IBM Selectric Typewriter & its digital to analogue converter

Using slow motion video Bill Hammack, the engineer guy, shows how
IBM's revolutionary "golf ball" typewriter works. He describes the
marvelous completely mechanical digital-to-analogue converter that
translates the discrete impulse of the keys to the rotation of the
type element. (This is the typewriter featured on the television series Mad Men.)

• Category

  Science & Technology

[mechcalc]

Superb video! Very clear explanation of a fabulously elegant (and remarkably robust) mechanism. However, is the IBM Selectric really a D-to-A converter? To be truly analogue, wouldn't it need a continuum of positions? The typeball has 88 discreet positions (96 for Selectric III) so it seems more like a digital-to-digital converter. It's like the difference between a dial or a digital display for a watch.

[engineerguyvideo]

This is a good question and I don't have a simple answer. Clearly its a mechanism (the whiffletree) for changing one kind of impulse into another so maybe D-to-A and A-to-D isn't the best way to think about it.

[drhenrikholm]

It is indeed a good question, with an answer. No D/A converter can produce a truly continuous value. Instead it will produce an approximation. Most D/A converters will have many more "positions" than the Selectric's 88 or 96 -- for instance a Compact Disc has 16 bits per sample which is roughly 65000 positions. This makes it "continuous" for practical purposes (i.e., our ears can't hear that it isn't continuous.) But it still has discrete positions, just many more than the typewriter has.

[engineerguyvideo]

Yeah .. when I research this video I came to a similar conclusion, but not stated as clearly as you did. You these discrete key strokes and the turn the ball, which could be "tuned" to any position (in fact they had to do adjust them from time to time). I recall even thinking like this: If the ball had a piece of lead it would draw a continuous line when rotated ....

[John Hascall]

Hi. I’m Jerry Hathaway with Everything. Tonight, we’re going to look at something that most of us take for granted. The Selectric, what does it look like?

[engineerguyvideo]

:-)

[engineerguyvideo]

. I should have highlighted one more of those levers in blue. The far right one hooked to the cable and so transfers the "pulls", but the lever second from the right is part of the whiffletree tabs. That would give 2x2x2x2 which is 16 and is more than enough for 11 rows. The shift lever is not shown in the drawing. Note that is is a TOTAL of 22 rows ... 11 on each side. So if the ball is set for lower case you need 11 positions; if it is in upper case 11 positions. Does this help?

[Nathan Cohorst]

I strongly agree!



[beag00]

The music is too loud and is distracting.

[TheRealEquinox]

I love seeing machines that you can see just enough of it working, to entice yourself to want to learn more. In a digital age with solid state electronics, it's harder to to interest someone into engineering without that enticement in everyday life. E.g. a belt drive overhead that powers all the rotary tools in a craftsman's shop compared to a 19.2v battery station of power tools from craftsman.

[Tore Sinding Bekkedal]

You might be interested to learn that the golfball mechanism actually made its first appearance at IBM as a printing computer console terminal. If I'm not wrong, for the IBM 7030 Stretch.

[GH123XL]

Honestly, I almost thought to use electric typewriter. However, I realize that nowadays we do not only incorporate texts into documents, but also pictures/diagrams. This is, in my humble opinion, something which could not be done by typewriter. In my fantasy, I would like to have an absolutely thin laptop. No hangky panky. Type only and chat like Blackberry Messenger. I have not seen such laptop. Otherwise, my life would have been simpler.

[HiAdrian]

That's so nifty! No wonder some people get nostalgic about old hardware.

[hashdfw]

One of the earliest "electronic" typewrites was made by IBM based on the selectric. They basically used the Selectric with a similar mechanical mechanism on the keyboard to encode the keystroke which went into 7 or 8 on/off switches. The encoding went into their electronics. Then their electronics fired solenoids that where connected to the levers that do the tilt and rotate. Basically the computer was shimmed onto the control rods.

[fasnachtday]

No, that's a misleading way to think of it. The type ball does have a continuum of positions-- the letters only happen to be printed at 88 points on that continuum, and the mechanism makes it pause at one of those points, but the ball still turns through every fraction of a degree in between them. The type ball's position is an analog signal regardless of how many letters are on it, just because its position is a physical property.