3,710,175
3 4
then, after the first effect, the eye will move to focus on In operation of the circuit of FIG. 2, the reading
the proper point for evaluation of the second effect. signal output of storage device 12 is modulated at a 4.3
However, if the second effect is delayed excessively, Hz twinkle rate by the continuous and substantially
the eye will jump to a somewhat different and improper sinusoidal voltage wave-form across capacitor 41. The
position as the initial step of a search pattern. When the 5 modulation index may be controlled by adjustment of
excessively delayed second effect does occur, the eye is potentiometer 44. If amplifier 28a comprises a pair of
not properly focussed and will move back to the proper sharp cut-off pentodes or a pair of field effect
point for evaluation of the third effect. transistors, then the modulation index may be made
The usable flicker frequency band of the human eye substantially equal to one hundred percent by adjusting
thus extends from somewhat less than the nystagmus 10 potentiometer 44 so that amplifier 28a is cut off at the
frequency to a safe margin below the flicker vertigo peak negative excursion of the voltage wave-form
frequency. across capacitor 41. The modulation index may be
Upon each retrace of vertical sweep generator 16, a reduced by adjusting the slider of potentiometer 44 to a
pulse is coupled through differentiating capacitor 20 to more positive potential, so that amplifier 28a is not cut
index ring counter 21 which provides an output pulse 15 off at the peak negative excursion of the modulating
once for each four retraces of vertical sweep generator wave-form across capacitor 41. It will also be ap
16. These output pulses from counter 21 index counter preciated that if amplifier 28a comprises a pair of
22. It will be noted that gate 28 is enabled by both the remote cut-off pentodes, then the modulation index
"1" and the "2" outputs of counter 22 through OR cir- 2Q will always be appreciably less then 100 percent ir
cuit crystals 24 and 25. Gate 28 is disabled when respective of the positioning of the slider of poten
counter 22 provides a "0" output. The modulation of tiometer 44. In the circuit of FIG. 2, switch 29 and vari
the reading signal output of storage device 12 is thus an able resistor 31 of FIG. 1 are no longer needed to effect
asymmetrical square-wave with a mark-space ratio of control over the index of amplitude modulation. In
two. When switch 29 is open and gate 28 is disabled, no ^ FIG. 2, selected targets will have higher than average il
reading signal from storage device 12 is coupled to the lumination for seven fields or three and one-half frames
intensity input of display device 18; and the index of and will have lower than average illumination for a suc
amplitude modulation is accordingly one hundred per- ceeding period of seven fields or three and one-half
cent. The index of amplitude modulation may be frames. Selected targets from storage device 12 will
reduced by closing switch 29 and adjusting resistor 31 30 thus be presented to display device 18 at a symmetrical
as desired, so that some portion of the reading signal 4.3 Hz twinkle rate and are readily distinguished from
from storage device 12 is always coupled through the steady background display of storage device 13.
switch 29, resistor 31, and resistor 33 to the intensity It will be seen that I have accomplished the objects of
input of display device 18. Selected targets from my invention. I have provided a twinkle effect display
storage device 12 will thus be presented to display 35 system in which the video signal is amplitude modu
device 18 at a 5 Hz twinkle rate. The selected targets lated at a rate within the usable flicker frequency band
will be highly illuminated for eight fields or four frames of the human eye. The amplitude modulation may be
and will be extinguished or only partially illuminated either symmetrical or asymmetrical and may be either
(depending upon the position of switch 29) for four continuous or in discrete steps. The index of amplitude
fields or two frames. The selected targets are thus 40 modulation may be varied from one hundred percent to
readily identified against the steady background display appreciably lower values which still provide a readily
from storage device 13. observable twinkling.
Referring now to FIG. 2, retrace pulses from dif- It will be understood that certain features and sub
ferentiating capacitor 20 at a 60 Hz rate are coupled to combinations are of utility and may be employed
a divide-by-seven ring counter 21a which provides out- 45 without reference to other features and subcombina
put pulses at an 8.6 Hz rate. The output pulses of ring tions. It will be further understood that various changes
counter 21a are applied to a divide-by-two ring counter in detail may be made without departing from the spirit
or counting flip-flop 22a which provides a symmetrical of my invention.
square-wave of unity mark-space ratio at a 4.3 Hz rate. Having thus described my invention, what I claim is:
The square-wave output of counting flip-flop 22a is 50 1. A twinkle effect display system including in com
coupled to a first integrating circuit comprising series bination:
resistor 37 and shunt capacitor 38. The voltage output a display device having video input means and having
across capacitor 38 is a symmetrical triangular wave- horizontal and vertical sweep inputs;
form having equal positive and negative slopes and is first and second video signal sources having video
applied to a second integrating circuit comprising se- 55 signal outputs;
ries resistor 40 and shunt capacitor 41. The voltage timing signal generating means coupled to the
output across capacitor 41 is a substantially sinusoidal horizontal and vertical sweep inputs of said display
wave-form and is applied to the gain control input of a device for controlling the horizontal and vertical
balanced video amplifier 28a. The positive terminal of ^ sweeps thereof at a cyclic rate;
a bias battery 43 is grounded. Battery 43 is shunted by a means coupling the video signal output of said first
potentiometer 44. The slider of potentiometer 44 is video signal source to the video input means of
coupled through a resistor 45 to the gain control input said display device;
of amplifier 28a. The reading signal output of storage means including modulation means for coupling the
device 12 is applied to the input of variable gain ampli- ^ video signal output of said second video signal
fier 28a; and the output thereof is coupled through source to the video input means of said display
summing resistor 33 to the intensity input of display device, said modulation means including a
device 18, as in FIG. 1. frequency divider connected to said timing signal
