So It Was Told To Me… [u]

Posted on by Larry

[ Updated June 9, 2019, to correct a math error regarding drop frame timecode. ]

Back in the early days, the 1930’s, when television was being invented, the engineers trying to create pictures out of thin air had a problem.

The method they were using to display a picture required zapping a high-voltage beam of electrons against a curved piece of plate glass the inside edge of which was painted with a phosphorescent solution.

When the voltage was high, around 15,000 volts, the phosphors would glow white. When the voltage was a little less, the phosphors would glow gray. And when the voltage dropped too low, the phosphors wouldn’t glow at all.

The goal was to create an image by “painting” this variable high-voltage beam in a series of rows from the top of the image to the bottom, causing phosphors to glow in its wake.

The problem was that these phosphors weren’t very high quality. The chemistry at the time meant that the phosphors would only glow for as long as it took for the beam to get about half-way down the back of the glass plate. Then, they’d fade away.

This meant that, instead of looking at a complete image, these engineers were looking at a rapidly shifting band of gray as the phosphors were refreshed and faded.

What to do?

Improved chemistry was out of the question. These phosphors were the best available. Instead, the engineers came up with a very clever work-around: instead of painting every line from top to bottom in one pass, they would paint every other line, so that as the phosphors in the first “odd field” started to fade, the second “even field” was already being painted and overlaid on top.

Instantly, the engineers had a complete image and interlacing was born.

The next problem was that there was no wire connecting the originating station with the TV receiver. Without a wire, how could the two synchronize so that the TV set would know when to start painting each frame?

Remember, this was thirty-years before the invention of the transistor and a digital clock.

What do do?

The engineers looked around for a stable clock that could be used anywhere in the United States and discovered the pulse built into 60-cycle AC current. Electricity was standardized across North America

By locking the first “odd” video field to one pulse, then locking the second “even” video field to the second pulse, these new TVs could sync the two fields to create a single video frame 30 times each second – and do it anywhere in the US. Pictures were transmitted, but sync came from the plug in the wall.

Instantly, 30 frames per second (fps) video was born.

NOTE: Film used 24 frames per second because it was the slowest frame rate film could run through a projector and still deliver both smooth movement and high-quality audio. In other words, this was a choice based on cost. TV picked 30 frames a second because it was a timing pulse universally available across North America; a choice based upon universality.

It took more than 80 years to finally invent technology that made interlacing and incompatible frame rates disappear. Mostly.

So where did 25 frames per second come from? Read this.

Except, that explains 30 fps and 25 fps; where did 29.97 fps come from?

Well, the Wizard of Oz, actually.

Back in the early days, television was only black-and-white. But, films like the Wizard of Oz, along with every MGM musical of the 1930’s, proved that the American public thought color was pretty darn cool.

So, David Sarnoff, head of both NBC and RCA, asked his engineers what it would take to create color television. They replied: “One and a half years and one and a half million dollars.” Bold, but wrong.

It took more than 15 years and more than 15 million dollars. And, in the process, both the engineers and marketers had problems.

In those days, the 1940’s and early 50’s, it was impossible for consumers to take out a loan to buy a TV set. You could only pay cash for your TV.

The problem was that a black-and-white TV cost the average American two years salary to purchase. The proposed cost of a color TV looked to require saving for EIGHT years in order to make the payment!

What to do?

The marketers, realizing that no one would be able to afford a color TV if something didn’t change, needed to find a way around cash-only sales of TV sets; because if no one owned a color TV, color television broadcasting would die.

This was the genesis of consumer credit cards and installment loans. Initially offered for dining, the banks expanded it to consumer credit in 1958, though it took until 1976 for Visa to appear. MasterCard appeared a few years later.

As the marketers began to solve how to afford to purchase a TV set, the engineers were struggling to figure out how to transmit a color signal. The easy solution was to create two TV networks: one for black and white programs and one for color. But, as David Sarnoff strongly reminded them, that would mean that no single television could see all the programs available. Anything that limited the potential market was not acceptable.

What to do?

The engineers invented a way to piggy-back a color signal on top of the black-and-white signal.

Instantly, 4:2:2 color was born; where the green channel carried the black-and-white signal, with red and blue channels overlaid on top of the green for those sets that could display color. On Nov. 3, 1956, the Wizard of Oz ushered in the new world of color television on the NBC network.

NOTE: I was a kid when that broadcast first aired. My family watched it, along with about 30 neighbors, packed into the living room of Katie Malvetz, the local furniture dealer who had the only color TV in our city. When black-and-white Kansas turned into the color of Oz, there wasn’t a dry eye in that entire stunned group. It was a life-altering experience for each of us.

The problem was that transmitting all this color information took time. 0.03 seconds every second, to be exact. Where was this extra time to come from? Adding it to the program meant that one hour shows would actually run one hour, three seconds and 18 frames. Not gonna fly.

What do do?

The engineers, in order for shows to run accurately to time, slightly altered the timing of each frame to compensate for the extra delay caused by transmitting a color signal.

Instantly, 29.97 fps and drop-frame timecode were invented.

Well, perhaps every solution wasn’t quite so “instant” but you get the idea. Decisions made in the distant past to solve problems created technology that, warts and all, we continue to use today.

And now you know where each of these originated.

Or so it was told to me.


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4 Responses to So It Was Told To Me… [u]

  1. Philip Snyder says:

    Wow, Larry! Thanks for the behind the scenes explanation of the genesis of color TV. My first encounter with it was in the early ’60s seeing the NBC peacock followed by “Bonanza”. I was blown away by the blue cavalry officers’ uniforms and, later on, by Perry Como’s colorful sweaters. This was not at home but at a local restaurant. The novelty of color TV was definitely viewed as a draw to attract customers.

  2. Terry says:

    Colour theory here in the uk is also mind numbing, it’s locked to the 50hz mains but they have an interesting way of correcting the colour signal to keep it the same as the broadcast output, PAL. all this controlled by its own crystal controlled occilator running at 4.43mhz, there not enough time to go into to the maths behind it and to many years have passed since I studeid it, this was in the days of mechanical delay lines and passive components.

    if you want to know more about go to youtube there will be some kind nurd who has passed an in depth video.

  3. Larry,
    I thought running color results in 108 seconds long each hour, which (at 30fps) results in an overage of 3.6 second long per hour or 3 seconds 18 frames?

    Yes, No?

    Lou

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