Test image

Test images are used to assess the image quality of televisions and monitors as well as to support image setting and troubleshooting. Some test images are only useful for older devices with picture tubes , others are only used for digital image transmissions.

Historic television with the universal test pattern from the 1950s

Test methods in photography (especially for image sharpness ) and in ophthalmic optics (e.g. Amsler grid ) are also referred to as test images .

history

Image adjustment at the Grundig company in the 1950s: Exact inspection of the television chassis just manufactured for its image quality.

In Germany in the 1950s, the Institute for Broadcasting Technology designed the “universal test pattern” for ARD , which was broadcast roughly until ZDF started broadcasting. It was a graphic. Then the first black and white test images, generated purely electronically by test image generators, were used. On July 12, 1950, before the start of its regular television program, the Northwest German Radio broadcast the test image for the first time. With the beginning of color television , the Dutch manufacturer Philips commissioned the Danish engineer Finn Hendil (1939–2011) in 1966 to design the color test image for television sets, the Philips test image PM5544 . It is probably the best-known, electronically generated color test pattern that has been used in several countries for many years. Before the introduction of 24-hour television programs, test images were shown on public television in the non-broadcast times after the end of the program . By the end of the 1980s, the test images were switched off a few minutes after the broadcast had ended and only switched on again in the late morning. The various test images were highlighted either with a measurement tone (ZDF) or the tone of a radio program (ARD). Especially after these regular test picture programs were discontinued in the years after around 1990 ( RTL and Sat.1 1991, ProSieben 1995, ARD and ZDF 1996), the test picture of various broadcasters also achieved a certain cult status and has developed into a popular collector's item in certain circles . The last German television program that regularly sent a test image was hr-fernsehen until the end of 1997 . Today you can often see test images on cable television if there is a fault, or on the broadcasters from other countries via satellite.

In some countries such as Sweden or Estonia , test images are still common today until the early afternoon, although the test image is now only sent briefly in Sweden. Instead, in times of low viewership, the program of a news broadcaster or an automated program preview is activated on public broadcasters, while private broadcasters increasingly run repetitions or call-in broadcasts during the night. In Italy there are technical rehearsals once a month, which last from 2.00 am to 7.00 am. The transmitter is switched off and a test pattern runs. Every first Monday of the month there are rehearsals on Rai 1 , every second Tuesday of the month on Rai 2 and every third Friday of the month on Rai 3 .

Today, if required, test images can be generated at any time using special test image generators, with minor restrictions also with a test image DVD in the DVD player or via a computer with a TV output.

A measurement of the transmission path during operation is not carried out with (nowadays hardly any) analog transmission paths with test images, but with special test lines . In the vertical blanking interval, a normally invisible image line is replaced by a special signal, for example a 2T pulse . The visible television picture is not changed.

to form

Universal test pattern

Basic structure of the universal test image (there were different variants; these e.g. without white triangles to mark the side edges)

A universal test pattern was broadcast in the 1950s and early 1960s . This was a slide that was scanned by a slide or a monoscope. The geometrical figures of the test image made it possible to test and adjust in particular

1. Center and edge sharpness
2. gradation
3. Black attitude
4. Image geometry
5. Black level
6. White level
7. Linearity
8. Frequency response
9. Transient behavior
10. Noise
11. Reflection (caused by the antenna)

Wedge-shaped fans of lines in the vertical and horizontal directions (so-called frequency brooms ) allowed an assessment of the vertical (number of lines) and horizontal resolution. In addition, there were markings with the inscriptions 200, 300, 400 and 600 on the compartments. When a television receiver achieved a vertical resolution of 550 and a horizontal resolution of 400 pixels in the early 1960s, the resolution was considered to be particularly good. Minimum values ​​for usable image quality were 450–500 (vertical) and 350 (horizontal).

Horizontal and vertical gray wedges with ten different brightness levels (from white to black) were available for assessing and setting brightness and contrast. Four small multiple circles in the corners of the test image were used for focusing. A white circle in the center of the picture enabled the correct setting of the aspect ratio to be assessed. White triangles on the side edges pointing outwards were used to check the correct size of the image. To assess the linearity, eleven vertical black lines were arranged in the middle at the upper, lower, left and right edges of the image, each marked with the number 2 .

Analog test image (FuBK test image)

Sketch of a FuBK television test picture, which was mainly used in Germany

The "normal" test image - FuBK stands for Radio Operations Commission  - consists of different colored and black and white fields, a large circle and a grid for geometry settings, a gray staircase for contrast and brightness control and four burst signals with sine curves 1 MHz, 2 MHz , 3 MHz and 4.433 MHz as a test of the horizontal resolution. With PAL , with the maximum displayable video bandwidth of around 5  MHz , individual lines should just be visible in all fields; if this is not the case (for example with a VHS video with around 3 MHz), they become increasingly closed a gray area. If PAL is used, there are two special gray fields, the so-called "achromatic fields". In one of these fields, during modulation in the area of ​​this field, instead of the carrier for the red color difference signal, the carrier for the blue color difference signal is switched in its phase position by 180 degrees. In the other field, the carrier of the red component is not switched. In both cases this creates an artificial phase error of 90 degrees. Due to this phase error, the color is completely canceled out during demodulation, since the red component is carried out normally every second line in the receiver. If the reverse rotation of the receiver is not 180 degrees or the angle between the red and blue components is not exactly 90 degrees, the cancellation would not be complete and the two fields would be colored.

This picture shows the pointer behavior for the achromatic field, in which the blue component instead of the red component in the phase position is switched by 180 degrees in every second line.

1. Vector diagram: line , normal phase position ${\displaystyle n}$
2. Vector diagram: line , instead of the carrier of the red color difference signal, the carrier for blue is rotated by 180 degrees ${\displaystyle n+1}$
3. Vector diagram: In the receiver, the carrier of the red color difference signal is nevertheless rotated
4. Vector diagram: vector addition of the color signals of the two lines
   Result: the color is erased - an achromatic field

Color bar test pattern

EBU color bars

In this color bar test pattern , a number of colored vertical bars are generated, starting with white through all colors that can be represented with two of the basic colors ( RGB ) and the basic colors themselves in decreasing brightness up to black. This image is particularly suitable for troubleshooting with an oscilloscope , since all image lines (except those in the image blanking interval) carry the same information. The oscilloscope can be operated with line frequency and with the appropriate triggering you can see a still picture when you are scanning a video signal, color difference signals or other oscilloscope, because since all lines are the same, all lines are written exactly one above the other by the oscilloscope. See also: television signal .

SMPTE test pattern

SMPTE color bars

The SMPTE test pattern is similar to the EBU color bar pattern and has its origin in North America. It has been expanded to include additional colors in the lower area, which are used to control the color setting in receivers according to the NTSC video standard . The first analog NTSC color televisions tended to have more color distortions due to the transmission method. Below the red bar there are three shades of black: "super black", normal black and very dark gray. If the brightness is set correctly, the first two fields should look the same, while the third should look a little brighter. If all fields look different, the brightness is too high; if they all look the same, the brightness is set too low.

Digital test images

Due to the increasing spread of digital television, special test images are used in this area. These are less used for adjusting display devices than for simple and mostly automatic control of the data transmission or the determination of the number of transmission errors. This applies above all to the display of movement, because due to the MPEG coding with integrated image memories, even a brief line interruption cannot be controlled with a simple still image. At least one picture element should contain a uniform movement (for example bars moving from left to right and vice versa) in order to also detect conversion errors (60 Hz / 50 Hz image frequency), for example.

A loss-free digital video transmission based on the Serial Digital Interface Standard (SDI signal), which is mainly used in studios and broadcasts around the world, uses test patterns similar to the SMPTE test pattern, but extends these in the lower area with certain color combinations which are used in the serial coding of the Image information leads to long logical 0 or logical 1 sequences. These data sequences are more difficult to receive due to the lack of clock information for SDI receivers and place higher quality requirements on the clock sources used, such as crystal oscillators . These test images are also known colloquially as clock crackers .

Grid test pattern

It consists of a white grid on a black background. This can be used to check the image geometry and the convergence of the picture tube . Convergence means that the three electron beams for red, green and blue are exactly on top of each other.

Monochrome test images

The color purity can be checked with monochrome test images in red, green and blue. Especially for liquid crystal displays can use it to pixel errors are detected.

Pump test pattern

A larger area of ​​the picture is constantly changing between white and black. There should be no change in image size. This test is only useful for cathode ray tubes . The cathode beam, which consists of electrons , forms a circuit with the high-voltage generator . If the picture becomes brighter, more current flows . If the stabilization of the high voltage is inadequate, it will sag significantly. The high voltage ( acceleration voltage ) is used to accelerate the electrons. With a lower voltage, their speed also decreases . Slower electrons are easier to deflect, so the image is shown larger. Due to the electrical capacitance of the picture tube and components of the high-voltage generation, the acceleration voltage changes with a delay , which means that the picture size changes in a characteristic way - called pumps - when there are light-dark  changes.

Special test images and elements

• Lena (test picture) - A test picture of computer graphics
• Frequency broom - A black and white element used to detect the resolution
• Siemens star - black and white test pattern for printers and digital cameras

Trivia

• In the series 1000 masterpieces there was a report about "The Testbild" of the Munich Institute for Broadcasting Technology by Paul-M. Sedlacek and Petra Neuwirth , the first broadcast of this episode was on October 2, 1994.
• The band Mittagspause from Düsseldorf published the song Testbild in 1979 , Die Toten Hosen, a title of the same name in 1988 on their album A Little Bit of Horror Show .
• On the ZDF digital channel ZDFneo , an EBU color bar runs between the individual programs , but after a few seconds it turns out that the bars are shown by actors in huge color bars. There are at least ten different episodes of it.
• In the show Zeigler's wonderful world of football , Arnd Zeigler regularly displays test images as image effects.
• A German band founded in 1978 is called DIN A Testbild .

literature

• Werner W. Diefenbach: television service . In: Handbook of radio and television repair technology . tape 2 . Franck'sche Verlagsbuchhandlung, Stuttgart 1961.
• Werner W. Diefenbach: Television service fault diagnosis based on test images and oscillograms . In: Handbook of radio and television repair technology . tape 3 . Franck'sche Verlagsbuchhandlung, Stuttgart 1961.

Web links

Commons : Test patterns  - collection of images, videos and audio files

Wiktionary: Test image  - explanations of meanings, word origins, synonyms, translations

• tv-testbild.com Links and pictures to the test picture
• Test image - The film documentation about the test image and about test image collectors
• Stefan Petermann: It's a shame you're not there anymore: The test image. In: JUSTmag. Archived from the original on November 29, 2013 . 
• Solveig Grothe: TV test images: Longing for the broadcast break. In: Spiegel Online . October 9, 2008 (with numerous international test images). 
• Oliver Klasen: 70 years of test image: from gap filler to design icon. In: sueddeutsche.de . July 12, 2020 . 

Individual evidence

1. ↑ Joachim Umbach: The good old test image. In: Schwäbische Zeitung (Ravensburg edition), July 22, 2000, p. 53.
2. ↑ Thomas Harder: Danskeren bag verdens mest sete TV-udsendelse er død. In: ekstrabladet.dk . February 2, 2011, accessed on July 12, 2020 (dk). 
3. ↑ Wolfgang Tunze: Photo series for: 60 years of television: Good evening, ladies and gentlemen! Image 4 of 7: There are no more test images today. In: FAZ.net . December 19, 2012, accessed July 12, 2020 . 
4. ↑ 1000 masterpieces. Institute for Broadcasting Technology: “The test image” on YouTube (video; 9:17 minutes).