Forgotten Memory

This is the memory of the first commercial computer, UNIVAC 1.

The following is taken from the IEEE Spectrum blog

“…J. Presper Eckert, co-­inventor of the UNIVAC, and four other members of the Institute of Radio Engineers wrote in 1949: ”In a delay-line memory, ­information is stored in the form of groups of electrical or acoustical impulses or signals ­circulating in an electric delay line or medium suitable for transmission of acoustic waves.

“The authors noted, ”Although considerable research is being done on electrostatic ­memories…the delay-line type of memory is more highly developed at the present time.” Of course, today essentially all memory is electrostatic.”

Links to radar
Interestingly, delay-line memory was developed from work on radar to filter out unwanted ‘noise’. This from Wikipedia; I’ve included it in its entirety as it gives a good description of the principles and fundamentals of radar. Geeks, read on…

“The basic concept of the delay line originated with World War II radar research, as a system to reduce clutter from reflections from the ground and other “fixed” objects.

A radar system consists largely of an antenna, a transmitter, a receiver, and a display of some sort. The antenna is connected to the transmitter, which sends out a brief pulse of radio energy before being disconnected again. The antenna is then connected to the receiver, which amplifies any reflected signals, and sends them to the display. Objects further from the radar return echos later in time than those located closer to the radar, which the display indicates visually.

Non-moving objects at a fixed distance from the antenna always return a signal after the same delay. This would appear as a fixed spot on the display, making detection of other targets in the area more difficult. Early radars simply aimed their beams away from the ground in order to avoid the majority of this “clutter”. This was not an ideal situation by any means; it required careful setup and aiming which was not very easy for smaller mobile radars, and did nothing to remove other sources of clutter like reflections off of certain terrain features.

In order to filter these returns out, two pulses were compared, and returns with common timing are removed. To do this, the signal being sent from the receiver to the display was split in two, with one path leading directly to the display, and the second leading to a delay unit. The delay was carefully tuned to delay the signals some multiple of the time between pulses (the pulse repetition frequency), that way the delayed signal from an earlier pulse would exit the delay unit at the same time as a newer pulse was being received from the antenna. One of the signals was then inverted, typically the one from the delay, and the two signals were then combined and sent to the display. Any signal that was at the same location was nullified by the inverted signal from a previous pulse, leaving only the moving objects on the display.

Several different types of delay systems were invented for this purpose, with one common principle being that the information was stored acoustically in a medium. MIT experimented with a number of systems including glass, quartz, steel and lead. The Japanese deployed a system consisting of a quartz element with a powdered glass coating that reduced surface waves that interfered with proper reception. The United States Naval Research Laboratory used steel rods wrapped into a helix, but this was useful only for low frequencies under 1 MHz. Raytheon used a magnesium alloy originally developed for making bells.[1]

The first practical de-cluttering system based on the concept was developed by J. Presper Eckert at the University of Pennsylvania‘s Moore School of Electrical Engineering. His solution used a column of mercury with piezo crystal transducers (a combination of speaker and microphone) at either end. Signals from the radar amplifier were sent to the piezo at one end of the tube, which would cause the transducer to pulse and generate a small wave in the mercury. The wave would quickly travel to the far end of the tube, where it would be read back out by the other piezo, inverted, and sent to the display. Careful mechanical arrangement was needed to ensure the delay time matched the inter-pulse timing of the particular radar being used.

All of these systems were suitable for conversion into a computer memory. The key was to recycle the signals within the memory system so they would not disappear after traveling through the delay. This was relatively easy to arrange with simple electronics.

Acoustic delay lines: Mercury
After the war Eckert turned his attention to computer development, which was a topic of some interest at the time. One problem with practical development was the lack of a suitable memory device, and Eckert’s work on the radar delays meant he had a major advantage over other researchers in this regard.

For a computer application the timing was still critical, but for a different reason. Conventional computers have a natural “cycle time” needed to complete an operation, the start and end of which typically consist of reading or writing memory. Thus the delay lines had to be timed such that the pulses would arrive at the receiver just as the computer was ready to read it. Typically many pulses would be “in flight” through the delay, and the computer would count the pulses by comparing to a master clock to find the particular bit it was looking for.

Mercury was used because the acoustic impedance of mercury is almost exactly the same as that of the piezoelectric quartz crystals; this minimized the energy loss and the echoes when the signal was transmitted from crystal to medium and back again. The high speed of sound in mercury (1450 m/s) meant that the time needed to wait for a pulse to arrive at the receiving end was less than it would have been with a slower medium, such as air, but it also meant that the total number of pulses that could be stored in any reasonably sized column of mercury was limited. Other technical drawbacks of mercury included its weight, its cost, and its toxicity. Moreover, to get the acoustic impedances to match as closely as possible, the mercury had to be kept at a temperature of 40 °C (100 °F), which made servicing the tubes hot and uncomfortable work.

A considerable amount of engineering was needed to maintain a “clean” signal inside the tube. Large transducers were used to generate a very tight “beam” of sound that would not touch the walls of the tube, and care had to be taken to eliminate reflections off the far end of the tubes. The tightness of the beam then required considerable tuning to make sure the two piezos were pointed directly at each other. Since the speed of sound changes with temperature (because of the change in density with temperature) the tubes were heated in large ovens to keep them at a precise temperature. Other systems instead adjusted the computer clock rate according to the ambient temperature to achieve the same effect.

EDSAC, designed to be the first stored-program digital computer, began operation with 512 35-bit words of memory, stored in 32 delay lines holding 576 bits each (a 36th bit was added to every word as a start/stop indicator). In the UNIVAC I this was reduced somewhat, each column stored 120 bits (although the term “bit” was not in popular use at the time), requiring seven large memory units with 18 columns each to make up a 1000-word store. Combined with their support circuitry and amplifiers, the memory subsystem formed its own walk-in room. The average access time was about 222 microseconds, which was considerably faster than the mechanical systems used on earlier computers.”

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