Polaroid SprintScan SS35, Repair Manual

Page: 45 / 187

Slide Scanner Repair Manual Functional Description
45
Analog Signal Processing
The VSP (Figure 2-5) contains a preamplifier with voltage controlled gain, a correlated double
sampler with offset adjustment, and an output amplifier. It also contains three individually
controlled sample-and-hold amplifiers and two 3-to-1 multiplexers which facilitate dark level
adjustment and white balance correction.
White balance corrections are determined during calibration by the microcontroller on the main
controller PC board and are written back to the signal processor through a digital-to-analog
(D/A) converter in the form of closed loop gain information.
Dark levels are also determined during calibration by the microcontroller on the main controller PC
board and are written back to the signal processor through a D/A converter in the form of offset
information.
In this way, a closed-loop white-balance and low frequency dark level correction is applied
to the analog signal before analog-to-digital conversion takes place.
Final digital correction occurs on the slide scanner main controller PC board. The output of the
VSP is filtered before it is fed into the A/D converter to avoid aliasing. The analog signal is
then converted at 10-bits per sample for SS35/SS35 ES or at 12-bits per sample for
SS35 PLUS.
Digital Signal Processing
Digital signal processing of image data consists of uniformity correction, lamp brightness and
color balance correction, reductions in resolution, area of interest selection, color correction,
and adaptive sharpening.
The digital signal processing is performed in two steps and by the primary and secondary
ASIC's.
Primary correction is performed in the primary correction ASIC concentrating on all processing
which can be performed on a single channel of red, green, or blue data.
Secondary color correction is performed in the ESCAPADE ASIC which performs operations
based on the color pixel triplets, including color processing, sharpening and color mode.
A 1 Meg DRAM (buffer) separates the two processing ASIC's. It is used to re-align the
three colors and serve as a transfer buffer between the synchronous data generation by the
CCD image sensor and the asynchronous host SCSI data transfer.
Primary Correction ASIC
The primary correction ASIC offers improves image quality and sensor yield. To the full,
half and quarter resolution modes, it adds a 1/8th averaging mode and a 3/4 two
dimensional shrink on all 4 binary resolution modes.