- 17 - 
where 
-sin 0 = sin (-0) 
Inspection of these equations reveals that the product wave form 
is composed of the resolved component of the contour angle and a second 
harmonic term. By filtering the second harmonic term, the remaining 
steering signals are simply the resolved components of the contour angle 
which will move the scan along the contour at each analog computation 
point. 
CORRELATOR DETAILS 
The major difficulty in constructing a wide-band electronic cor 
relator involves the analog multiplication of the electrical signals de 
rived from the scanning of each diapositive. As a result of the band 
width requirements, relatively few types of analog multipliers are appli 
cable. The types of multipliers commonly used are either time-division 
or quarter-square multipliers. Either type is inherently capable of accu 
rate wide-band operation. 
Both correlators employed on the stereoplotter utilize circuit 
configurations which have proved to be simpler and more versatile than 
the more conventional multipliers already described. These improved 
correlators employ digital circuit techniques and offer high-speed opera 
tion with a minimum of circuit components (5). 
An interface problem develops when one considers the form of 
the electrical signals obtained from electronic scanning of diapositives 
as compared to the pulse signals required by digital circuitry. The analog 
electrical signals, derived from scanning, must be converted to a digital 
form and, simultaneously, loss of information due to the conversion pro 
cess must be minimized. 
The analog-to-digital conversion process has been considered, 
and it can be shown that only a small amount of information is lost if 
only the polarity signs of the analog signals are retained. The conversion 
process is then simply one of detecting zero-crossings of the analog sig 
nal. The input to the digital correlator will consist of a series of pulses, 
having constant amplitude, whose polarity and duration contain the