Early Color Television
RCA Dot Sequential System (US, 1949)
RCA's approach to a compatible electronic color system was a progression of previous work demonstrated in 1946. RCA’s 1946 system used simultaneous transmission of full bandwidth green and limited bandwidth red and blue signals in a non-compatible 12 Mhz channel, the same bandwidth as CBS’s 1946 field sequential system. High frequency components of red and blue were mixed into the green channel, the concept of ‘Mixed Highs’ by Alva Bedford.
The 1949 RCA compatible 6 Mhz bandwidth system used red, green, and blue channels equally limited to 2 MHz bandwidth and sampled sequentially at 3.8 Mhz, then later at 3.6 MHz. Color sync to the receiver was provided by wiggling the horizontal sync pulse trailing edge, then later as a color sine wave sync, or ‘burst’. The high frequencies of a monochrome mix of RGB, from 2 to 4 MHz, were passed around the sequential RGB switching, using the Mixed Highs method.
RCA’s dot-multiplexing system combined with Mixed Highs was the breakthrough that would allow B&W television owners to receive the compatible broadcasts, but defects such as ‘dot crawl’ produced a noisy image on monochrome sets.
The RCA Dot-Sequential and Mixed Highs concept was replaced by the concept of Shunted Monochrome invented by Bernard Loughlin at Hazeltine Corp. at Long Island, NY. In Shunted Monochrome the red, blue, and green signals are combined in proportion to create a full bandwidth monochrome signal without sampling, and the color channels produce no luminance information, as used today in all color systems. The Shunted Monochrome is received as a normal monochrome image on B&W receivers. Hazeltine demonstrated the improved system to the FCC and to RCA engineers in April, 1950. RCA engineers quickly modified their system to use Loughlin’s method.
Hazeltine engineers also found that the 2 MHz bandwidth of the red and green channels of RCA”s system was not necessary and contributed to noise. Hazeltine engineers also developed the concept of deriving RGB with two color difference signals, evolving into the two-axis color modulation system of NTSC and other color systems today.