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Very clever glassware...

Before the days of semiconductors, some really ingenious things were done using electric glassware, including analogue to digital conversion and digital memory devices.

Also, check out this Russion pulse-generator tube

Analogue to digital conversion by glassware!

This is an extract from an article on pulse code modulation, from Electronic Engineering magazine, April 1953, describing a very ingenuious use of a cathode-ray tube to perform 7-bit analogue to digital conversion, outputting a serial binary data stream from an analogue input.

Meacham and Peterson* have described a system which uses a special form of cathode-ray tube for the coder. The signal of each channel is sampled every 125 microseconds and the outputs from 12 channel modulators are commoned to form a pulse-amplitude modulated T.D.M. system. Because the operation of the coder takes longer than 10.4 microseconds two coders are provided, each dealing with alternate channels.

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The essential features of the coding circuit are shown in Fig. 38 and photographs of the coding tubes' and its aperture plate are shown in Fig. 39. The aperture plate is perforated with the 128 combinations of 7 binary digits with holes for units and solid metal for zeros. When-the electron beam is switched on it is deflected by the stored sample voltage to the corresponding vertical height and is moved across the aperture plate by a linear sweep. As the beam passes the holes in the aperture plate current flows to the collector plate producing the coded group of output pulses; the beam is switched off and retraced in readiness for coding the next sample. In front of the aperture plate is the quantizing grid of 129 wires between which are spaces in line with the row of holes in the aperture plate. The wires are used to centre the beam accurately on a row of holes in the aperture plate: without it the beam might wander between two rows during the scan and false coding would result. During the scan a bias voltage is applied which tends to move the beam upward, but when it strikes one of the wires of the quantizing grid secondary electrons are emitted which are attractzd to the collector. The signal from the collector is fed back to the vertical amplifier and deflects the beam downwards so that it is kept in a stable position just beneath the quantizing wire.

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The aperture of the coding tube viewed from the gun end; (below) the complete tube

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*Meacham, L.A.,Peterson,E. An Experimental Multi-channel Pulse Code Modulation System of Toll Quality. Bell Syst. Tech. J. 27,1 (1948)

Thanks to Joe Sousa  for the following info : I was particularly impressed by the 8 bit glass ADC from 1948, because I have been designing ADC's for 20 years. You should know that in the 1980's Tektronix sold a 7912AD digital storage oscilloscope that used a similar glass CRT to digitize single-shot waveforms up to 500MHz. This link will show a picture of the scope.  This scope had GPIB digital output or NTSC video output, where the 512x512 diode array inside the CRT face was read at NTSC video rates. Just imagine how wonderfully geeky it would be to hook up this scope to a 32"(81cm) home TV. I have never seen the actual ADC tube.

Below is a device along similar lines, from Wireless World, March 1964

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A variety of applications are envisaged for the Raytheon CK1378 cathode-ray tube. The electron beam produced by a normal gun is deflected into one of thirty-six sectors on the "screen" of the tube, each sector being electrically isolated from the rest. Output leads from the sectors are taken to four E9 nine-pin bases mounted on the tube face, which is also made fluorescent in the normal way, so that the position of the beam can be seen. The tube can be used for multiplexing, frequency multiplication, analogue-to-digital conversion and many other applications.

wpe39.jpg (14818 bytes)And while we're on the subject of wacky CRT's.... how about this - a tube with EIGHT independent electron guns! Made by 20th Centuary Electronics, from Wireless World, June 1957.






On a similar note, chack out this Interesting 1958 article on generating numeric characters on a CRT using entirely analogue methods

Glass memory

wpe9.jpg (63185 bytes)RCA's Selectron memory tube. 256 bits of RAM in a bottle!

wpe12.jpg (18674 bytes)Thanks to Enrique Viseras-Marin for this photo. More info Here.

This Wired computer history article mentions the 'Johnniac' - an early computer using 80 of these devices. (search for 'Selectron' on page)

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