Digital systems are discussed which have the capacity to handle the large amounts of information contained in a typical image. It was used with a high gain pulse counting television camera tube, with a silicon target image detector and an analog to digital converter between the detector and the memory

Dennison, E. W.

English

NASA Technical Reports Server

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MEMORY SYSTEMS FOR SIGNAL GENERATING
PHOTOELECTRIC IMAGE DETECTORS
Edwin W. Dennison
Hale Observatories
Pasadena, California USA
INTRODUCTION
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The recent developments in signal generating electronic
image detectors have necessitated a corresponding~development
of digital memory systems which have the capacity to handle
the large amount of information contained in a typical image.
Current memory technology enables the practical use of memories " |
v
containing 4 x 109 bits and larger which can be used as image
integrators and storage devices. A memory of this type was
built at the ,Hale Observatories as part of a joint project with
the University of Princeton Observatory. It has been success-
fully used with a high gain pulse-counting television camera
tube, and also with a silicon target image detector with an
analog-to-digital converter between the detector and the memory.
In both cases the memory clearly demonstrated the power of this
form of image integration. Some valuable experience was gained
with this memory and a recommended list of design criteria for
future memories has been made. Alternative systems are poss-ible
for photoelectronic detectors, using a slow scan readout.
(CNJ A*
https://ntrs.nasa.gov/search.jsp?R=19750002336 2020-03-23T03:23:20+00:00Z
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THE PRINCETON-HALE DIGITAL MEMORY
The memory elements of this system are dynamic MOS
shift registers. The maximum number of words is 65,536,
the minimum number is 2,048. The memory size can be
changed in binary increments between these Limits. The
memory words are serially accessed with a two coordinate
raster type addressing system. The total number of pixels
(picture elements) per scan line, and the total number of
scan lines per frame can be set at between 8 and 2,048 in
binary increments. The memory has, in fact, always been
used in a 256 x 256 raster pattern. The. additional flexi-
bility was intended to allow the use of this memory with
rectangular spectroscopic images.
To increase the time available to input new data the
memory was divided into 8 banks which are sequentially'
shifted. The input data, which can be in the form of 1 to
16 bits, is clocked into a multiplexer which distributes the
data to the memory banks. - This arrangement gives- the cir- •--
cuitry eight-times the pixel-period for-accessing, adding,... -
storing, and shifting the data. In the usual mode of opera-
tion the pixel period is 208 n-sec (4.8 MHz), but the operating
circuitry has approximately 1.7 y sec to process each memory
word. The currently addressed memory word is also multiplexed
out to an 8-bit digital-analog converter, which the operator
may set to any 8 contiguous bits.
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The pixel or word frequency can be set over the range
from 9.6 KHz to 4.8 MHz. The fact that the memory elements
are dynamic in operation means that there is necessarily a
lower limit to the pixel rate and also that the controlling
circuitry must allow for computer access, horizontal and
vertical blanking periods, and flexibility in data acquisition,
without'stopping the memory shift clocking long enough to
degrade the data. The master timing circuitry which controls/ *
the format of the data clocking also generates sync
pulses, blanking pulses, and standard clock pulses. All of
these timing pulses are available for synchronizing the
circuits which are external to the memory. The clock pulses
are transmitted to the camera unit and back, to automatically
allow for propagation delays in the 700 feet of coaxial
cables connecting the camera unit and the memory (for the 200"
operation). For most of the actual display operations, the
composite video and sync signals were sent directly to a
standard video monitor. This monitor allows the operators to
see the image in. the memory as it changes during the integration
period.
The total integration time can be set from 1 to 9,999
seconds. Other operator controls can be used to start, or
stop, the integration, suspend the integration, record the data,
etc. All of these controls have been arranged to provide the
operator with the maximum possible flexibility and convenience.
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The image data, telescope coordinates, time, object
name, and other relevant information are all recorded on
9-track industry-compatible magnetic tape under the control
of a Raytheon computer. The image memory interface circuits
have provision for integration control and data transfer to
the computer, but do not allow data to be transferred into
the memory from the computer.
t
Some of the circuitry performs dynamic tests on the
memory to ensure that the memory data stays in synchronizm
with the clock pulses. Although the system as a whole has
not been entirely trouble-free, no clocking or word address
problems were ever encountered.
APPLICATIONS
The two camera systems which have been used with this
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memory are being described elsewhere at this symposium. -'
In the first case "(Lowrance et al) the system was used in
the pulse-counting mode, i.e., the words in the memory were
incremented by 1 if the detector sensed the presence of a
signal during the time interval corresponding to each pixel.
In the second case (Alsberg et al) the video signal was
processed in a 8-bit digitizer during each pixel time, and
the resulting value was added into the corresponding memory
word. In both cases the memory was run at the 4.8 MHz pixel
rate and a standard TV monitor was used to evaluate the image
during the integration. All other system conditions were
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essentially the same except that in the first case the data
were read out by the computer, and in the second case a "line-
snatch" technique was used to read out the data, one line at
a time. The final recording was on magnetic tape in both
cases. In the second case, a thorough analysis of the noise
spectrum established that the system was operating as a perfect
integrator.
OPERATIONAL EXPERIENCES
To summarize our operational experiences I will list the
specifications which I would use if I were building an image
memory at this time.
1. Static data storage — The memory should be capable of
storing information in a static form. The important problem
to avoid is the requirement that the data be shifted periodi-
cally to insure its retention.
2. Random access — Serial memory devices are difficult to
reconfigure for different image -formats and are generally
constructed from finite length modules.
3. 65,536 words expandable to 262,144 words — This allows
the- initial format to be 256 x 256 pixels with possible
expansion to-512 x 512 pixels.
4. At least 16 bits per word — Larger words may be desirable
but there appears to be no justification for a word size
larger than 24 bits.
5. Data transfer in and out of memory — This two way data
transfer capability is most valuable for memory diagnostic
tests, reviewing previously recorded images, and possibly for
some data reduction operations at the observatory site.
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6. 10 MHz maximum word rate -
7. Input of 12 bit data word - Preferably provision should
be made for an input of the full memory word size.
8. Real time video or amplitude display - During the image
integration period or during image analysis it is extremely
valuable to have a visual presentation of the image.
9. Changeable operating parameters - The entire range of
possible operating parameters cannot be built into the
original unit, but provision must be made for easy modifica-
tion of the central control circuits. This could be by the
use of micro-computers or an interchangeable control module.
10. Manual and computer interface for controls - The actual
memory system operation should be controllable by either a
computer or a simple operator panel. The status of all
counters and registers must be computer readable.
11. Camera system I/O registers - These registers should
provide for either computer or manual camera controls and
sratus sensing. /" '"" " - - - • -• -
12". " Easily operable at laboratory-and observing- locations
- In most cases this implies that the system must be self-
contained, although in some cases standard computer
installations may be available. It is also possible that
a system could be developed with a standard CAMAC interface.
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. . - GENERAL COMMENTS
It would appear that no single photoelectron image
detector system has been developed to the point where it
can be used for all applications and image detector develop-
ment will continue for a number of years. Our experience
does indicate that it is possible to construct a memory
system which can be effectively used on a variety of image
detectors. • The advantages of digital memory systems are
that they provide
a) Ideal integrators;
b) " Real time display of images during data integration;
c) Re-display of images after the data-recording;
d) The possibility of performing rapid image processing
at the observing location.
It is interesting to note that the above specifications
can be met for a slow scan readout system (approximately 20
KHz) by the use of a mini-computer with a disk, magnetic tape
output, and some form of video display. The performance of
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a system of this type will be inferior to that of a separate
memory system, but in many cases, this reduced performance may
be an acceptable consequence of budget or time limitations.
This work has been supported by the National Science
Foundation and the National Aeronautics and Space Administration.
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REFERENCES
1) Paper Number 30 The I-Sit-Isocon photon counting
TV system. J. L. Lowrance, G. Renda and P.
Zucchino, Princeton University Observatory,
Princeton, New Jersey.
2) Paper Number 25 High resolution electron microscope
imaging with silicon diode array target vidicons.
H. Alsberg and R. E. Hartman, Jet Propulsion
Laboratory, California Institute of Technology '-
Pasadena, California.


Memory systems for signal generating photoelectric image detectors

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