Field-sequential color system

A field-sequential color system (FSC) is a color television system in which the primary color information is transmitted in successive images and relies on the human vision system to fuse the successive images into a color picture. One field-sequential system was developed by Dr. Peter Goldmark for CBS, which was its sole user in commercial broadcasting. It was first demonstrated to the press on September 4, 1940,[1] and first shown to the general public on January 12, 1950.[2] The Federal Communications Commission adopted it on October 11, 1950 as the standard for color television in the United States, but it was later withdrawn.[3]

The use of sequential color systems for moving images predates the invention of fully electronic television. Although usually known at the time simply as "additive" rather than sequential color systems, two-color Kinemacolor, in commercial use since 1906, and its predecessor three-color format, invented by Edward Raymond Turner and patented in 1899, were both sequential natural color systems for use with motion picture film. They utilized black-and-white film and rotating color filter wheels to record the amount of each color in the scene on alternating frames of the film, so that when the frames were projected by light of similar colors at a sufficiently rapid rate, those colors blended together in the viewer's eye and produced a wider range of hues. Due to litigation by William Friese-Greene, Kinemacolor ended up in the public domain in 1915, after which several derivative sequential color processes (such as Friese-Greene's Biocolour and the original Prisma Color) were developed. Some were brought to the point of being publicly shown, but during the 1920s they could not compete with rival bipack and other subtractive color processes, which were free of color flicker and did not require special projection equipment—the final multicolored images were right there on the film as transparent coloring matter.

Operation

Patent diagrams of CBS field-sequential color system: Fig. 1 the transmission system, Fig. 2 the receiving system, Fig. 3 the color filter disk.

The CBS field-sequential system was an example of a mechanical television system because it relied in part on a disc of color filters rotating at 1440 rpm inside the camera and the receiver, capturing and displaying red, green, and blue television images in sequence. The field rate was increased from 60 to 144 fields per second to overcome the flicker from the separate color images, resulting in 24 complete color frames per second (each of the three colors was scanned twice, double interlacing being standard for all electronic television: 2 scans × 3 colors × 24 frames per second = 144 fields per second), instead of the standard 30 frames/60 fields per second of monochrome. If the 144-field color signal were transmitted with the same detail as a 60-field monochrome signal, 2.4 times the bandwidth would be required. Therefore, to keep the signal within the standard 6-MHz bandwidth of a channel, the image's vertical resolution was reduced from 525 lines to 405. The vertical resolution was 77% of monochrome, and the horizontal resolution was 54% of monochrome.[4]

Because of these variances in resolution and frame rate from the NTSC standards for television broadcasting, field-sequential color broadcasts could not be seen on existing black and white receivers without an adapter (to see them in monochrome), or adapter-converter (to see them in color).[5]

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