R. J.. Reiman, Historian

By 1949, monochrome television had become a commercial success, 10 million sets had been sold, and programs were available to the general public. A change to color television would only be licensed if the color broadcast signal could also be received as a monochrome signal on these sets. This greatly complicated the technology. RCA was the leading company in the television field, with CBS a distant second, but CBS advocated the field sequential color system which utilized rotating disk on which red, green and blue color filters are mounted. The system was not "compatible" with the monochrome requirement cited above, but it was practical, especially under laboratory conditions, and overcame the limitation that the color television tube, which can integrate three channels of signals (dot sequential), had not been invented.

CBS was led by William S. Pauley, who was opposed by RCA's David Sarnoff. CBS executives Frank Stanton and Peter Goldmark, were opposed by RCA's Elmer Engstrom and George H. Brown. RCA had an technical staff edge, more development funds, and the virtually unlimited determination of Sarnoff to make the RCA's dot sequential color system the winner.

The field sequential system (See Figure 1) displayed red, green, and blue television images in sequences, and depended upon the retentivity of the eye to merge these into a single color picture. If, however, flicker and picture sharpness were to be maintained at the level of monochrome television, a field sequential broadcast signal would require three times the bandwidth of monochrome. A compromise or trade off was reached by increasing the bandwidth from 4 to 5 MHz, number of frames were reduced from 30 to 20 per second, and scanning lines reduced from 525 to 343. RCA labeled this system as "mechanical", which was true of the color tube system only.

RCA's dot sequential system approach to solve the bandwidth limitation (see Figure 2), was one proposed by Alda Bedford of RCA, the use of "mixed highs." This relied on the limitation of the eye's relative insensitivity to the fine detail of color, the portion of the picture that requires the transmission of higher frequency components. Bedford proposed that these components be separated from the three color signals, mixed, and then added to the GREEN signal. The bandwidth of the red and blue signals could then be reduced substantially. Another addition, the use of a burst (train of 8 cycles of a sine wave) to the color signal provided a solid synchronism between camera source and receiver, and overcame noise which would cause instability. Field tests brought about the change of color to orange-red and blue-green to take advantage of the eye's insensitivity to fine detail in the blue-green region, thereby narrowing the blue-green band.

CBS won approval of its field sequential system and started expensive color broadcasts. However, the sets did not sell and had no audience. CBS was able to gracefully back-out by the intervention of the Korean War, and the ban on strategic material.

Figure 1 - The Field Sequential Color System.
(Top) Operation of Field Sequential Color System; (Bottom) Color Transmission Sequence in Field Sequential Systems.

The basic operation of the field sequential color system is shown above. Light from the scene passes through a rotating disk on which red, green and blue color filters are mounted. Thus a camera tube is exposed in sequence to the red, green and blue color components of the scene. A disk at the receiver, similarly equipped with color filters, rotates in synchronism so that the light from the kinescope passes through the red filter, for example, while the camera tube is being exposed to red light from the scene.

The bottom portion of Figure 1 shows the color sequence in successive fields of the color signal as proposed by CBS in the 1949 hearing. Note that only two colors are included in each frame; for example, frame 1 has red odd lines and green even lines. Six fields, or 1/8 second, was required to scan all lines in all three colors. This caused fast-moving white objects to exhibit color break-up -- that is, to appear as a series of colored objects.

Figure 2
Principle of the dot sequential system.

Red, green and blue color signals are produced continuously and simultaneously. These signals are then sampled in sequence at a rapid rate, nominally 3.6 MHz. The output of the sampling process is a series of pulses, each having an amplitude proportional to the amplitude of the corresponding color signal at that point in the picture. This signal produces a series of tiny (approximately 0.03" wide) colored dots on a tricolor kinescope. These are perceived by the eye as a single color with a hue determined by the relative amplitude of the red, green and blue pulses at that point.