Ed Reitan's Color Television History
RCA Dot Sequential Color System
RCA Dot Sequential Colorcast from Washington, D.C.
Color Television System Development during the FCC Hearings 1949-1950
With increased interest by the industry in color television, the Federal Communications Commission on July 11, 1949 called for hearings to determine the feasibility of introducing color service. Hearings began on September 26, 1949 and were, to last until May 26, 1950 with 10,000 pages of testimony and 265 exhibits submitted for the record. Three competing methods of color were proposed: the Field Sequential method by CBS; the Dot Sequential approach of RCA; and the Line Sequential proposed by Color Television Incorporated.
RCA Dot Sequential Color System
RCA first announced its compatible dot sequential system on August 25, 1949. The world's first compatible color broadcasts began in September on NBC's WNBW Channel 4 in Washington, D.C. Three tube, image orthicon live pickup cameras and flying spot slide pickup cameras were used in the Wardman Park Hotel studios. Cameras were developed at RCA Laboratories by Richard C. Webb.
The dot sequential color system involved high speed time-division multiplexing, whereby a sharp sampling commutator selected 2-MHz lowpass filtered red, blue, and green channels in turn. The sampling frequency was first 3.8 MHz, later changed to 3.6 MHz, and then refined to 3.583125 MHz. Initially, synchronization of the sampling frequencies between the transmitter and receiver was accomplished by varying the timing of the trailing edge of horizontal horizontal sync pulse in a manner to control sampling phasing and color dot interlace on a line-to-line basis. The instability of this method was later overcome by Al Bedford's suggestion that a reference burst of the color sampling frequency be transmitted during the rear porch of the horizontal sync waveform - a method later adopted for the later NTSC system, and which has served well for 47 years. Bedford's burst was on a dc pedestal above the blanking level. Video representing mixed highs (the 2-4 MHz high pass-filtered sum of the red, green, and blue channels - a concept also devised by Bedford), was added to the sampled sequence to complete the dot sequential signal.
RCA first demonstrated its system using a number of bulky three-tube projection receivers and "triniscope" direct view receivers (using 10-inch picture tubes with images combined by dichroic mirrors). One of these receivers had 108 tubes. Dr. George Brown, in his entertaining and informative autobiography, describes one triniscope receiver, using 16-inch CRTs, that was 6-feet tall, 6-feet deep, and 31-inches wide. (ref.16). Considering the bulk of these RCA receivers, it is understandable why the competing CBS color system with its large rotating color wheel was not immediately derided as impractical.
To reduce the complexity of these triniscope receivers, RCA built a number of receivers that operated using the theory of two-color primary reproduction. The images from two (orange and cyan) CRTs were combined by a dichroic mirror to achieve a limited color range that was not unacceptable to a public experienced with two-color motion pictures made between the 1920s and 1950s.
One of the three known surviving triniscope receivers was deposited with the UCLA Collection of Television Technology by the RCA Laboratories of Princeton, N.J. (arranged by Al Pinsky and Julie Maddocks). The model started in development in March 1950 and a quantity of ten was completed by May. This is the last of the series of triniscope models. Technical details of this triniscope model is contained in the RCA Laboratories Developmental Color Receivers page at this color television history site.
Shortly thereafter, RCA unveiled its 16-inch metal cone shadow mask color kinescope to the FCC and the press on March 29, 1950. After that time, all RCA receiver work concentrated on the development of shadow mask crt receivers. RCA Broadcast continued to refine the laboratory model studio equipment into producible live and film cameras.
Copyright 1997, Ed Reitan