Cathode ray tube screen
A cathode ray tube screen is a screen , which on the cathode ray tube by Ferdinand Braun based (Braun tube). Frequently it is also called cathode ray, tube or CRT (abbr. For English Cathode Ray Tube ) screen called. It can be used in a variety of devices such as oscilloscopes , televisions and monitors (e.g. PC monitors, surveillance systems, etc.) and many other areas. However, this technology has been largely replaced by flat screens since the 2000s .
Screens are made in different sizes. The diagonal of the screen is used as a measure. Screens for modern cash registers have a diagonal of approx. 23 cm, while larger computer screens reach up to approx. 56 cm. Usually these diagonals are not given in cm, but in inches . More pixels and thus more information units can be displayed on larger screens than on smaller models, since the image resolution cannot be increased at will. In the entertainment sector (televisions), screen sizes of up to 82 cm are available. The decisive factor here is not the resolution (the number of pixels is determined by the respective television standard), but the viewing distance.
functionality
With the exception of the oscilloscope and other scientific equipment, the screen is used to display raster graphics . The image information is transmitted in a sequence of successively transmitted information per pixel ( television signal ). This is then processed by the electronics within the device and used to display the original image on the luminous layer. The television signal is only one way of getting the information to the screen. In computer technology, the information for the primary colors is transmitted on separate signal paths, as is the information for the synchronization of the position of the electron beam on the luminous layer.
The reason for the separation and joint transmission of the signals is that only small cable-connected distances have to be overcome from the signal generation on the graphics card to the screen. Therefore the effort of mixing and unmixing the complex analog signals, which is always associated with losses, is not necessary here. From the television studio to the television viewer at home there is usually only one transmission channel available, which must ensure transmission over long distances. The effort is worthwhile here.
Structure and mode of operation
The most important component in color monitors or color television sets is the cathode ray tube . By glow emission from heated glow cathodes with subsequent electrostatic focusing, three electron beams are generated which, through fluorescence, generate a more or less bright light spot on the light-emitting layer .
On the way from the beam generation system to the luminous layer, these electron beams are deflected together by magnetic fields, so that a grid is created.
The brightness of a pixel depending on its position on the luminescent screen gives the picture content.
The television signal mentioned at the beginning is used in the screen to control this brightness information as a function of the position of the electron beam.
Horizontal and vertical deflection frequencies
The respective frequencies with which the two magnetic fields deflect the beam in the horizontal (horizontal) and vertical (vertical) direction (= line frequency and refresh rate ), as well as the pixel clock (also called video bandwidth and, on PC monitors, as RAMDAC frequency) the properties of the grid: number of lines or pixels, aspect ratio of the pixels and how often per time a pixel is made to glow again.
The European television standard provides for a horizontal frequency of 15 625 Hz and a vertical frequency of 50 Hz. The image is created using the interlacing method . The 50 Hz are known for the so-called "flicker" in tube televisions.
Most computer monitors (almost all built from around 1990) can adapt these two frequencies to the input signal within certain limits. These are between approx. 30 and 130 kHz in the horizontal direction and between 60 and 200 Hz in the vertical direction.
In computer technology, efforts are made to set the vertical frequency to more than approx. 80 Hz. This is the only way to ensure an eye-friendly, flicker-free display. The limit of freedom from flicker depends on several factors:
- Afterglow time of the luminous layer. Newer picture tubes have luminous layers with very short afterglow times (a few dozen µs). Black and white screens, on the other hand, have long persistence times in the three-digit µs range.
- From the viewer. A few people perceive screens with a vertical frequency of 60 Hz to be flicker-free, while others still recognize a slight flicker even at 85 Hz.
The increase in the line frequency therefore also increases the rate at which the brightness information must be transmitted and processed (pixel clock, see above). In the computer sector, this effect can be seen e.g. B. quite clear when inferior cables are used for signal transmission. A corresponding image with clear contrasts appears increasingly blurred the higher the playback frequencies are set with the same resolution.
Image composition
A distinction is made between two techniques of image construction:
- When interlaced (engl. Interlace ) is initially only every second row of the image, thus only the odd-numbered lines shown, in the subsequent vertical pass then the even-numbered rows. The refresh rate is doubled, so to speak, which creates a less flickering image. A similar method is used with cinema projectors, where each image (24 images per second) is projected twice onto the screen through a screen. Silent films were filmed at 18 frames per second and each image was shown three times. The interlace method is z. B. used in analog TV.
- The alternative is the full-screen process ( progressive scan ). The image is generated line by line in full resolution. Thanks to twice the number of lines, progressive scan delivers better images, but also requires more expensive technology, since the horizontal deflection unit has to deliver twice the frequency. The technique is z. B. used in computer monitors, sometimes also in HDTV .
Advantages and disadvantages
- advantages
- Good black level
- Color display almost completely independent of the viewing angle, even with dark areas of the picture
- No predetermined ideal resolution
- Fast reaction time
- Long durability
- disadvantage
- Big and heavy
- requires a lot of storage space
- Possible influence from external magnetic fields, such as motors, location near overhead lines such as B. Deutsche Bahn (color falsification, flickering, possibly also collapse of the picture etc.)
- "Flickering" and "beeping" especially with older devices
- Afterglow of the luminescent screen, which is normally only noticeable when switching directly to black and in darkened rooms, but under these circumstances it can mean that the last image can still be seen on the screen for about 1–2 seconds. However, the time until the eye has adapted to the new brightness conditions is significantly longer.
- Weak X-rays are emitted from the device. However, newer devices (monitors from TCO 99 ) are practically completely shielded against radiation leakage.
- Possibly. geometric distortions due to non-linearities in the course of the deflection fields over time. However, these can largely be compensated for by appropriate (complex) shaping of the associated voltage curves. With modern devices, these settings can also be fine-tuned by the user.
- Color fringes due to inaccurately adjusted compensation measures to cover the three electron beams.
- Disposal of the picture tube is problematic: Many different materials are combined here, which makes recycling complex.
- Wear of the picture tube ( burn-in , decrease in the emissivity of the cathodes), an exchange is usually not economically worthwhile.
- High power consumption; a 17 "tube monitor typically consumes around 60 watts .
Native screen resolution
Computer screens with cathode ray tubes can, due to their design, display different screen resolutions without significant scaling losses, such as those found in e.g. B. from LC screens are known. With low screen resolutions, the electron beam touches several screen pixels at the same time and thus does the scaling . Cathode ray screens are therefore particularly suitable for barrier-free computer workstations where a low screen resolution with large fonts is required due to better legibility. With particularly small resolutions, however, the space between the written lines becomes noticeable as a horizontal pattern of black lines, since the sharpness of the beam remains constant even at a lower resolution.
Device-specific
Televisions
The first devices were equipped with black and white picture tubes in 4: 3 format, the size of which could be increased to 63 cm by the mid-1970s. Larger screens require a more stable structure of the tube through thicker glass constructions, which is reflected in the weight.
In principle, a cathode ray tube has a certain installation depth. This has been reduced again and again with increasing deflection angle, but will never be able to achieve the low installation depth of modern flat screen technologies.
By the late 1960s, color television development was complete enough that commercial equipment became affordable. At the beginning of the 1990s, attempts were made to introduce the 16: 9 aspect ratio , but this failed. From 2000 it was tried again, with the success that around 20% of all picture tube televisions were sold in 16: 9 format. Some picture tubes were designed for HDTV, which has been popular in Japan and North America since the 1990s .
Computer monitors
There were computer monitors from the 1960s with the monochrome luminous colors white, green and amber. Amber monitors in particular showed a very calm image display due to a long afterglow period.