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Smith-Corona Ultrasonic

What do you think the term "Ultrasonic" in the name of this Smith-Corona electric typewriter model stands for?
Published June 20, 2022
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Despite owning some mechanical typewriters I don't know much about them. (I bought them based on their look. :D) However, I'd never really been interested in electrical typewriters – although ballpoint typewriters and some typewriter caps compatible with common mechanical switches (e.g. Triumph Adler models) raised my attention. Here comes the short story of why I changed my mind and bought my first electrical one last week. :D

So the other day I came across the listing of a 1983 Smith-Corona Ultrasonic and I immediately fell in love with the keycaps with a strong VIC-20 vibe.

The problem was this is a 110V US model, I can't even plug it into my wall socket. Of course the caps aren't MX compatible either. Still, the machine looked dope and a quick search brought up HaTaa's post from 2014 (here) which convinced me about buying it immediately.

So what's so interesting about this model? The type of actuation.

You all know mechanical switches. Optical ones too. Many of you may be familiar with Hall-effect and capacitive mechanisms to register keypresses. But what about an acoustic method? Ladies and gents, this contraption has sonic "switches".

It arrived in a bizarre packaging and it's a real miracle it survived the travel without any visible damage.


First impressions: Beautiful. And very heavy. I thought the case is metal but it's thick plastic. "Double-wall vacuum-cast." Two lids on the top, they are quite dark smokey transparent ones and move very nicely. One is for the paper feed and the other lid covers the typing head. Opening the latter opens the first one too.

The model I bought is the original Ultrasonic. It seems Smith-Corona made several revisions like the Ultrasonic I Plus, Ultrasonic II Dual-pitch, Ultrasonic III Messenger, and the Ultrasonic 450 Messenger.

Double-shot caps, but as already told, not compatible with any common switch types.

Pic: Smith Corona Ultrasonic doubleshot keycap

Smith Corona Ultrasonic doubleshot keycap

I'm not sure about the material of the ribbon but it is used very effectively. I mean it becomes fully transparent where it was hit, clearly revealing the typed character. Just by taking a look at the exposed part of the ribbon I can tell that the last words typed with this almost 40-year old device were: "Malibu CA90265". And I could probably restore everything typed by unwinding the ribbon which means privacy wasn't the greatest strength of this thing.

It also has a correction ribbon with similarly zero-privacy readability. :D Memory and correction for at least the last word (press and hold Correction button) (source) – however, I'm not sure this particular model is capable of doing that or it was introduced in a later revision.

Sound & feel

Typing on this (unplugged) sounds and feels somewhere between a box switch with click bar and an IBM Model F – and you'll see in a minute why.

This is the original video posted by HaTaa:

Acoustic sensing

HaTaa has a presentation (Keycon West 2014) on switch types, and this Ultrasonic model is the example for acoustic actuation sensing.

This method uses TDOA (Time Difference of Arrival). There is a transfer bar in the back of the keyboard part of the typewriter. Each key hits a position on this bar, a few mms away from one another. And two transducers at the two ends of the rod sense the strike and convert the two sound wave fronts into a first and secondary output signal which can be translated into actual keys/characters.


Here is the patent filed in 1979:

Encoding apparatus for a business machine includes a resilient striker for inducing diverging sound waves within a rod by impact with it at a given location. Transducers positioned along the rod on each side of the striker and at unequal distances from it, convert the sound wave fronts into a first and a second output signal having a predetermined time interval between them which is dependent upon the location of impact with the rod. The transducers are connected to a logic unit which includes an oscillator-driven binary counter. The first output signal starts the counter and subsequent arrival of the second output signal determines the elapsed time, the counter output at that instant being a binary code value usable for any desired purpose: control, display, etc. The encoding apparatus may be incorporated in a keyboard having a number of keys and a corresponding number of strikers differentially positioned along the rod such that each key produces output signals with a predetermined time interval between them and accordingly a unique binary code may be obtained upon actuation of each key.

And here is it in action:


One benefit of this mechanism could be that it doesn't need debouncing – and with only two sensors (two pins?) at the ends of the rod you can pretty much handle all the keys. (Well, shift/caps lock requires another trick but you know what I mean.)

Before you rush to make a modern keyboard based on what you've learned, keep in mind that behind the contraption in the photo there's almost a full typewriter mechanism with all the type bars and even more. So it's probably not a coincidence that we don't see these kind of keyboards nowadays.

For much more photos of the interior check HaTaa's original post.

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Published on Mon 20th Jun 2022. Featured in KBD #83.


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