Alexanderson and Color Television

Alexanderson was by now experimenting with color television, and is showing the apparatus he uses to interested members of the USA’s National Television Systems Committee for television, including Peter Goldmark, responsible for similar experiments at Columbia Broadcasting System and George Henry Payne, member of the Federal Communications Commission. The show took place in the basement of Alexanderson’s house at Adams Road in Schenectady in November, 1940.

The equipment consisted of an ordinary black and white receiver and in front of the monitor was placed a rotating disk. The disk, 24 inch in diameter, consisted of two transparent color filters, one orange-red and the other green-blue. The disk spun at 1,800 r.p.m and at this speed the color reproduction of the transmitted program became very natural.

On the transmitting side a rotary disk was placed in front of the iconoscope tube with corresponding transparent color filters and with the same rotation speed as the receiver had. The other equipment was like a black and white transmission set-up. Alexanderson had earlier tried with three color disks, but then “color flicker” had appeared. As the difference in color reproduction according to Alexanderson was minor, the system with two colored filters was chosen.

RCA took increasing interest in the development of TV technology in the USA and in Alexanderson’s experiments in color television. Rotary multi-colored disks can be seen as one of his active interests within this area. However he followed with interest other developments and after the Second World War gave Swedish engineers his views on television’s possibilities in Sweden, which we will touch on later.

True to his habits however, Alexanderson came back to television, and in February 14, 1952 — several years after retirement — he handed in an application for a patent on an entirely new receiver for color television. The intention with the new design was that the device should be able to receive both black and white and color television, and be suited for RCA’s system of color TV transmission. The receiver was based on an adaptation of an extra unit to a conventional black and white receiver. The idea was the method of making it possible to do the color selection. In his proposal for the design he retains the original scanning system for both black and white television and color use.

Instead of seeking, as previously, geometric conformity between the color phosphor line pattern and the “scanning lines” or “raster,” Alexanderson suggests letting the printed color phosphor lines and scanning raster of the electron beams cut each other at a 20-degree angle in accordance with the sketch (Fig 1). A and B indicate two lines in the raster, moving over the picture screen as a fast wave motion over the RGB (red, green and blue) of the printed phosphors of the color lines. The left part, indicated as 1, represents a red picture section, the right, 2, a white section (all colors together). [In essence the scanning electron beam is “position modulated” to place it over RGB phosphors as needed.]

At 3 the cut-off point between the red signal and the horizontal grid is shown. One can also see that the color signal wave between adjacent scanned lines is offset by 180 degrees in phase. The result is that the horizontal electron beam of the scanning-line A overlaps the red printed phosphor lines with even intervals as in 4, leaving the following intervals unexposed. The electron scanning-line B, however, fills in the gap 5 left by scanning-line A, causing the red printed phosphor line to light up for all of its length.

At 8 in the lower right-hand picture, the original receiver for black and white is represented by a “black box” of an amplifier controlling the grid of the electron gun 9. 15 shows three pentodes serving as gates for red, green and the blue signals and 16 consists of three detectors which distinguish the color signals and open the “gates.”

If the photocell, 11, connected to the picture tube, is not switched on, the receiver gives a common black and white picture with a horizontal line grid. If, on the other hand, the photocell is connected and the specially modulated light from the picture screen activates it, the amplifier 12 gives a signal of two MHz starting the oscillator 13 and giving energy to the 3-phase network, 14. In this way three phase-displaced alternating voltages are obtained, to represent each color red, green and blue. The voltages control the grids of the three pentodes, which make up the color gates 15. The other grids are controlled by the three detectors, 16.

RCA was offered the invention, but declined, as the company already was concentrating on another system NTSC (National Television Systems Committee, from which SECAM (Systeme Electronique Couleur Avec Memoire) and later PAL (Phase Alternation Line) were developed, the two systems that are used in Europe.

Alexanderson’s interesting, but in practice untested, contributions to the development of color television was granted a patent on February 8, 1955.