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 the 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.
Both 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 full 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 display 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 the 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 conveyed 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 RGB 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 determining 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 information, 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 between 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 have 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 intensity, 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 you 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 time 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 them 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 with
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.
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