TV Engineering
COLOR TELEVISION CAMERA
Figure shows a simple block schematic of a color TV camera. It
essentially consists of three camera tubes in which each tube receives
selectively filtered primary colors.
Each camera tube develops a signal voltage proportional to the respective
color intensity received by it. Light from the scene is processed by the
objective lens system. The image formed by the lens is split into three
images by means of glass prisms.
These prisms are designed as diachroic mirrors. A diachroic mirror passes
one wavelength and rejects other wavelengths (colors of light).Thus red,
green, and blue color images are formed. The rays from each of the light
splitters also pass through color filters called trimming filters.
These filters provide highly precise primary color images which are
converted into video signals by image-orthicon or vidicon camera tubes.
Thus the three color signals are generated. These are called Red (R),
Green (G) and Blue (B) signals. Simultaneous scanning of the three
camera tubes is accomplished by a master deflection oscillator and sync
generator which drives all the three tubes.
The three video signals produced by the camera represent three primaries
of the color diagram. By selective use of these signals, all colors in the
visible spectrum can be reproduced on the screen of a special (color)
picture tube.
Color Signal Generation
At any instant during the scanning process the transmitted signal must
indicate the proportions, of red, green and blue lights which are present in
the element being scanned. Besides this, to fulfil the requirements of
compatibility, the luminance signal which represents the brightness of the
elements being scanned must also be generated and transmitted along with
the color signals.
Figure illustrates the method of generating these signals. The camera
output voltages are labelled as V R, V G and V B but generally the prefix
V is omitted and only the symbols R, G, and B are used to represent these
voltages. With the specified source of white light the three cameras are
adjusted to give equal output voltage.
Gamma Correction
To compensate for the non-linearity of the system including TV camera
and picture tubes, a correction is applied to the voltages produced by the
three camera tubes.
The output voltages are then referred as R′, G′ and B′. However, in our
discussion we will ignore such a distinction and use the same symbols i.e.,
R, G and B to represent gamma corrected output voltages.
Furthermore, for convenience of explanation the camera outputs
corresponding to maximum intensity (100%) of standard white light to be
handled are assumed adjusted at an arbitrary value of one volt. Then on
grey shades, i.e., on white of lesser brightness, R, G and B voltages will
remain equal but at amplitude less than one volt.
Def:
A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of
electrical transformer. It was initially designed to generate high voltage sawtooth signals at a
relatively high frequency. In modern applications, it is used extensively in switched-mode power
supplies for both low (3 V) and high voltage (over 10 kV) supplies.
Functions:
The flyback transformer is used in the operation of CRT-display devices such as television sets
and CRT computer monitors. The voltage and frequency can each range over a wide scale
depending on the device. For example, a large color TV CRT may require 20 to 50 kV with a
horizontal scan rate of 15.734 kHz for NTSC devices. Unlike a power (or "mains") transformer
which uses an alternating current of 50 or 60 hertz, a flyback transformer typically operates with
switched currents at much higher frequencies in the range of 15 kHz to 50 kHz.
Q..What is image masking?
Image masking is an image processing technique that is used to remove the background from
which photographs those have blurred/fuzzy edges, transparent or hair portions.
Q..Shadow Masking:
Q.. Phosphor Masking:
The process of producing Blacker Black & Whiter white is known as Phosphor masking.