The CCD pickup assembly is similar
to a small video camera, using a long object-to-lens working distance and a
short-focal-length objective lens. Our first unit has the lens on one mount,
while the CCD, drive electronics and video output amplifier are contained on a
circuit card, which is mounted separately as a self-functioning unit. On
production units, both of the above sub-assemblies will form one module, which
will be mounted and focused by the lens tube. The lens tube will be mounted in
accordance with current practice, and the electronics will be housed in the
existing exciter-lamp compartment. Conversely, the exciter lamp, second-stage
condenser and IR shield form an assembly, to be mounted within the sound drum
on the existing solar cell bracket. Both of these units can operate with long
cables and can therefore be treated like their conventional counterparts within
the sound head.
Boundary Scan Reading.
This
technique affords a simple method of reading a multiplicity of recorded
tracks, with very high stability and minimal crosstalk. Presently only four
tracks are used. Readback is by transversal scanning of the moving image edges of a
modulated track, recognizing and converting the edge transition point, to the
rise or fall components of a variable-area constant-amplitude electrical pulse.
The average duration of the pulse in time then is proportional to the average
illuminated width of the image slit. Many signal tracks or moving edges become
width-modulated pulse trains (Fig. 12) the position of which within the scan
window can be recognized by the CCD. For the Colortek system, each pulse train
is derived from a modulated track edge, or recorded reference edge. These pulse
edges, when compared to an internal time reference, give variable-area samples,
which are each commuted to individual charge-holding circuits or channel
receptors. This arrangement then gives a serial edge-to-parallel-track
conversion. The channel commutations are filtered to yield individual signals,
the amplitude of which is proportional to the modulation area of each
individual track. Since these events are each isolated in time, the system
channel-to-channel crosstalk is very low. More important, if the time-reference
circuits follow the average track positions, or a recorded reference edge, the
readback is insensitive to where the pulse train begins or ends, within the
scan window. Hence the film position and weave errors only move the pulse
train, as a group, with respect to the scan window. The timing and information
segments of the train remain unchanged, as does the commuted signal and its
channel-to-channel crosstalk.
Electronic Slicing.
The edge boundary of the
modulated track as projected and focused on the CCD varies from a minimum
luminous transmittance on one side to a maximum transmittance on the other side
(Fig. 13a). Between these points, the edge is defined by a curve, the shape of
which depends upon the nature of the film emulsion, its exposure, processing
and the nature of the recorded edge object image and its focus. Microscopic
examination has shown that the transition from a 10% to 90% of the minimum to
maximum transmittance usually occurs in less than 4 µm for
commercial prints and less than 1 µm for optimum experimental negatives.
The slope, or sharpness, appears to be independent of small transmittance
changes that can occur within the accepted tolerances of normal production
printing. Any desired transmittance value, falling along the 10% to 90% curve,
can be converted to the beginning or end of a pulse. This value, termed the
"slicing point," produces an equivalent recorded image size, since
the electrical pulse width changes with the setting. Adjustment of the slicing
point alters the electrical readback image size by as much as 4 µm. Cross-modulation
distortion, audibly recognizable as sibilant blocking, is at its lowest when
the slicing point sets the scaled pulse width equal to the correct object image
size, determined by optimum negative exposure, in accordance with standard
cross-modulation testing procedures. Optimal selection of