|
Delay line memory is a form of computer memory, now obsolete, that was used on some of the earliest digital computers. 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 sequential-access. Analog delay line technology had been used since the 1920s to delay the propagation of analog signals. When a delay line is used as a memory device, an amplifier and a pulse shaper are connected between the output of the delay line and the input. The memory capacity is determined by dividing the time taken to transmit one bit into the time it takes for data to circulate through the delay line. Early delay-line memory systems had capacities of a few thousand bits, with recirculation times measured in microseconds. To read or write a particular bit stored in such a memory, it is necessary to wait for that bit to circulate through the delay line into the electronics. The delay to read or write any particular bit is no longer than the recirculation time. 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. Eckert and John Mauchly applied for a patent for a delay line memory system on October 31, 1947; the patent was issued in 1953. This patent focused on mercury delay lines, but it also discussed delay lines made of strings of inductors and capacitors, magnetostrictive delay lines, and delay lines built using rotating disks to transfer data to a read head at one point on the circumference from a write head elsewhere around the circumference. ==Genesis in radar== 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 principally of an antenna, a transmitter, a receiver, and a display. 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 like prominent hills, 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), as 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.〔J.P. Eckert, Jr., A Survey of Digital Computer Memory Systems, Proceedings of the IRE, October 1953〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「delay line memory」の詳細全文を読む スポンサード リンク
|