Digital system processes spacecraft television pictures by converting images sensed on a photostorage vidicon to pulses which can be transmitted by telemetry. This system can be applied in the processing of medical X ray photographs and in electron microscopy

Wong, R. Y.

English

NASA Technical Reports Server

July 1967	 Brief 67-10246 
•	 NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151: 
Improved Television Signal Processing System 
Photostorage 
Vidicon
Selecti
DC 1jJ	 Signal I / J 
-vi
Restorer Gate	 I	 I TtR 	
Preamp 
Chopper Circuit 	 L 
—v2
Signal Driver kø4
	 Clock Frequency Signal Gat 
8-Bit Linear 
Counter 
Amp 2	 V3 fer 
onstant	 rent 
Source	
8 i Shift 
Register 
Amp 3 Discharge 
JfrinPut Signal	
Amplifier 
LL
Fixed Frequency 
Clock Signal
Digital 
Output 
Signal 
An improved digital system has been developed for 
processing spacecraft television pictures. In this sys-
tem, the image sensed on a photostorage vidicon is 
digitized, ac-amplified, data-compressed, dc-restored, 
and then converted to pulses of width accurately 
proportional to signal amplitude. These pulses are 
used for gating a frequency-standard clock to a 
binary counter, which yields a count that is more 
accurately representative of camera signal levels. 
The counts are shifted serially, by transfer gate and 
register, into a magnetic tape recorder from which
they are subsequently transmitted by telemetry at 
slow scan rates. 	 - - 
An illuminated pattern of the optical image of the 
subject is stored on the photostorage surface of the 
vidicon
'
The vidicon represents a constant current 
source that generates a video signal across a load 
impedance RL. Capacitance C 1 represents the sum of 
the photostorage and other stray capacities. The video 
signal is obtained by scanning the photostorage sur-
face with an electron scanning beam. V 1
 is the voltage 
source supplying a current to generate the video 
(continued overleaf) 
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signal, and V2 is the voltage source for generating 
the electron scanning beam. To obtain a signal of 
higher signal-to-noise ratio, the electron beam is 
chopped by the chopper circuit in the beam cathode 
return lead to turn the beam on and off at the rate of 
fc times per second. Thus, the signal developed across 
the load resistor is an amplitude-modulated signal 
with a carrier frequency of fc cycles per second. The 
light intensity of the illuminated image, therefore, 
contains in the envelope peaks of the carrier signal. 
The modulated signal is then amplified by the capaci-
tor-coupled preamplifier, and the dc component ol the 
signal is removed. 
Because of the limited data storage capacity of the 
digital tape recorder that will store the video data, 
only the necessary minimal number of video signal 
cycles are processed and digitized so that an accurate 
reconstruction of the television pictures can be made. 
The signal selecting gate is used to select the desired 
signal cycle at a regular interval (for example, every 
7th signal cycle). Each of the selected signal cycles is 
then dc-restored to its original photometric amplitude 
by the dc restorer. 
The dc restored signal is converted to a time siknal 
by the peak amplitude-to-time signal converter. The 
converter consists of amplifiers 2 and 3, a discharge' 
amplifier, timing capacitor C2, and a constant current 
source. The converter processes the input signal so 
that the timing capacitor C2 is discharged by the 
discharge amplifier as the input signal reaches its peak. 
The net voltage zV=V3—V4, across C2, as a result of 
the discharge, is equal to the peak of the input signal 
V, or zV=V. As the input signal reaches its peak 
and begins to drop, C2 begins to be recharged to its 
initial voltage level of V3 by a constant current I from 
the constant current source. Therefore, the time, At= 
(C2/I)V,for the timing capacitor to be fully recharged
is proportional to the peak of the input signal. The 
signal driver is used to generate a gate signal whose 
pulse width is equal to the charging .time At of capaci-
tor C2. The gate signal is then used to control the 
clock frequency signal gate so that the 8-bit linear 
counter can be used to count the number of clock 
frequency cycles during each gating. The resulting 
8-bit linear word from the counter is used to provide 
256 distinctive video photometric levels. 
When each counting is completed, the content of the 
counter is transferred in parallel by the 8-bit transfer 
gate to the 8-bit shift register. The transferred content 
in the shift register is then shifted out to the digital 
recorder bit by bit in series for recording. While the 
content is being shifted out of the register, a new input 
signal can be digitized by the digitizing circuitry. 
Notes: 
I. This system may be applicable to the taking of 
pictures for processing by digital computer to 
enhance contrast and correct distortions in the 
pictures. An example of such application, in the 
processing of medical X-ray photographs, is de-
scribed in NASA Tech Brief 67-10005. This system 
may also be useful in electron microscopy. 
2. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 
Reference: B67-10246 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Robert Y. Wong 
Jet Propulsion Laboratory
(NPO-10 140)
[I: 
. ^ 
Brief 67-10246	 Category 01


Improved television signal processing system

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