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IBM Selectric Typewriter

Using slow motion video Bill Hammack shows how the mechanical digital-to-analogue converter of IBM's revolutionary "golf ball" typewriter works.

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I marvel at IBM's Selectric Typewriter. I still recall clearly the day my parents brought one home! I thought at the time "that's a magnificant machine" and, you know, I still do.

It paved the way for computer printers because the Selectric had an early version of a digital to analogue converter. Let me show you.

The converter controls this ball, it distinguished the Selectric from all other typewriters. Instead of an individual lever for each letter, they'll all gathered together on a type element: Lower case on one hemisphere, uppercase on the other.

Now to type a letter takes only a fraction of a second, but if slowed down we can see the motion of the type element.

The letter "g", for example, is located on the lower left hand side of the type element.

To access that letter the type elements needs to rotate ... and then tilt and strike. Now let's look at the very clever mechanism that controls the motion of this ball.

There are two cables buried in here that are kind of bured in here. They're a little hard to see, but they control the tilt and rotation of the ball. Pull on the top one and it tilts to one of four rows, and pull on the bottom one and it will make it rotate to one of twenty-two columns.

An amazing mechanical digital-to-analogue controls these cables. Its buried in the typewriter: I can just barely see the top of it down here, but its a set of arms designed to give the cables precise tugs.

Its based on an old device called a "whiffletree". Its a

contraption that distributes forces evently through a set of rigid linkages. Here the typewriter keys pull on the colored tabs, which the whiffletree translates into a precise rotation of these pieces, which in turn tugs on the two cables I showed earlier. The yellow tabs control the tilt, the blue ones the rotation.

Let's look at only the tilt mechanism. It requires two bits of information to access the four rows on the ball: The home position plus three tilt positions. The characters z t half and j make up the front row on the ball - no rotation is needed, only tilt.

The letter z requires no tilt and is represented by the bits zero zero. The type element simply needs to move forward and strike.

Yank on this tab and you pull that tilt cable six hundreds of an inch this happens in seven milliseconds and gives one "unit" of tilt to select the letter t.

Pull on this tab and the cable is yanked tweleve hundreds - twice the distance - and you get two units of tilt for the character half. And lastly pull on both tabs get three unit of tilt to access the bottom row and type the letter j.

The rotation works the same, but more complex because there are twenty-two positions. The importance of all the is that the discrete motions of the keys are converted to an analogue movement of the ball, which means that its possible to hook a computer to a Selectric and use it as an output device. It wasn't easy, but it started us on the word processing revolution.

I'm Bill Hammack - the engineer guy.