- 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