I remember those. They were made by Summagraphics. We purchased a 
large format one (about 4 feet X 5 feet) to digitize apparel 
patterns. They had linear microphones along the top and left sides of 
the table.  You had to be careful not to put your free hand between 
the spark pen and the microphones. I recall reading about 3-D 

The tablets were accurate to a few hundredths of an inch but were not 
that reliable. I think they simply started two counters when the 
spark went off and stopped each when the microphone registered a 
sound.  If I remember right the did around 5 points per second. We 
eventually switched to mechanical technology.

The noise from a spark probably has a much faster rise time than a 
keyboard click, but with modern signal processing it might well be 
feasible to resolve key presses. Of course, if one can get access to 
the room where the computer is used, it is probably easier to bug the 
keyboard directly.  Still it may be time to add mouse-based 
passphrase input as an option to programs like PGP.

Arnold Reinhold

At 10:24 AM -0500 1/15/2001, Trei, Peter wrote:
>I've seen an existance proof which indicates that this is possible.
>Back when I was first getting involved with computers (circa 1972),
>some digitizer tablets worked by speed-of-sound measurements.
>The stylus tip contained a small  spark gap which was energized
>when the stylus pressed on the  tablet. This created a spark,
>and the spark a minuscule roll of  thunder. Microphones situated
>along the edges of the tablet recorded the arrival times of the sound,
>and the location of the stylus calculated within a millimeter or two.
>This was a peripheral for a DEC PDP-8E.
>This was calculating a position over about 20 cm to a millimeter,
>in real time, in 1972. Doing so to a resolution of a centimeter or
>two, in 2001, ever several meters sounds feasible.
>Peter Trei 
>> ----------
>> From:        Ray Dillinger[SMTP:[EMAIL PROTECTED]]
>> Sent:        Friday, January 12, 2001 4:37 PM
>> To:  John Young
>> Subject:     Re: NONSTOP Crypto Query
>> On Fri, 12 Jan 2001, John Young wrote:
>> >Wright also describes the use of supersensitive microphones
>> >to pick up the daily setting of rotors on cryptomachines of the
>> >time, in particular the Hagelins made by CryptoAG.
>> Hmmm.  That sounds like a trick that could be brought up to
>> date.  If you get two sensitive microphones in a room, you
>> should be able to do interferometry to get the exact locations
>> on a keyboard of keystrokes from the sound of someone typing. 
>> I guess three would be better, but with some reasonable
>> assumptions about keys being coplanar or on a surface of known
>> curvature, two would do it.  Interesting possibilities.
>>                              Bear
>> [A quick contemplation of the wavelength of the sounds in question
> > would put an end to that speculation I suspect. --Perry]
> >

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