liquid 
crystals with the light propagation axis to make the lens transmissive. 
Synchronization between the operation of the glasses and the display is 
provided by a transmitter 17 connected to the monitor and a receiver 18 
connected to the glasses. The transmission medium can be any suitable 
medium such as infrared, electro acoustic, radio frequency or a cable. If 
desired, the transmitter can be built into the monitor, and the receiver 
can be constructed in miniaturized, battery powered form and mounted on 
the frame of the glasses. 
As noted above, the left and right images are displayed on the screen of 
the monitor during alternate fields of the picture. In the standard NTSC 
format, the picture is displayed at a rate of 30 frames per second, with 
two fields per frame and 262.5 horizontal lines per frame. The left image 
is displayed during the first field of each frame, and the right image is 
displayed during the second. In order to present the left and right images 
to the proper eyes, the left lens is rendered transmissive during the 
first field of each frame, and the right lens is rendered transmissive 
during the second. 
Referring now to the waveform of the NTSC video signal shown in FIG. 4, it 
will be noted that the vertical blanking interval between successive 
fields of the picture includes a first equalization pulse interval 
followed by a vertical sync pulse interval and a second equalization pulse 
interval. Each of these intervals has a duration equal to one half of the 
horizontal scan period. In the first field of each frame, the first 
equalization pulse interval has 6 negative going pulses, and the second 
has 7. During the second field of each frame, the first equalization pulse 
interval has 7 negative going pulses, and the second has 6. Thus, by 
monitoring the pulses during one of the equalization pulse intervals, it 
is possible to determine which field is being displayed. This information 
is utilized to control the synchronization of the glasses. 
As illustrated in FIG. 2, the transmitter includes a sync separator 21 
which receives signals from a video source 22 such as a video recorder, 
the video circuits of a television receiver, or a computer. This circuit 
is of conventional design, and it obtains the horizontal sync pulses and 
the vertical blanking signals from the video signal. The output of the 
sync separator is connected to the input of an inverter 23. 
The sync pulses and blanking signals from inverter 23 are applied to the 
input of a vertical sync pulse interval detector 24. This circuit is shown 
in detail in FIG. 5, and it includes a pair of NAND gates 26, 27 connected 
as inverters with a diode 28 connected between them. A resistor 29 and a 
capacitor 31 are connected between the supply voltage V.sub.DD and the 
input of inverter 27. A positive transition at the input of inverter 26 
produces a negative transition at the output, which turns on diode 28 to 
provide a charging path for capacitor 31 through the inverter to ground. 
On a negative transition at the input of the inverter, the diode is cut 
off, and the capacitor discharges through resistor 29. The time constant 
of resistor 29 and capacitor 31 is greater than the vertical sync pulse 
interval, and the capacitor goes through charging and discharging cycles 
during the vertical sync pulse interval. 
The output of vertical sync pulse interval detector 24 is connected to the 
input of an envelope detector 32 which is similar to detector 24 except it 
has an RC time constant such that the capacitor remains charged during the 
vertical sync pulse interval. 
The outputs of envelope detector 32 and inverter 23 are connected to the 
inputs of an AND gate 33. The output of this gate is connected to the 
input of a detector 34 which similar to detectors 24 and 32. The input to 
this detector consists of the pulses from the vertical sync pulse interval 
and the second equalization pulse interval, and the output is the inverted 
pulses from the second equalization interval. This signal is restored to a 
positive going pulse train by an inverter 36. 
The output of inverter 36 is connected to the input of a divide-by-2 
counter 37, which is reset at the end of each vertical sync pulse interval 
by the output of vertical sync pulse interval detector 24. The counter 
thus counts the pulses during the second equalization pulse interval. 
After counting these pulses, the output of the counter remains in a logic 
1 state if the interval contains an odd number of pulses, and it remains 
in a logic 0 state if there are an even number of pulses. Thus, during 
field 1, when the second equalization interval contains 7 pulses, the 
output of counter 37 is high, and during field 2, when the second interval 
contains 6 pulses, the output is low. By changing in level in accordance 
with the field of the picture, this signal provides positive 
identification of the field being displayed at any given time. This signal 
can be utilized as desired to control the operation of the glasses and, 
hence, the passage of the displayed images to the eyes of the viewer. 
In the embodiment illustrated, the output of counter 37 is connected to the 
ENABLE input of a divide-by-10 counter 38 which counts the horizontal sync 
pulses and the vertical blanking pulses from sync separator 21 to provide 
an output signal having a suitable pulse rate for processing in a low 
power receiver. The output of counter 38 is applied to a pulse transmitter 
39 which transmits a burst of pulses during the first field of each frame. 
As noted above, these pulses can be transmitted over any suitable medium 
such as infrared, electro acoustics, radio frequency or a cable. 
As illustrated in FIG. 3, receiver 18 includes an input stage 41 which 
includes one or more detectors for receiving the pulses from transmitter 
39. The detectors and the remainder of the stage are selected in 
accordance with the medium employed for transmission and can be of 
conventional design. In one presently preferred embodiment, the detectors 
are directional, and they pick up the signal from transmitter 17 only when 
the viewer wearing the glasses is looking toward the monitor. 
The pulses from input stage 41 are increased in level by an amplifier 42. 
The output of the amplifier is connected to the input of an envelope 
detector 43 which is similar to detector 24 and envelope detector 32, with 
a longer RC time constant to match the pulse width which is 10 times 
greater in the receiver than in the transmitter. 
The output of envelope detector 43 is connected to the input of a 
left/right selector 44 which comprises another envelope detector similar 
to detector 43. The output of selector 44 changes in level in 
synchronization with the control signal from counter 37 in the 
transmitter, and it is connected to the inputs of drivers 46, 47 to 
control the application of drive signals from an oscillator 48 to the left 
and right liquid crystal lenses 14, 16 in glasses 13. In the embodiment 
illustrated, the oscillator is connected directly to the back plane of 
each lens, and the drivers control the phases of the signals applied to 
the front planes. As noted above, when the signal applied to the front 
plane is in phase with the signal applied to the back plane, the lens is 
transmissive, and when the signals are out of phase, the lens is opaque. 
When no signals are applied to the lenses, e.g. when the oscillator is 
turned off, the lenses are clear or transmissive. 
A transmission detector 49 monitors the signal in amplifier 42 and turns 
off oscillator 48 when no signals are being received, for example when the 
viewer is looking away from the monitor. This permits the viewer to move 
about safely or to direct his attention to something else which requires 
unobstructed vision without having to remove the glasses. It also 
conserves battery power in the receiver. 
Means is provided for limiting the intensity or brightness of the displayed 
images to which the eyes of the viewer are exposed in order to reduce or 
eliminate flickering of the images as perceived by the viewer. Such 
flickering occurs because of persistence in the human eye when exposed to 
bright objects. In the embodiment illustrated in FIG. 2, a brightness 
limiter 51 is connected between the video source and the display circuits 
of monitor 11 to limit the intensity or brightness of the displayed images 
to eliminate flicker when the images are viewed through the glasses. 
As illustrated in FIG. 6, the brightness limiter includes a sync separator 
52 similar to sync separator 21. It obtains the horizontal sync pulses and 
vertical blanking signals from the video signal and inverts the sync phase 
amplitude relationships. 
The output of sync separator 52 is connected to the input of a horizontal 
sync detector 53, and the output of detector 53 is connected to the 
control inputs of a reference black level detector 54 and a reference 
white level detector 56. The reference level detectors also receive the 
video signal as inputs, and they provide DC output signals corresponding 
to the black and white levels in the video signal. 
The outputs of the black and white reference level detectors are applied to 
the inputs of a DC restorer 57, together with the video signal. The DC 
restorer sets the maximum peak brightness for the video being processed 
for each field as it passes through the circuit. It then compresses the 
brightness range on a line-by-line basis such that the peak values remain 
within the eye's ability to separate the images clearly, without 
compromising the contrast settings in the display. 
The output of the DC restorer is connected to one input of a sync restorer 
58, and the horizontal sync signal from sync detector 53 is applied to a 
second input of the sync restorer. The sync restorer reinserts the sync 
signals into the signal delivered to the video display. 
Alternatively, the intensity or brightness to which the viewer's eyes are 
exposed can be limited by increasing the shuttering rate of the glasses to 
decrease the amount of light which the eyes receive during each field. 
This can, for example, be done by doubling the shuttering rate so that eye 
receives two shorter exposures for each field. The increase in the 
shuttering rate is readily effected by doubling or otherwise increasing 
the rate of the control signal from envelope detector 43. The increased 
shuttering rate can be employed either in place of or in addition to the 
brightness limiting circuitry described above. When utilized by itself, 
the increased shuttering rate eliminates the need for the brightness 
limiting circuitry, and this can reduce the cost of the system. 
Operation and use of the system, and therein the method of the invention, 
are as follows. It is assumed that the transmitter is connected to a 
source of video signals in the standard NTSC format and that signals for 
the left and right images of a stereoscopic picture are present during the 
first and second fields, respectively, of each frame. At the outset of 
each field, detectors 24, 32 and 34 and AND gate 33 separate the second 
equalization interval pulses from the rest of the sync and blanking 
pulses, and these equalization pulses are counted by counter 37. During 
the first field of each frame, there is an odd number of pulses, and the 
output of the counter is high. During the second field, there is an even 
number of pulses, and the output of the counter is low. 
When the output of counter 37 is high, counter 38 passes the pulses from 
sync separator 21 to pulse transmitter, and the transmitter transmits a 
series of pulses which are picked up by the receiver 18. Upon receipt of 
these pulses, left/right selector 44 conditions drivers 46, 47 to apply 
in-phase signals to the front and back planes of the left liquid crystal 
lens 14 and out-of-phase signals to the front and back planes of the right 
lens. The in-phase signals render the left lens transmissive, and the 
displayed image is passed to the left eye. The out-of-phase signals render 
the right lens opaque, and the left image does not reach that eye. 
During the second field of each frame, no pulses are transmitted, the 
in-phase signals are applied to the right lens 16, and the out-of-phase 
signals are applied to the left lens. During this field, the image 
displayed on the monitor screen is passed to the right eye and blocked 
from the left eye. 
Flickering of the images seen by a person wearing the glasses is prevented 
by limiting the intensity or brightness of the images displayed on the 
monitor to which the eyes of the viewer are exposed in the manner 
discussed above. 
Although the invention has been described with specific reference to a 
video signal in the NTSC format, it can be employed with signals in any 
suitable format. Likewise, while the left image has been disclosed as 
being displayed in the first field of each frame, the order can be 
reversed, if desired, with the right image being displayed in the first 
frame. 
The invention has a number of important features and advantages. It 
overcomes one of the most serious problems of stereoscopic video systems 
heretofore provided, namely the loss of synchronization between the video 
display and the shuttering device for the eyes. By monitoring the 
equalization pulses, each field is positively identified, and proper 
synchronization is assured. Flicker is eliminated without perceptible loss 
of contrast by limiting the intensity or brightness to which the eyes of 
the viewer are exposed. The system can be manufactured economically, and 
it can be employed with existing television receivers, video recorders, 
computers and monitors. 
It is apparent from the foregoing that a new and improved stereoscopic 
video system and method have been provided. While only certain presently 
preferred embodiments have been described in detail, as will be apparent 
to those familiar with the art, certain changes and modifications can be 
made without departing from the scope of the invention as defined by the 
following claims.