Color Television:

A color TV screen differs from a black-and-white screen in three ways:

• There are three electron beams that move simultaneously across the screen. They are named the red, green and blue beams.

• The screen is not coated with a single sheet of phosphor as in a black-and-white TV. Instead, the screen is coated with red, green and blue phosphors arranged in dots or stripes. If you turn on your TV or computer monitor and look closely at the screen with a magnifying glass, you will be able to see the dots or stripes.

• On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen.


When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.

Concept of colors in TV: 

  • The standard color wheel is the key to understanding many issues in color television.
  • Red, green and blue are TV's primary colors, and yellow, magenta, and cyan are considered secondary colors.
  • If any two colors exactly opposite each other on the color wheel are mixed, the result is white.
  • Note that instead of canceling each other as they did with subtractive colors, these complementary colors combine for an additive effect. 
  • It may be obvious at this point that by combining the proper mixture of red, green and blue light any color of the rainbow can be produced.
  • Therefore, in color television only three colors (red, green and blue) are needed to produce a full range of colors in a color TV picture.

Television (TV) Camera Tube






A TV camera tube may be called the eye of a TV system or video camera. A TV camera tube is a transducer which converts the variations of light intensity into the variation of electrical current or voltage, known as video signals. It is an essential part of a video camera.

Basic Principle of Camera Tube:

An opto electrical converter is used to translate brightness variations into an electrical picture signal. Different converter systems are available, but only pickup tubes with a photosensitive semiconductor layer are really important for TV technology.
Any picture appears to be composed of small elementary areas of light or shade, which are known as picture elements. The elements thus contain the visual image of the scene. 
The purpose of a TV pick-up tube is to sense each element independently and develop a signal in electrical form proportional to the brightness of each element. Light from the scene is focused on a photosensitive surface known as the image plate, and the optical image thus formed with a lens system represents light intensity variations of the scene. By making use of photoelectric properties, the image plate then converts different light intensities into corresponding electrical variations.

Types of Camera Tubes:

The first developed storage type of camera tube was ‘Iconoscope’ which has now been replaced by image orthicon camera tube. Because of its high light sensitivity, stability and high-quality picture capabilities. The light sensitivity is the ratio of the signal output to the incident illumination. 
Next to be developed was the vidicon camera tube and is much simpler in operation. Similar to the vidicon camera tube is another tube known as plumbicon. The latest device in use for image scanning is the solid-state image scanner.





PAL TV Receiver:

Phase Alternating Line (PAL) is a colour encoding system for Analog television used in broadcast television systems in most countries broadcasting at 625-line / 50 field (25 frame) per second (576i). It was one of three major analogue colour television standards, the others being NTSC and SECAM.




The TV receiver has a VHF and UHF tuner on the front. It has a tuned circuit that allows you to choose the channel you want. 
The antenna signal is amplified and transformed into an IF signal, which is then sent into the video IF amplifier.
Since the tuner's output signal isn't strong enough to drive the video detector, it is amplified to the appropriate level via cascaded IF amplifiers.
From the modulated composite video stream, the video detector recovers the original video signal.
The luminance signal ‘Y’ is applied to the Y amplifier which is a wideband video amplifier. It is further passed through a delay network providing 64 µsec delay and applied to the RGB matrix.

Despite the intensity of the input signal, the Automatic Gain Control (AGC) circuit keeps the output signal at consistent amplitude.
The detected U and V signals are applied to a resistive matrix to produce the (R-Y), (B-Y) and (G-Y) signal which are applied to RGB matrix along with luminance signal ‘Y’ to produce R, G and B signals.

Advantages :

-The phase error causing error in reproduction of colour is eliminated
-Bandwidth of U & V is same .This simplifies filtering action
-Studio mixing is easy as compared to SECAM
-Use of delay lines before demodulators isolates U and V signals from each other reduces crosstalk type of interference in colours better than NTSC-results in better picture quality.

Disadvantages:

-Design is complex as compared to others.
-Delay line technique reduces vertical resolution of chroma signal


Difference between PAL, NTSC and SECAM:

There are only three television standards in the world: NTSC, PAL, and SECAM







 Plasma and conduction of charge:

A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Now a days Plasma TV lost nearly all market share due to competition from low-cost LCDs and more expensive but high-contrast OLED flat-panel displays. Plasma displays are bright, have a wide color range, and can be produced in fairly large sizes—up to 3.8 metres (150 in) diagonally.

Design:

A panel of a plasma display typically comprises millions of tiny compartments in between two panels of glass. These compartments, or "bulbs" or "cells", hold a mixture of noble gases and a minor amount of another gas (e.g., mercury vapor). Just as in the fluorescent lamps, when a high voltage is applied across the cell, the gas in the cells forms a plasma. Many tiny cells located between two panels of glass hold an inert mixture of noble gases (neon and xenon). The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light.





Working: 

Essentially plasma is an electrically conductive gas that contains free-flowing ions (positively charged) and electrons (negatively charged). If you introduce more electrons by applying a voltage through the gas then they will begin to collide with atoms, knocking off electrons and turning them into ions. Then negatively charged particles will start to move towards the positively charged area, and vice versa. This causes the atomic equivalent of a motorway pile up, with particles smashing into each other and the xenon and neon gases used in plasma screens releasing photons of light. Most of this light is ultraviolet light which is invisible, but this is turned into visible light by painting the tiny cells with phosphoric material.

Advantages:

  • Color reproduction is very similar to that of CRTs.
  • Gives a superior contrast ratio than LCDs
  • Wider viewing angles than those of LCD
  • Faster response time
  • Gives good brightness level
  • They were less expensive for the buyer per square inch than LCD

Disadvantages:

  • Plasma displays are generally heavier than LCD
  • Does not work as well at high altitudes above 6,500 feet
  • Uses more electrical power
  • Signal processing in Plasma TV receivers: A PDP is built from two glass substrates, with a gas mixture between them. A ‘barrier’ structure between the glass plates divides the panel into separate cells or channels that are coated with phosphorescent material. In the gas, usually based on Neon and Xenon, an ion discharge can be induced by applying a voltage above the ‘ignition’ threshold (typically around 100V). This discharge emits UV-light, which is converted to visible light by phosphorescent materials.