Thursday, March 07, 2013

Backwards Compatibility I - Presented In Color

Journey to the Past

Let's take a journey back in time - well before smart phones and buzzwords like "social media" (ugh). Before Nintendo (pick any version), before Atari 2600, and even before Pong. Turn back your mental clock to a time when people would listen to shows on the radio. It was in this period that owning a newfangled television set became something of a status symbol.

Television was a new phenomenon and a fascinating novelty. There were still many great enhancements left to discover. Nobody had yet figured out recorded video, instant replay, picture-in-picture, infomercials, reality TV, daytime talk shows, or re-runs. Perhaps most surprising of all was that viewers might like to see their shows in color instead of just black and white. Even while lacking all of these things that we take for granted today, television was clearly a hit. Consumers far and wide opened their wallets to buy one of these shiny entertainment machines.

How Televisions Work

Those readers who are familiar with how CRT televisions and monitors work will probably like to skip my simplified explanation in this section. Go ahead and we'll catch up with you in the next section. For those who are learning about this for the first time, pull up a comfortable chair and grab an icy-cold drink and read on. Readers who are truly in the know will find my description to be a slight oversimplification.

The earliest televisions used a technology called cathode ray tubes (CRT). This was the primary television technology for many years and you will still find them haunting the corners of basements, rec rooms, and man caves today. CRTs function very differently from the modern LCD, LED, plasma or other more exotic televisions available today.

In CRT screens, an electron gun fires a beam of electrons at the screen. A thin coating of phosphors are layered on the inside of the screen. When these phosphors are hit by the electrons, they glow for a brief time. A powerful electromagnet directs the stream of electrons in a zig-zag pattern across the screen and from top to bottom. The electron gun modulates the intensity of the beam from high to low. When the intensity is high, the phosphors on the screen will glow brightly (white). By reducing the intensity, the display can glow in shades of gray or even fade to black. A further, more detailed description is available here.

All of the above happens many times per second - 60 per second for North America and 50 per second for most of Europe. In order to determine how intense the electron beam should be, the television is tuned to a video signal. In original black-and-white televisions, this only indicated two things: 1. When to begin each pass. 2. The intensity of the electron beam at each moment. If your television is tuned in to a strong enough signal, this will result in a clear image on your screen.

The Trouble with Color

To those who skipped the previous section, welcome back. To those who chose to read on, I hope you learned something new.

Now, color was obviously a desirable feature. Inventors quickly began working toward this goal. Unfortunately, color added a whole new level of complication to the system. Achieving color television would require modifications to both the hardware used to display those colors and to the video signal that transmits them.

The very first commercial color broadcast used a newly-invented signal format. Televisions designed to receive these signals worked perfectly well with the new signal, but all of the old black-and-white sets already installed in viewers' living rooms were completely unable to view these broadcasts. Needless to say, this caused a serious chicken-and-egg problem. Consumers would need to buy new televisions to watch in color, but they couldn't watch black-and-white broadcasts. Broadcasters had to choose whether to broadcast each show in the sparsely-adopted color format, or in the well-established black-and-white format.

Backwards Compatibility

What if we could find a way that old black-and-white televisions could watch new color broadcasts (minus the color) while new color televisions could still receive old black-and-white transmissions? Broadcasters and consumers could stop worrying about the tired old question of black-and-white vs. color and move on to inventing and voting on singing competition shows respectively. As nice as it would be, surely this idea was just a naive pipe dream.

Amazingly enough, Georges Valensi found a clever way to make this dream a reality. He designed a new video signal that added additional information to the old format. The first part of the video signal indicated the overall light intensity (just like before). The second, additional, part of the video signal indicated the relative intensity of the red, green, and blue color components. This meant that the old black-and-white televisions could safely ignore the second part of the signal and still display a reasonable image. New color televisions were able to perform additional processing on the signal to fill in the corresponding colors.

This example of the color television signal format is just one of many thousands of examples of backwards compatibility. When implemented correctly, backwards compatibility allows old, existing systems to continue functioning while adding new, desirable features and capabilities to newly-designed systems. Sometimes protocols can be designed with future flexibility in mind. Other times, clever thinking is required to find a compatible new solution. In any case, backwards compatibility is a fabulous way to create something new without breaking a system that is already in use and working well.


Joshua Ganes

This post was an introduction to the topic of backwards compatibility. My next article on this topic: Backwards Compatibility II - Fixing Bugs.

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