Compatible Color: The Ultimate Three-For-One Special

How do you take an existing technological standard, and make it do three times as much “stuff”, all the while without really changing it? This was the task given to RCA in developing their compatible color television system. A black and white television set need only know how bright part of an image is, but a color television needs to know how much red is in an image, how much green, and how much blue. These three combined images will appear in full color, but any one of them on their own wouldn

’t look right on a black and white television. And RCA didn’t have more bandwidth at their disposal as the standards had already been established for black and white TV. They couldn’t just send three discrete images even if they wanted to. So they needed to take the existing television signal, and somehow add more information to it, and still make it compatible with existing black and white televisions. RCA was working towards a color television system that could do just that. They had gotten th

e fundamentals figured out on their compatible color system, and were competing with CBS’s field-sequential color wheel system for FCC approval in 1950. You can learn more about that system in the previous video. RCA’s system was based on the 1938 work of Georges Valensi of France. The National Television Systems Committee, or NTSC, chose RCA’s system as their standard, and they lobbied hard for its approval. To recap, the CBS system used a conventional black and white picture tube and camera. B

oth of which were fitted with a spinning wheel in front of them. The wheel contained repeating sections tinted red, green, and blue. If you show each color sequentially and do it fast enough, persistence of vision will kick in prevent you from noticing. Synchronize the discs between camera and receiver and you’ll get a full color image. But to be convincing, the color wheel has to spin very fast. CBS increased the field rate from the standard 60 hz to 144 hz, allowing the complete R-G-B image to

be seen 48 times per second. But in doing so, they threw out any possibility of being compatible with existing televisions. RCA, meanwhile, was using a new type of picture tube which contained a repeating pattern of red, green, and blue phosphor dots in front of a shadow mask, which worked in conjunction with three separate electron guns in the tube to simultaneously show a red, green, and blue image in one device. These groupings are not in any way discrete pixels, which I feel bears repeating

from my last video. They simply serve as a regular pattern with which the shadow mask can prevent the electron guns from hitting the wrong color. This new shadow mask picture tube was the key to compatibility. If all three electron guns ran at the same intensity relative to each other it would function exactly the same as an ordinary black and white television, making an image using a repeating pattern of horizontal lines. But if you control them independently, the picture tube can now make a f

ull color image on its own, while still building the image in fundamentally the same fashion as a standard television. However, RCA’s color system required a more complicated camera with three separate video tubes, each of which was behind a filter to provide it only with red, green, or blue light. The three separate image components provided by the tubes were combined and encoded in such a way that the transmitted signal appears to be a black and white transmission. A black and white TV would d

isplay a perfectly acceptable image from these transmissions. But a color television set could recover the color information from the camera using some clever tricks. Now, I’m going to do my best at describing how NTSC color is encoded. It’s very technical and hertz to think too much about, so I’m going to tell you some basic info and do my best to explain how it allows the TV to form a color image from an apparently black and white transmission. The beauty of NTSC color was how it dealt with th

e outputs of the three separate camera tubes. Instead of deal with them directly in an RGB encoding scheme, the outputs from the tubes were used to create two separate intermediary signal components: luminance, a brightness component, and chrominance, a separate color component. The luminance part of the signal was created by adding together the red, green, and blue channels from the camera. This would be compatible with existing black and white televisions, as it was simply a measure of overall

brightness. The most genius part of NTSC color was that the chrominance component was almost completely hidden using some clever encoding methods. No extra bandwidth was required because by using a little math, a color television can derive the chrominance component from within the luminance component itself. So first, some methodology. The luminance signal is created by adding together the the R, G, and B components from the camera in this ratio. Luminance is referred to as Y. Chrominance is c

onveyed as two signals, I and Q. I is created by taking 60 percent of the Red signal, subtracting 28 percent of the Green signal, and further subtracting 32 percent of the Blue signal. Q is created by taking 21% of the red signal, subtracting 52% of the green, and then adding 31% of the blue. Yes this is all confusing and I don’t even want to try and work out how these ratios fit into each other but the fact of the matter is so long as the TV set can see I and Q, it can bring back the original R

GB values seen in the camera by applying a bit of analog algebra to the known luminance, that’s Y. With I and Q signals, a television can use the same ratios the camera did to produce I, Y, and Q to bring back R, G, and B, and it will then be able to adjust the output of the three electron beams in order to follow the original RGB ratios the camera picked up. If the television doesn’t see I and Q, it will simply fire all three electron guns with the same intensity to produce only black and white

. Now for the REALLY complicated bit. You might have asked where I and Q are coming from. He keeps talking about these apparently nonexistent values! What is he going on about?! Well, I and Q are hidden within the luminance component. Using a process called QAM, or quadrature amplitude modulation, the luminance component can serve as a vessel for I and Q. To actually extract them, the luminance carrier is multiplied by a 3.579545 MHz reference signal. The phase of this signal is used in determin

ing the hue of the color, so it was critical that the television receive a perfectly timed reference of this frequency from the transmission itself, otherwise the hue would be all wrong. Just before each line, in the back porch as it’s called, is the colorburst. This is a brief transmission of that carrier. The television set’s own crystal oscillator will lock onto and synchronize with the carrier to ensure the colors are decoded correctly. The colorburst doesn’t actually include any color infor

mation, but it’s vital to the television’s ability to correctly extract the color. If the television sees the color burst, it will begin demodulating the I and Q signals with the help of that crystal oscillator, now in a phase-locked loop with the colorburst. Actually, this is easier to explain in graph form. A black and white television signal looks like this. These low points are the horizontal blanking intervals between the lines, and this squiggly bit is the part you see. The brief section b

etween the blanking interval and the start of the visible portion is called the back porch. Along the visible portion, the higher the line goes, the brighter the image is drawn on the screen. Remember, the image is made of lines, which can you learn more about, here. Now, a color transmission looks nearly exactly the same, but this little squiggle, the colorburst, is tucked in just before the line in the back porch. This colorburst is simply the carrier that I and Q are modulated on. The carrier

is suppressed during transmission except for during the colorburst, which eliminates most of the interference between the chrominance and luminance signals, allowing a black and white television to display the image. A color TV, though, will use this colorburst to synchronize its own crystal oscillator. Once it’s locked on, the television set’s oscillator will recreate the carrier. Imagine the colorburst continuing on throughout the entire process. The TV will then use its own generated carrier

to complete the task of demodulating the color. As the three electron beams fly along the face of the tube, the circuitry multiplies both the sine and the cosine of the carrier wave with the luminance value, which will now provide the I and Q values. Then the derived I and Q values, along with the original Y value are passed through a matrix circuit applying the ratios we learned earlier, which will then spit out the original RGB values encoded by the camera. With this recovered, we finally hav

e the ability to mix the three electron guns separately into a coherent RGB output. Now, limited bandwidth meant that the resolution of the chrominance component is not as high as the luminance. In effect, the picture is still mostly monochrome but with gobs of loosely-applied coloring thrown on top. Think of it as the ratio of the three electron beams--thus the apparent color--being locked in by the color signal over large chunks of the line, but still with the ability to in unison change inten

sity, and thus brightness, along with the luminance signal. We’re much more sensitive to changes in brightness than in color, so the image would seem completely natural. In fact, many digital formats such as MPEG-2 exploit our eye’s poor color detection in a similar way, dedicating more data to brightness than color to allow for compression. Anyway, I’m really not going to elaborate further on QAM because oh my goodness there’s a lot of math involved but if you’re intrigued I cordially invite yo

u to visit some links down below. But to finish our story we started with the last video, when the FCC had given the go-ahead for CBS’s color wheel system in 1950, RCA’s color wasn’t working too well. In fact at the beginning of the “competition”, they were still pursuing a three tube optical system, but they were investing heavily in their replacement system using a shadow mask CRT. Regardless, the image quality from the CBS system was far superior to what RCA and others could muster at the tim

e with their early shadow mask offerings. RCA fought very hard to get approval for their system, but it didn’t come. But two things happened to make sure they would eventually succeed. First was the reformation of the NTSC. The National Television Systems Committee created the original black and white television specifications, and having seen RCA’s work on a compatible color system coming down the pipeline, they got back together and worked hard to try and get the FCC to approve this new system

. This very much annoyed the FCC because they had already given permission to CBS to proceed. However, the Korean War threw CBS’s plans under the bus. The National Production Authority declared that color television production must cease on November 20th, 1951--exactly one year after permission was granted to begin color broadcasts. And in the meantime, the NTSC and RCA remained hard at work improving their new compatible color system. By 1953, the shadow mask CRT was sufficiently developed and

was producing excellent results, and funny enough it was one of CBS’s subsidiaries that made the first breakthrough. It became obvious that CBS’s sequential color wheel system didn’t make any sense whatsoever if compatible color was producing similar results. In March of 1953, CBS testified before congress that they were abandoning their color TV efforts, afterwhich time the ban on producing color televisions was lifted. The NTSC restarted their efforts for FCC approval, and it was granted to th

em on December 17th, 1953. The first nationwide color transmission of the new compatible system occurred on January 1, 1954, a broadcast of that year’s Tournament of Roses parade. In the US, color TV wouldn’t really take off for about 10 years. Early color sets were insanely expensive, costing about as much as a new car, and their vacuum tube electronics were very finicky. These color TVs required lots of patience. Though it’s amazing to me that this amount of complexity could even be achieved w

ith 1950s technology. But the fact was, most TV programming was still in black and white. It wasn’t until 1965 that the majority of TV broadcasts were in color. Until that point, owning a color TV was a luxury that only rarely saw its true value. It wouldn’t be until 1972 that color TVs outsold black and white units in the US. And now, some odds and ends. Actually, later, some odds and ends. I hate to do this, but I’ve found over 10 minutes of stuff that I want to talk about. I’m sorry, this was

supposed to be two parts, but it’s getting longer still. In the next video, I’ll touch on the differences between NTSC and PAL, we’ll go over chroma dots and color restoration, Guillermo Gonzalez Camarena’s dubious invention, the 29.97 frames per second nonsense we deal with here in the States, and more. Thanks for watching, and thanks for putting up with another cliffhanger. If you like this sort of video and are new to the channel, why not subscribe? I’d also like to thank all of my Patreon s

upports for keeping this channel possible, especial these folks who get their name in lights. You can support this channel and help keep these videos coming more frequently. Patreon Supporters have allowed me to focus on the channel to a much greater extent. If you’re interested in helping out, please check out my Patreon Page through the link on your screen or down below in the description. Thank you for your consideration, and I’ll see you next time!