Retrotechtacular: The Forgotten Vacuum Tube A/D Converters Of 1965

Jean J. Sanders

In any era, the tale of electronics has quite much been about figuring out how to make a thing come about with what is out there at the time. And as is usually the case, the most fascinating developments occur from occasions when wants exceed what’s available. Which is when authentic innovation takes position, even if situation conspire to maintain the innovation from ever taking keep in the marketplace.

This gem of a online video from the Antique Wireless Association has a ideal instance of this: the lengthy-lost analog-to-electronic converter vacuum tube. Like nearly every mid-20th-century innovation in electronics, this one particular traces its roots back again to the Bell Laboratories, which was keenly fascinated in increasing bandwidth on its huge community of copper strains and microwave one-way links. As early as 1947, 1 Dr. Frank Gray, a physicist at Bell Labs, experienced been operating on a vacuum tube that could directly convert an analog signal into a digital representation. His alternative was a cathode ray tube related to the CRT in an oscilloscope. A beam of electrons would glow down the size of the tube onto a shadow mask that contains holes arranged in a “reflected binary code,” which would later be known as a Gray code. The analog signal to be digitized was applied to a pair of vertical deflector plates, which moved the beam into a posture along the plate corresponding to the voltage. A pair of horizontal deflector plates would then scan the beam throughout the shadow mask exactly where electrons fell on a gap, they would go via to an output plate to be registered as a little bit to be established.

Quickly forward twenty decades, and Dr. Gray’s essential plan was leveraged to create a 224 Mb/s analog-to-electronic converter that just wasn’t attainable with the transistors of the working day. The innovation with this tube was to parallelize the output — rather of a one electron beam remaining rastered across the shadow mask at the suitable situation, a ribbon of electrons fell on an overall 9-little bit row just before placing an array of output detectors.

As standard for Bell Labs, the tube carried out excellently, nearly matching the theoretical signal-to-noise ratio. But alas, one more project in the lab to establish an all-sound-point out ADC had gained traction though the tube was remaining perfected, and a great deal of what drove each tasks fell aside as AT&T concentrated on microwave guideline and optical fiber for their electronic networking requires.

As progressive as the vacuum tube ADC was, it never appears to have observed use in any manufacturing networks. But it’s still a amazing illustration of what is attainable under constrained circumstances. We’d enjoy to see 1 of these tubes, if any even now exist, resurrected and set through its paces.

Many thanks to [Mark Erdle] for the idea.

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