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Why are test cards so important in broadcast history? Did you know?
Test cards, also known as test patterns or start/stop tests, have always been an integral element of television broadcasts. These test signals are typically broadcast when the transmitter is enabled but not playing any programs, allowing the user to adjust the television's display. From the earliest physical cards to later electronic test signals, the evolution of test cards not only optimized broadcast quality, but also laid the foundation for the development of subsequent technologies.
The history of test charts cannot be traced back to the origins of television broadcasting. They were the first step in adjusting image quality.
The early test cards were physical cards, and the cameras were adjusted to these cards to ensure the best image quality. With the advancement of technology, beginning in the 1950s, the image of the test card was built into the monochrome tube, freeing up the camera and avoiding the trouble of needing to constantly rotate the camera to broadcast the physical test card. By the late 1960s, electronically generated test patterns were introduced, making broadcast quality testing more accurate and reliable.
Electronically generated test patterns detect different parameters including color decoding, synchronization, frame rate and frequency response.
These modes are not only for image adjustment, but can also be paired with other image test equipment such as vector oscilloscopes, making fine adjustment of imaging equipment possible. The audio played by the test card is usually a sine wave tone, or accompanied by music from the radio station, which adds a certain cultural background to the testing process.
With the rise of digital television, digitally generated test charts have emerged with specific features, such as checking for error correction, color subsampling, and audio settings. Today, the use of test cards is not limited to televisions, but also extends to large LED screens and other digital display devices such as video projectors.
Technical details of the test card
A test chart usually contains a set of patterns that allow television cameras and receivers to be adjusted to display the picture correctly. Most modern test charts contain calibrated color bars that create characteristic patterns on a vectorscope, allowing for precise adjustment of color and tone.
These color bars cover the range from black (7.5 IRE) to completely white (100 IRE) and include the so-called "sub-black" bit (0 IRE), which represents the lowest low-frequency transmission acceptable in NTSC broadcasts Voltage.
As test cards became more popular, many broadcasters began using them to determine the actual coverage of new broadcast antennas and networks. In the 1950s, the British Independent Television Authority (ITA) commissioned Belling & Lee to design a series of test transmission cards for testing by potential viewers and enthusiasts. Not only that, the BBC began testing its new UHF network in 1964 with Test Card F.
Historical evolution of test cards
Test cards have grown up with television since the early days of broadcasting. The earliest documented records can be traced back to the BBC's mechanical 30-line broadcast in 1934. With the advancement of technology, the pattern of the test chart has also continued to evolve, adding graticules for resolution testing, grids to help with geometric adjustment of pictures, and gray scales for brightness and contrast adjustment.
In North America, the famous Indian Head test patterns became a household name in the 1950s and 1960s, but with the development of 24-hour television networks, they gradually faded from the market.
With the introduction of color television, electronically generated test charts became popular, and many test charts were named after the devices they generated, such as Grundig VG1000 or Philips PM5544. With the financial considerations of commercial television, non-commercial broadcasts have to face the same requirements as commercial television in time scheduling, making test cards increasingly scarce in today's quiet periods.
The application and impact of test cards today
Today, test cards are not only used in television broadcasts, but also in various digital display devices. For example, in large-scale concerts and live events, the functionality of the test card also extends to professional video settings, adjusted according to specific technical parameters.
The advancement of automation technology has made test cards no longer exclusive to the broadcast industry, but has extended to more digital display fields.
Whether it is used for digital proof of calibration or test images tailored for extended setups in large-scale installations, the diverse application examples of test cards have repeatedly proven their importance in today's media environment . So, in future technology evolution, will we see test cards play a role in a wider range of verification applications?
