CRT Televisions (Revisiting Old Tech)

by Mayank
crt-tv

Overview

It’s been about five years since Videocon’s last CRT TV was produced in 2015. Since the beginning of the 21st century, flat-panel technologies (such as LCD, LCD-, OLED & QLED) have almost completely replaced CRTs in all major markets. Plasma TVs were popular for a while, but they have also ceased to exist since 2007.

The reason people started to favor LCDs was their lower cost, improved picture quality and small size. However, like many of the old technology, such as Radio, which people still use as a nostalgia product, these TVs will still be a fun memory of the period when we watched The Simpsons on holiday season (ask any 90s kid and you’ll know what I’m talking about).

Here we are revisiting these TVs and giving you some of those old memories with a tech flavor.

CRT Televisions

The world of television actually started with the introduction of CRT TVs to the market. They were these big boxy TVs which made such an impact on the world that even today you can find them in some homes. These TVs, as the name states used the CRT (Cathode Ray Tube) to display pictures. The CRT was the most important part of these TVs. Let us dive into some details about CRT.

Working of a Cathode Ray Tube

The cathode-ray tube is a tube made from a thick, lead-glass envelope with a funnel-shaped end. They’re long, deep, and quite bulky. That’s why CRT TVs are big and bulky. The CRT consists of electron guns and a phosphorescent screen for the display of images. The interior of these tubes is highly evacuated to ensure the free flow of electrons inside the tube.

A Cathode Ray Tube

The entire front area of the CRT is scanned repeatedly at certain intervals. This is done to capture images from the video signal and reconstruct those images on the TV screen.

Electron Guns

Electron guns shoot narrow, parallel electron beams on the screen of the tube.

The color CRTs use 3 electron guns, one for each color — red, green, blue. The mixture of these three main colors will form other colors. These three guns are factory arranged in a Delta or Triangular configuration.

Electron guns used commercially deploy a hot cathode that operates under the thermionic emission principle. In such electron guns, the cathode is heated by electric current using a filament. This thermal effect causes the cathode to release electrons that move through a collimator which causes the electron beams to become parallel with each other. The intensity of these electron beams can be controlled according to the video signal.

An Electron Gun

So, now we’ve got three electron beams for three additive colors (red, green & blue). Let’s think for a second that if these beams (which move in a straight line) are allowed to hit the phosphor screen directly, you’d just get three colors in the center of the screen. But we don’t want to do that. We want the picture to be displayed on the whole screen. We thus need to deflect these beams in some way so that they can cover the entire screen.

To accomplish this in CRT TVs, we use magnetic deflection (in which a pair of permanent magnets with varying magnetic fields are mounted on the neck of the tube, one on either side of the tube).

Phosphorescent Screen

The Phosphorescent screen is the funnel-shaped end of the CRT. Color CRTs use three distinct phosphors, each emitting red, green and blue lights. The three separate electron streams formed by the three electron guns are required to hit the phosphor of their intended color.

To make this easier, a shadow mask is used, which is a metal plate with 3 holes on its surface. This shadow mask ensures that every electron beam hits the correct phosphor. The electrons that do not pass through the holes are reflected back. This prevents the electrons from hitting the wrong spot on the screen.

Magnified view of a delta-gun shadow mask color CRT

The electron beams are deflected accordingly with the video signal as a reference. Other colors are formed on the screen by mixing the three primary colors (RGB) in varying intensities. The result is the final image we can see on the screen.

Sometimes the electron beams do not mix as intended, due to some alignment errors in the electron guns. This causes problems with what is called “Color Convergence.” Because of this, we sometimes see color shadows/ghosts along the edges of the TV. Similarly, due to the magnetization of the metal shadow mask, the beams are further deflected and hit the wrong phosphor, which creates problems in the so-called “Colour Purity” which causes the images to look out of color. These problems are resolved to some extent by the use of weak permanent magnets, but sometimes they still happen.

So that was a brief description of how CRTs work.

Problems which led to the demise of CRT TVs in the 21st century

  • The key explanation for this was the introduction of LCD TVs that were lightweight compared to CRT TVs that were big and bulky. Also, CRT TVs were produced with screen sizes of up to 45 inches, while LCD TVs could be of more than 100 inches!
  • Another explanation was the issue of recycling. CRTs consist of chemicals that are hazardous to the environment, such as lead, phosphorus, cadmium, etc. These needed to be disposed of properly.
  • Another problem in CRTs occurs due to the heavy evacuation of the CRT (or vacuum tube). So if a part of the glass tube is damaged during the flow of electron beams, this will cause the atmospheric pressure to collapse the tube internally in a process called implosion. This will result in fragments of glass flying violently outside, which can harm the viewer.

Although CRT televisions are no longer on the market now, they were the pioneers of bringing movies and shows to our homes that we could enjoy while sitting on the couch with a bag of Doritos. Thus, CRT TVs will always be remembered as a piece of retro-tech that we can’t ever forget.

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