Retrotectacular: The 1965 Forgotten Vacuum Tube A / D Converter

In any age, the story of electronics was about finding out how something happened with what was available at that time. And as is often the case, the most interesting developments come from events when needs are available that go beyond. That’s when real innovation happens, even when the market can’t keep up.

A video from the Antique Wireless Association contains a perfect example of this gem: the long-lost analog-to-digital converter vacuum tube. Like almost every other invention in the mid-twentieth century in electronics, it also has its roots back in Bell Laboratories, which was interested in improving bandwidth over a vast network of copper lines and microwave links. In early 1947, Dr. Frank Gray, a physicist at Bell Labs, was working on a vacuum tube that could directly convert an analog signal into a digital representation. His solution was a cathode ray tube similar to CRT in an oscilloscope. A ray of electrons will illuminate the length of the tube over a perforated shadow mask arranged in a “reflective binary code”, which will later be known as a gray code. To digitize, the analog signal was applied to a pair of vertical deflector plates, which moved the beam to a position along the plate relative to the voltage. A pair of horizontal deflector plates will then scan the beam across the shadow mask; Where electrons fell into a hole, they had to go through an output plate so that it could be registered as a set bit.

Fast forward twenty years, and Dr. Gray’s basic concept was used to create a 224 Mb / s analog-to-digital converter that was not possible with today’s transistors. The invention of this tube was to make the output parallel – instead of raster a single electron beam across the shadow mask in the appropriate position, a ribbon of electrons fell into a full 9-bit row before hitting the output detector array.

As usual for Bell Labs, the tube has performed excellently, almost matching the theoretical signal-to-noise ratio. But alas, another lab project to create an all-solid-state ADC gained traction while perfecting the tube, and AT&T focused on microwave guidelines and optical fiber for their digital networking needs. .

The vacuum tube was as innovative as ADC, it is not seen to be used in any manufacturing network. But it is still a great illustration of what is possible in a limited situation. We want to see one of these tubes, if any, still exists, has been resurfaced and has to go through its motion.

Thanks [Mark Erdle] For the tip

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