# Storage tube

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Storage tubes are a class of cathode-ray tubes (CRTs) that are designed to hold an image for a long period of time, typically as long as power is supplied to the tube.

The cathode-ray tube (CRT) is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures, radar targets, or other phenomena. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer.

## Contents

A specialized type of storage tube, the Williams tube, was used as a main memory system on a number of early computers, from the late 1940s into the early 1950s. They were replaced with other technologies, notably core memory, starting in the 1950s.

The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory. It was the first random-access digital storage device, and was used successfully in several early computers.

A computer is a device that can be instructed to carry out sequences of arithmetic or logical operations automatically via computer programming. Modern computers have the ability to follow generalized sets of operations, called programs. These programs enable computers to perform an extremely wide range of tasks. A "complete" computer including the hardware, the operating system, and peripheral equipment required and used for "full" operation can be referred to as a computer system. This term may as well be used for a group of computers that are connected and work together, in particular a computer network or computer cluster.

Storage tubes made a comeback in the 1960s and 1970s for use in computer graphics, most notably the Tektronix 4010 series. Today they are obsolete, their functions provided by low-cost memory devices and liquid crystal displays.

Computer graphics are pictures and films created using computers. Usually, the term refers to computer-generated image data created with the help of specialized graphical hardware and software. It is a vast and recently developed area of computer science. The phrase was coined in 1960, by computer graphics researchers Verne Hudson and William Fetter of Boeing. It is often abbreviated as CG, though sometimes erroneously referred to as computer-generated imagery (CGI).

The Tektronix 4010 series was a family of text and graphics computer terminals based on the company's storage tube technology. There were several members of the family introduced through the 1970s, the best known being the 11-inch 4010 and 19-inch 4014, along with the less popular 25-inch 4016. They were widely used in the CAD market in the 1970s and early 1980s.

## Operation

### Background

A conventional CRT consists of an electron gun at the back of the tube that is aimed at a thin layer of phosphor at the front of the tube. Depending on the role, the beam of electrons emitted by the gun is steered around the display using magnetic (television) or electrostatic (oscilloscope) means. When the electrons strike the phosphor, the phosphor "lights up" at that location for a time, and then fades away. The length of time the spot remains is a function of the phosphor chemistry.

An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode ray tubes (CRTs), used in nearly all television sets, computer displays and oscilloscopes that are not flat-panel displays. They are also used in field emission displays (FEDs), which are essentially flat-panel displays made out of rows of extremely small cathode ray tubes. They are also used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, and gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation, by emission mechanism, by focusing, or by the number of electrodes.

A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness, and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark materials, whereas fluorescent materials are common in cathode ray tube (CRT) and plasma video display screens, fluorescent lights, sensors, and white LEDs.

Television (TV), sometimes shortened to tele or telly, is a telecommunication medium used for transmitting moving images in monochrome, or in colour, and in two or three dimensions and sound. The term can refer to a television set, a television program, or the medium of television transmission. Television is a mass medium for advertising, entertainment and news.

At very low energies, electrons from the gun will strike the phosphor and nothing will happen. As the energy is increased, it will reach a critical point, ${\displaystyle V_{cr1}}$, that will activate the phosphor and cause it to give off light. As the voltage increases beyond Vcr1 the brightness of the spot will increase. This allows the CRT to display images with varying intensity, like a television image.

Above Vcr1 another effect also starts, secondary emission. When any insulating material is struck by electrons over a certain critical energy, electrons within the material are forced out of it through collisions, increasing the number of free electrons. This effect is used in electron multipliers as found in night vision systems and similar devices. In the case of a CRT this effect is generally undesirable; the new electrons generally fall back to the display and cause the surrounding phosphor to light up, which appears as a lowering of the focus of the image.

Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.

An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary-emissive material, induce emission of roughly 1 to 3 electrons. If an electric potential is applied between this metal plate and yet another, the emitted electrons will accelerate to the next metal plate and induce secondary emission of still more electrons. This can be repeated a number of times, resulting in a large shower of electrons all collected by a metal anode, all having been triggered by just one.

Night vision is the ability to see in low-light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range. Humans have poor night vision compared to many animals, in part because the human eye lacks a tapetum lucidum.

The rate of secondary emission is also a function of the electron beam energy, but follows a different rate curve. As the electron energy is increased, the rate increases until it reaches a critical threshold, Vcr2 when the number of secondary emissions is greater than the number supplied by the gun. In this case the localized image rapidly fades as energy is removed by the secondary electrons.

In any CRT, images are displayed by striking the screen with electron energies between these two values, Vcr1 and Vcr2. Below Vcr1 no image is formed, and above Vcr2 any image rapidly fades.

Another side effect, initially a curiosity, is that electrons will stick to the phosphor in lit up areas. As the light emission fades, these electrons are likewise released back into the tube. The charge is generally far too small to have a visual effect, and was generally ignored in the case of displays.

### Storage

These two effects were both utilized in the construction of a storage tube. Storage was accomplished by striking any suitably long-lived phosphor with electrons with energies just above Vcr1, and erased by striking them with electrons above Vcr2. There were any number of varieties of mechanical layouts used to improve focus or cause the image to be refreshed either internally to the tube or through off board storage.

The easiest example to understand are the early computer memory systems as typified by the Williams tube. These consisted of World War II surplus radar display CRTs connected to a computer. The X and Y deflection plates were connected to amplifiers that converted memory locations into X and Y positions on the screen, in most cases such that positions along the X axis represented individual bits within a word, while Y locations were different words.

To write a value to memory, the address was amplified and sent to the Y deflection plates, such that the beam would be fixed to a horizontal line on the screen. A timer then set the X deflection plate to increasing voltages, causing the beam to be scanned across the selected line. The gun was set to a default energy close to Vcr1, and the bits from the computer fed to the gun to modulate the voltage up and down such that 0's would be below Vcr1 and 1's above it. By the time the beam reached the other side of the line, a pattern of short dashes was drawn for each 1, while 0's were empty locations.

To read the values back out, the deflections plates were set to the same values, but the gun energy set to a value above Vcr2. As the beam scanned the line, the phosphor was pushed well beyond the secondary emission threshold. If the beam was located over a blank area, a certain number of electrons would be released, but if it was over a lit area, the number would be increased by the number of electrons stuck to that area. In the Williams tube these values were read by measuring the capacitance of a metal plate just in front of the display side of the tube. As the reading process also erased any stored values, the signal had to be regenerated through associated circuitry. A CRT with two electron guns, one for reading and one for writing, made this process trivial.

### Imaging systems

The earliest computer graphics systems, like those of the TX-2 and DEC PDP-1, required the entire attention of the computer to maintain. A list of vectors stored in main memory was periodically read out to the display to refresh it before the image faded. This generally occurred frequently enough that there was little time to do anything else, and interactive systems like Spacewar! were tour-de-force programming efforts.

For practical use, graphical displays were developed that contained their own memory and an associated very simple computer which offloaded the refreshing task from the mainframe. This was not inexpensive; the IBM 2250 graphics terminal used with the IBM S/360 cost \$280,000 in 1970. [1]

A storage tube could replace most or all of the localized hardware by storing the vectors directly within the display, instead of an associated local computer. Commands that previously caused the terminal to erase its memory and thus clear the display could be emulated by scanning the entire screen at an energy above Vcr2. In most systems, this caused the entire screen to quickly "flash" before clearing to a blank state. The two main advantages were:

• Very low bandwidth needs [2] compared to vector graphics displays, thus allowing much longer cable distances between computer and terminal
• No need for display-local RAM (as in modern terminals), which was prohibitively expensive at the time.

Generally speaking, storage tubes could be divided into two categories. In the more common category, they were only capable of storing "binary" images; any given point on the screen was either illuminated or dark. The Tektronix Direct-View Bistable Storage Tube was perhaps the best example in this category. Other storage tubes were able to store greyscale/halftoned images; the tradeoff was usually a much-reduced storage time.

Some pioneering storage tube displays were MIT Project MAC's ARDS (Advanced Remote Display Station), the Computek 400 Series Display terminals (a commercial derivative), [3] which both used a Tektronix type 611 storage display unit, and Tektronix's 4014 terminal, the latter becoming a de facto computer terminal standard some time after its introduction (later being emulated by other systems due to this status).

The first generalized computer assisted instruction system, PLATO I, c. 1960 on ILLIAC I, used a storage tube as its computer graphics display. PLATO II and PLATO III also used storage tubes as displays.

## Notes

1. "Computer Display Review", Keydata Corp., March 1970, pp. V.1980, V.1964
2. Michael L. Dertouzos (April 1967). "Phaseplot: An On-Line Graphical Display Technique". IEEE Transactions on Electronic Computers. IEEE. EC-16 (2): 203–209. doi:10.1109/pgec.1967.264817. The main advantage of this technique is graphical data compression.
3. Michael L. Dertouzos (April 1967). "Phaseplot: An On-Line Graphical Display Technique". IEEE Transactions on Electronic Computers. IEEE. EC-16 (2): 203–209. doi:10.1109/pgec.1967.264817. This article describes the principle used in the graphical output portion of the Computek series 400 Display Terminals (added to a reprint of the article distributed by Computek)

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