Scan Generator 
   
  
  
  
   
  
  
   
  
    
  
Transferred CRT Raster 
and Spot 
Phototube 
Video 
Distortion Error mplifier 
Control Signals 
X and Y Scan 
Signals 
Video 
Stereo 
Diapositive 
Feedback Signals Image Error 
Signals 
Figure 9 Basic Electronic Correlation System 
diapositive, the light it transmits is modulated by the 
image in the diapositive. The phototube detects this 
light and converts it to an electrical signal. The signal 
is then amplified and filtered by the video amplifier 
to conform with the dynamic range and bandpass of 
the electronic video correlator. 
In the correlator the two video signals are pro- 
cessed together with the X and Y scan signals to 
derive the alignment and distortion error signals. A 
"block diagram of the video correlator is shown in 
Figure 10. The basic elements in the correlator are 
the multiplier, filter, and quadrature circuits. An elec- 
tronic multiplier-filter combination is the instru- 
mentation required for the implementation of a cor- 
relation measurement. The first multiplier-filter mea- 
sures the correlation quality of the stereo image pair. 
This measurement, a gross indicator, indicates 
whether two similar images are in the aperture of the 
correlator. The quadrature circuits perform the 
mathematical operation of differentiation on elec- 
trical signals. In the correlator they are used in two 
different ways. The first quadrature circuit works on 
the two video signals and changes their relative form. 
The changes in the signals are such that, when they 
are correlated, the correlation function they produce 
has the shape of the desired alignment error signal— 
positive valued when one image is to the left of its 
centered conjugate image, and negative valued for the 
reversed condition. The remaining quadrature circuits 
are used to produce two signals that indicate the x 
and y direction of the scanning spots as they move 
across the diapositives. The two scan direction signals 
are obtained by differentiating the X and Y scan 
signals from the flying-spot scanner. They are used to 
resolve the alignment error signals from the previous 
quadrature video signal correlation process. A discus- 
sion of this problem was ignored previously so as not 
to complicate the description of the basic system. 
78 
When the stereo images are slightly misaligned in 
the aperture of the flying-spot scanner, the quadra- 
ture correlator produces an oscillating misalignment 
error signal instead of the desired constant value 
signal. This is due to the back and forth motion of 
the scanning spots as they follow the raster pattern. 
To the correlation process, since it effectively sees 
only the video time signals, one image appears to be 
misaligned to the right of the other image when the 
spots are moving in one direction and then to the left 
when they are moving in the opposite direction. In 
the correlator system this problem is rectified by 
multiplying the quadrature correlator output by the x 
and y scan direction signals derived from the quadra- 
ture circuits. The filtered outputs of the multipliers 
are corrected X and Y alignment error signals. 
The X alignment error signal is processed one step 
further to obtain the two image distortion error 
signals. The processing is similar to the quadruplex 
correlation scheme described earlier. Here the X and 
Y scan signals are used to change the signs of the 
error signal. Instead of summing fixed outputs as in 
the optical correlator, the sign of the X alignment 
error signal is changed when the scanning spots are in 
the appropriate quadrant of the correlator aperture. 
This is done by multiplying the X-error signal by the 
sign of the scan signal. The scan signal multiplication 
process provides the appropriate phase reversal of the 
error signal such that the filtered multiplier outputs 
provide a measure of the relative X scale difference 
and skewing of the stereo images. 
PERFORMANCE COMPARISON 
The ultimate comparative indicator is the signal- 
to-noise ratio performance of both correlators. This 
signal-to-noise performance of the correlators will be 
Quadrature 
V4 Circuits 
Multipliers Filters 
  
    
  
  
  
    
  
Dual Video 
Flying Spot Output L 
Scanner Signals r12 (Ax, Ay) 
    
System 
  
  
  
  
X; | Raster 
Signals 
  
  
  
  
Figure 10 Block Diagram of the Electronic 
Correlator Circuits 
KOWALSKI