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Storage tube

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   The Tektronix 4014 uses a storage tube for its display.

   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.

   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.

   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.
   [ ]

Contents

     * 1 Operation
          + 1.1 Background
          + 1.2 Storage
          + 1.3 Imaging systems
     * 2 See also
     * 3 References

Operation[edit]

Background[edit]

   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.

   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,
   [MATH: <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle
   displaystyle="true" scriptlevel="0"> <msub> <mi>V</mi> <mrow
   class="MJX-TeXAtom-ORD"> <mi>c</mi> <mi>r</mi> <mn>1</mn> </mrow>
   </msub> </mstyle> </mrow> <annotation
   encoding="application/x-tex">{\displaystyle V_{cr1}}</annotation>
   </semantics> :MATH]
   V_{{cr1}} , that will activate the phosphor and cause it to give off
   light. As the voltage increases beyond V[cr1] the brightness of the
   spot will increase. This allows the CRT to display images with varying
   intensity, like a television image.

   Above V[cr1] 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.

   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,
   V[cr2] 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 leaving the display through secondary electrons is
   greater than the rate it is being supplied by the gun.

   In any CRT, images are displayed by striking the screen with electron
   energies between these two values, V[cr1] and V[cr2]. Below V[cr1] no
   image is formed, and above V[cr2] 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[edit]

   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 V[cr1], and erased by
   striking them with electrons above V[cr2]. 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.

   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 time base generator then set the X deflection
   plate to increasing voltages, causing the beam to be scanned across the
   selected line. In this respect, it is similar to a conventional
   television scanning a single line. The gun was set to a default energy
   close to V[cr1], and the bits from the computer fed to the gun to
   modulate the voltage up and down such that 0's would be below V[cr1]
   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 V[cr2]. 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 previously 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. Electrons leaving the front of
   the CRT hit the plate and changed its charge. 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[edit]

   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 V[cr2]. 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.

See also[edit]

     * Direct-View Bistable Storage Tube (DVBST)
     * Cathode ray tube (for an explanation of how analog storage tubes
       worked)
     * Williams tube and Selectron tube both used the term "storage tube"
       for early computer memory devices
     * Electronic paper

References[edit]

    1. ^ "Computer Display Review", Keydata Corp., March 1970, pp. V.1980,
       V.1964 Archived at the Wayback Machine
    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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