Delay line memory
Encyclopedia
Delay line memory was a form of computer memory
used on some of the earliest digital
computer
s. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory
, but as opposed to modern random-access memory
, delay line memory was serial-access
. In the earliest forms of delay line memory, information introduced to the memory in the form of electric pulses was transduced
into mechanical waves that propagated relatively slowly through a medium, such as a cylinder filled with a liquid like mercury
, a magnetostrictive coil, or a piezoelectric crystal
. The propagation medium could support the propagation of hundreds or thousands of pulses at any one time. Upon reaching the other end of the propagation medium, the waves were re-transduced into electric pulses, amplified
, shaped
, and reintroduced to the propagation medium at the beginning, thus refreshing the memory. Accessing a desired part of the propagation medium's memory contents required waiting for the pulses of interest to reach the end of the medium, a wait typically on the order of microseconds
. Use of a delay line for a computer memory was invented by J. Presper Eckert
in the mid-1940s for use in computers such as the EDVAC
and the UNIVAC I
.
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 farther 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, did nothing to remove other sources of clutter like reflections off certain terrain features, and in the worst case would allow low-flying enemy aircraft to literally fly "under the radar".
To filter these returns out, two pulses were compared, and returns with common timing are removed. To do this, the signal 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 wave
s 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.
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
transducer
s (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.
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 constant temperature. The system heated the mercury to a uniform above-room temperature setting of 40 °C (100 °F), which made servicing the tubes hot and uncomfortable work. (Alan Turing
proposed the use of gin
as an ultrasonic delay medium, claiming that it had the necessary acoustic properties.)
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 bit
s (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 amplifier
s, the memory subsystem formed its own walk-in room. The average access time was about 222 microsecond
s, which was considerably faster than the mechanical systems used on earlier computers.
CSIRAC
, completed in November 1949, also used delay line memory.
; small pieces of a magnetostrictive material, typically nickel
, were attached to either side of the end of the wire, inside an electromagnet. When bits from the computer entered the magnets the nickel would contract or expand (based on the polarity) and twist the end of the wire. The resulting torsional wave would then move down the wire just as the sound wave did down the mercury column. In most cases the entire wire was made of the same material.
Unlike the compressive wave, however, the torsional
waves are considerably more resistant to problems caused by mechanical imperfections, so much so that the wires could be wound into a loose coil and pinned to a board. Due to their ability to be coiled, the wire-based systems could be built as "long" as needed, and tended to hold considerably more data per unit; 1k
units were typical on a board only 1 foot square. Of course this also meant that the time needed to find a particular bit was somewhat longer as it traveled through the wire, and access times on the order of 500 microseconds were typical.
Delay line memory was far less expensive and far more reliable per bit than flip-flop
s made from tubes
, and yet far faster than a latching relay. It was used right into the late 1960s, notably on British commercial machines like the LEO I
, Highgate Wood Telephone Exchange
, and various Ferranti
machines. Delay line memory was also used for video memory in early terminals, where one delay line would typically store 4 lines of characters. (4 lines x 40 characters per line x 6 bits per character= 960 bits in one delay line) They were also used very successfully in several models of early desktop electronic calculator, including the Friden
EC130 (1964) and EC132, the Olivetti
Programma 101
desktop programmable calculator
introduced in 1965, and the Litton Monroe Epic
2000 and 3000 programmable calculators of 1967.
A similar solution to the magnetostrictive system was to use delay lines made entirely of a piezo material, typically quartz. Current fed into one end of the crystal would generate a compressive wave that would flow to the other end where it could be read out. In effect, piezoelectric delays simply replaced the mercury and transducers of a conventional mercury delay line with a single unit combining both. However these solutions were fairly rare; building crystals of the required quality in large sizes was not easy, limiting them to small sizes, and thus small amounts of data storage.
A better and more widespread use of piezoelectric delays was in European television sets. The European PAL
standard for color broadcasts compares the signal from two subsequent lines in order to avoid color shifting due to small phase shifts. By comparing two lines, one inverted, the shifting is averaged out and returns a signal more closely matching the original even under interference. In order to compare the two lines, a piezo delay tuned to the timing of the lines, 64 µs, is inserted in the signal path. The delay unit is shaped to "fold" the beam multiple times through the crystal, greatly reducing its length and producing a small square-ish device.
Electric delay lines are used for shorter delay times (ns to several µs). They consist of a long electric line or are made of discrete inductors and capacitors, which are arranged in a chain. To shorten the total length of the line it can be wound around a metal tube, getting some more capacitance against ground and also more inductance due to the wire windings, which are lying close together.
Other examples are:
Another way to create a delay time is to implement a delay line in an integrated circuit
storage device
. This can be done digitally or with a discrete analogue method. The analogue one uses bucket-brigade devices or charge coupled devices (CCD), which transport a stored electric charge stepwise from one end to the other. Both digital and analog methods are bandwidth limited at the upper end to the half of the clock frequency, which determines the steps of transportation.
In modern computers operating at gigahertz speeds, millimeter differences in the length of conductors in a parallel data bus can cause data-bit skew, which can lead to data corruption or reduced processing performance. This is remedied by making all conductor paths of similar length, delaying the arrival time for what would otherwise be shorter travel distances by using zig-zagging traces.
, and encode the memory as pulses on the laser beam. With light taking around 2.5 seconds for a round trip to the Moon, and using terahertz pulse rates, the beam had a capacity of several trillion bits. This made it both the longest delay line and the highest capacity memory of any sort available at the time, although of course the apparatus only worked when the Moon was above the horizon.
"Memory System", Eckert-Mauchly Computer Corporation
, filed October 1947, patented February 1953
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
used on some of the earliest digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory
Memory refresh
Memory refresh is the process of periodically reading information from an area of computer memory, and immediately rewriting the read information to the same area with no modifications. Each memory refresh cycle refreshes a succeeding area of memory. Memory refresh is most often associated with...
, but as opposed to modern random-access memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
, delay line memory was serial-access
Sequential access
In computer science, sequential access means that a group of elements is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example if it is on a tape...
. In the earliest forms of delay line memory, information introduced to the memory in the form of electric pulses was transduced
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
into mechanical waves that propagated relatively slowly through a medium, such as a cylinder filled with a liquid like mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, a magnetostrictive coil, or a piezoelectric crystal
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
. The propagation medium could support the propagation of hundreds or thousands of pulses at any one time. Upon reaching the other end of the propagation medium, the waves were re-transduced into electric pulses, amplified
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
, shaped
Pulse shaping
In digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
, and reintroduced to the propagation medium at the beginning, thus refreshing the memory. Accessing a desired part of the propagation medium's memory contents required waiting for the pulses of interest to reach the end of the medium, a wait typically on the order of microseconds
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
. Use of a delay line for a computer memory was invented by J. Presper Eckert
J. Presper Eckert
John Adam Presper "Pres" Eckert Jr. was an American electrical engineer and computer pioneer. With John Mauchly he invented the first general-purpose electronic digital computer , presented the first course in computing topics , founded the first commercial computer company , and...
in the mid-1940s for use in computers such as the EDVAC
EDVAC
EDVAC was one of the earliest electronic computers. Unlike its predecessor the ENIAC, it was binary rather than decimal, and was a stored program computer....
and the UNIVAC I
UNIVAC I
The UNIVAC I was the first commercial computer produced in the United States. It was designed principally by J. Presper Eckert and John Mauchly, the inventors of the ENIAC...
.
Genesis in radar
The basic concept of the delay line originated with World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
research, as a system to reduce clutter
Clutter (radar)
Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.- Backscatter coefficient...
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
Radar display
Modern radar systems typically use some sort of raster scan display to produce a map-like image. In the past, notably during the early days of radar development, such displays were difficult to produce for a number of reasons. Several different display types were developed during this...
. 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 farther 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, did nothing to remove other sources of clutter like reflections off certain terrain features, and in the worst case would allow low-flying enemy aircraft to literally fly "under the radar".
To filter these returns out, two pulses were compared, and returns with common timing are removed. To do this, the signal 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
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
), 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.
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
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 wave
Surface wave
In physics, a surface wave is a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave guided by a refractive index gradient...
s that interfered with proper reception. The United States Naval Research Laboratory
United States Naval Research Laboratory
The United States Naval Research Laboratory is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a program of scientific research and development. NRL opened in 1923 at the instigation of Thomas Edison...
used steel rods wrapped into a helix, but this was useful only for low frequencies under 1 MHz. Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...
used a magnesium alloy originally developed for making bells.
The first practical de-cluttering system based on the concept was developed by J. Presper Eckert
J. Presper Eckert
John Adam Presper "Pres" Eckert Jr. was an American electrical engineer and computer pioneer. With John Mauchly he invented the first general-purpose electronic digital computer , presented the first course in computing topics , founded the first commercial computer company , and...
at the University of Pennsylvania
University of Pennsylvania
The University of Pennsylvania is a private, Ivy League university located in Philadelphia, Pennsylvania, United States. Penn is the fourth-oldest institution of higher education in the United States,Penn is the fourth-oldest using the founding dates claimed by each institution...
's Moore School of Electrical Engineering
Moore School of Electrical Engineering
The Moore School of Electrical Engineering at the University of Pennsylvania came into existence as a result of an endowment from Alfred Fitler Moore on June 4, 1923. It was granted to Penn's School of Electrical Engineering, located in the Towne Building...
. His solution used a column of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
with piezo crystal
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s (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.
Mercury delay lines
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
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
was used because the acoustic impedance
Acoustic impedance
The acoustic impedance at a particular frequency indicates how much sound pressure is generated by a given air vibration at that frequency. The acoustic impedance Z is frequency dependent and is very useful, for example, for describing the behaviour of musical wind instruments...
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
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
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 constant temperature. The system heated the mercury to a uniform above-room temperature setting of 40 °C (100 °F), which made servicing the tubes hot and uncomfortable work. (Alan Turing
Alan Turing
Alan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...
proposed the use of gin
Gin
Gin is a spirit which derives its predominant flavour from juniper berries . Although several different styles of gin have existed since its origins, it is broadly differentiated into two basic legal categories...
as an ultrasonic delay medium, claiming that it had the necessary acoustic properties.)
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
EDSAC
Electronic Delay Storage Automatic Calculator was an early British computer. The machine, having been inspired by John von Neumann's seminal First Draft of a Report on the EDVAC, was constructed by Maurice Wilkes and his team at the University of Cambridge Mathematical Laboratory in England...
, 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
UNIVAC I
The UNIVAC I was the first commercial computer produced in the United States. It was designed principally by J. Presper Eckert and John Mauchly, the inventors of the ENIAC...
this was reduced somewhat, each column stored 120 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s (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 amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s, the memory subsystem formed its own walk-in room. The average access time was about 222 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s, which was considerably faster than the mechanical systems used on earlier computers.
CSIRAC
CSIRAC
CSIRAC , originally known as CSIR Mk 1, was Australia's first digital computer, and the fourth stored program computer in the world. It was first to play digital music and is one of only a few surviving first-generation computers .The CSIRAC was constructed by a team led by Trevor Pearcey and...
, completed in November 1949, also used delay line memory.
Magnetostrictive delay lines
A later version of the delay line used metal wires as the storage medium. Transducers were built by applying the magnetostrictive effectMagnetostriction
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of material's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ...
; small pieces of a magnetostrictive material, typically nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, were attached to either side of the end of the wire, inside an electromagnet. When bits from the computer entered the magnets the nickel would contract or expand (based on the polarity) and twist the end of the wire. The resulting torsional wave would then move down the wire just as the sound wave did down the mercury column. In most cases the entire wire was made of the same material.
Unlike the compressive wave, however, the torsional
Torsion (mechanics)
In solid mechanics, torsion is the twisting of an object due to an applied torque. In sections perpendicular to the torque axis, the resultant shear stress in this section is perpendicular to the radius....
waves are considerably more resistant to problems caused by mechanical imperfections, so much so that the wires could be wound into a loose coil and pinned to a board. Due to their ability to be coiled, the wire-based systems could be built as "long" as needed, and tended to hold considerably more data per unit; 1k
Kilobit
The kilobit is a multiple of the unit bit for digital information or computer storage. The prefix kilo is defined in the International System of Units as a multiplier of 103 , and therefore,...
units were typical on a board only 1 foot square. Of course this also meant that the time needed to find a particular bit was somewhat longer as it traveled through the wire, and access times on the order of 500 microseconds were typical.
Flip-flop (electronics)
In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic...
s made from tubes
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
, and yet far faster than a latching relay. It was used right into the late 1960s, notably on British commercial machines like the LEO I
Leo I
- People :* Pope Leo I , also known as Pope Saint Leo the Great* Leo I the Thracian, 5th century Byzantine Emperor* Leo I, Prince of Armenia - People :* Pope Leo I (400–461), also known as Pope Saint Leo the Great* Leo I the Thracian, 5th century Byzantine Emperor* Leo I, Prince of Armenia -...
, Highgate Wood Telephone Exchange
Highgate Wood Telephone Exchange
The Highgate Wood Telephone Exchange was a telephone exchange built in the London suburb of Highgate Wood. It was the site of a trial for an electronic telephone exchange built by members from Joint Electronic Research Council , which was formed in 1956 and consisted of the British Post Office GPO,...
, and various Ferranti
Ferranti
Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. Known primarily for defence electronics, the Company was once a constituent of the FTSE 100 Index but ceased trading in 1993.The...
machines. Delay line memory was also used for video memory in early terminals, where one delay line would typically store 4 lines of characters. (4 lines x 40 characters per line x 6 bits per character= 960 bits in one delay line) They were also used very successfully in several models of early desktop electronic calculator, including the Friden
Friden, Inc.
Friden Calculating Machine Company was an American manufacturer of typewriters and electronic calculators. It was founded by Carl Friden in San Leandro, California in 1934. Friden electromechanical calculators were robust and popular....
EC130 (1964) and EC132, the Olivetti
Olivetti
Olivetti S.p.A. is an Italian manufacturer of computers, printers and other business machines.- Founding :The company was founded as a typewriter manufacturer in 1908 in Ivrea, near Turin, by Camillo Olivetti. The firm was mainly developed by his son Adriano Olivetti...
Programma 101
Programma 101
The Programma 101 was the first commercially produced "desktop computer". Launched by Olivetti at the 1964 New York World's Fair, volume production started in 1965. A futuristic design for its time, the Programma 101 was priced at $3,200...
desktop programmable calculator
Programmable calculator
Programmable calculators are calculators that can automatically carry out a sequence of operations under control of a stored program, much like a computer. The first programmable calculators such as the IBM CPC used punched cards or other media for program storage...
introduced in 1965, and the Litton Monroe Epic
Monroe Epic
The Monroe EPIC was a programmable calculator that came on the market in the 1960s. It consisted of a large desktop unit which attached to a floor-standing logic tower, and was capable of being programmed to perform many computer-like functions...
2000 and 3000 programmable calculators of 1967.
Piezoelectric delay lines
A better and more widespread use of piezoelectric delays was in European television sets. The European PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
standard for color broadcasts compares the signal from two subsequent lines in order to avoid color shifting due to small phase shifts. By comparing two lines, one inverted, the shifting is averaged out and returns a signal more closely matching the original even under interference. In order to compare the two lines, a piezo delay tuned to the timing of the lines, 64 µs, is inserted in the signal path. The delay unit is shaped to "fold" the beam multiple times through the crystal, greatly reducing its length and producing a small square-ish device.
Electric delay lines
Other examples are:
- short coaxial or microstrip lines for phase matching in high frequency circuits or antennaAntenna (radio)An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s - hollow resonator lines in magnetrons and klystronKlystronA klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...
s as helices in travelling wave tubes to match the velocity of the electrons to the velocity of the electromagnetic waves - undulatorUndulatorAn undulator is an insertion device from high-energy physics and usually part of a largerinstallation, a synchrotron storage ring. It consists of a periodic structure of dipole magnets . The static magnetic field is alternating along the length of the undulator with a wavelength \lambda_u...
s in free electron laserFree electron laserA free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
s
Another way to create a delay time is to implement a delay line in an integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
storage device
Storage device
Storage device may refer to:*Box, or any of a variety of containers or receptacles*Data storage device, a device for recording information, which could range from handwriting to video or acoustic recording, or to electromagnetic energy modulating magnetic tape and optical discs* Object storage...
. This can be done digitally or with a discrete analogue method. The analogue one uses bucket-brigade devices or charge coupled devices (CCD), which transport a stored electric charge stepwise from one end to the other. Both digital and analog methods are bandwidth limited at the upper end to the half of the clock frequency, which determines the steps of transportation.
In modern computers operating at gigahertz speeds, millimeter differences in the length of conductors in a parallel data bus can cause data-bit skew, which can lead to data corruption or reduced processing performance. This is remedied by making all conductor paths of similar length, delaying the arrival time for what would otherwise be shorter travel distances by using zig-zagging traces.
Optical delay line memory
In the 1980s a team associated with Prof. William Wolf at University of Colorado-Boulder were experimenting with all-optical computers and needed memory that was fast enough to keep up with the optical switches of the processing unit. Their solution was to bounce a laser beam off the corner reflector that had been left on the Moon by the Apollo astronautsLunar laser ranging experiment
The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...
, and encode the memory as pulses on the laser beam. With light taking around 2.5 seconds for a round trip to the Moon, and using terahertz pulse rates, the beam had a capacity of several trillion bits. This made it both the longest delay line and the highest capacity memory of any sort available at the time, although of course the apparatus only worked when the Moon was above the horizon.
External links
- Acoustic Delay Line Memory – has an image of a Ferranti wire-based system about half-way down the page
- Delay line memories – contains a diagram of the magnetostrictive transducer
- Litton Monroe Epic 3000 - Shows details of the torsion delay lines inside this electronic calculator of 1967.
- Magnetostrictive memory, still used in a German computer museum
"Memory System", Eckert-Mauchly Computer Corporation
Eckert-Mauchly Computer Corporation
The Eckert–Mauchly Computer Corporation was founded by J. Presper Eckert and John Mauchly, and was incorporated on December 22, 1947. After building the ENIAC at the University of Pennsylvania, Eckert and Mauchly formed EMCC to build new computer designs for commercial and military applications...
, filed October 1947, patented February 1953
- Display Terminal built with 32 TV delay lines Complete description.