Television and Video Recording Technologies: CRT, LCD, Plasma, VCR, DVD
Television: Cathode-Ray Tube (CRT)
A TV operates through an electronic process that converts electromagnetic waves into a quick sequence of images and sounds that replicate reality. The received signal contains information with different frequencies for color, light, and sound.
Traditional CRT-type TVs have a cathode-ray tube in which the received signal with the antenna creates a three-beam radiation of electrons. These are accelerated by the anode to the screen, covered with phosphorescent substances (red, green, and blue) that are arranged in close proximity to one another and emit light on the other side of the screen. A thin sheet of metal called a shadow mask, perforated with very fine holes, directs electron beams to a small area of the screen where each beam strikes the point of phosphorescence corresponding to the primary color. The effect on sight is a single point of light, with the resulting color, which depends on the intensity of each beam of electrons, so-called red, green, and blue beams.
By the action of copper coils, electron beams make continuous and fast sweeps of the screen, similar to how the human eye reads a page from left to right in each line from top to bottom in the 625 lines of the screen. Each sweep creates thousands of tiny points of light on the TV screen, which remains a unique retinal image. At a rate of 25 images per second, you get a clear and distinct perception of the emission due to the persistence of the images on the retina.
Liquid Crystal Display (LCD)
Liquid crystals contain molecules. These molecules move from disorder to order, a fixed liquid crystal order, when subjected to an electric field, aligned along its direction. They act as optical switches: when they are aligned along an electric field, they do not allow the passage of light, thus activating a pixel. In calculators, the pixels are the numbers we see on the screen. Unlike an LCD calculator, the screen of an LCD TV can produce color images. To achieve this, it is backed by millions of cells or liquid crystal pixels. Each pixel has three subpixels with color filters: red, green, and blue.
The different light intensities are achieved by controlling the flow received by each pixel. It uses a new technology called TFT that utilizes millions of transistors to control the lighting of each pixel.
Plasma TV
Plasma screens for televisions operate using a scheme that combines features of CRT and LCD. The colors are created by exciting phosphorescent materials as in CRT, and the activation of the pixels is similar to LCD screens, using electric fields. In plasma, there is a gas, usually xenon and neon, contained in hundreds of thousands of cells, located between electrodes between which an electric field is created, causing the flow of electrons. This gas is called plasma because it has the property that, when a stream of electrons crosses it, it emits light, colliding with the phosphorus contained in each cell.
The photons emitted by the gas excite red, blue, and green phosphors lining the inside of cells, emitting light in color. Each cell represents a pixel, divided into three subpixels. The varying intensity of the discharge of electrons in each subpixel makes the overall pixel pitch and loudness perfectly controlled.
Video Cassette Recorder (VCR)
The VCR, used to record images and sounds together on video in 1956, transformed television programs, which were hitherto limited to live broadcasts. Home video appeared around 1970. Video systems have evolved from analog videotape to digital video.
In videotape, the signals captured by the camera or television broadcast generate electrical signals that reach the recording heads of VCRs. These magnetize the tape, recording the image signals in the center and the sound on the top and bottom edges. During reproduction, magnetized particles on the tape induce currents in the heads, which are converted into images and sound.
Digital video cameras can record on tape or memory cards and have connectors to a TV or computer. They have a liquid crystal display to display the recording.
Digital Versatile Disc (DVD)
Emerged around 1995, DVDs seem similar to audio CDs but may have a capacity 25 times higher. DVDs provide high-quality video and audio. They may be single or double-sided, and each side may have one or two layers of overlapping information, reaching a maximum capacity of 17 GB. However, a digital movie can employ over 300 GB of information, so data compression known as MPEG-2 must be performed.
There are three variants of DVD:
- DVD-Video
- DVD-ROM, used in computers
- DVD-Audio, which, in addition to including photos or slideshows, gives a sound quality superior to CDs and DVD-Video.
Most players read all DVDs.
To burn to DVD, there are various computer formats:
- DVD-R is a recordable disc that can be written to once.
- DVD-RW is rewritable many times at high speed.
- DVD-RAM is a disc that can be read and written to at high speed.