Editing Delay-line memory (section)

==Genesis in radar==
The basic concept of the delay line originated with World War&nbsp;II [[radar]] research, as a system to reduce [[Clutter (radar)|clutter]] from reflections from the ground and other non-moving objects.

A radar system consists principally of an antenna, a transmitter, a receiver, and a [[Radar display|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 than those closer to the radar, which the display indicates visually as a "blip", which can be measured against a scale in order to determine range.

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 to avoid the majority of this "clutter". This was not an ideal situation; it required careful aiming, which was difficult for smaller mobile radars, and did not remove other sources of clutter-like reflections from features like prominent hills, and in the worst case would allow low-flying enemy aircraft to literally fly "under the radar".

To filter out static objects, two pulses were compared, and returns with the same delay times were 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 be some multiple of the time between pulses, or "[[pulse repetition frequency]]". This resulted in the delayed signal from an earlier pulse exiting the delay unit the same time that the signal from a newer pulse was received from the antenna. One of the signals was electrically 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 [[acoustics|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&nbsp;MHz. [[Raytheon]] used a magnesium alloy originally developed for making bells.<ref>J. P. Eckert, Jr., A Survey of Digital Computer Memory Systems, Proceedings of the IRE, October 1953.</ref>

The first practical de-cluttering system based on the concept was developed by [[J.&nbsp;Presper Eckert]] at the [[University of Pennsylvania]]'s [[Moore School of Electrical Engineering]]. His solution used a column of [[mercury (element)|mercury]] with [[piezoelectricity|piezo crystal]] [[transducer]]s (a combination of speaker and microphone) at either end. Signals from the radar amplifier were sent to the transducer at one end of the tube, which would 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 transducer, inverted, and sent to the display. Careful mechanical arrangement was needed to ensure that the delay time matched the inter-pulse timing of the radar being used.

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