Editing Delay-line memory (section)

{{Memory types}}
{{short description|Early type of computer memory}}
'''Delay-line memory''' is a form of [[computer memory]], mostly obsolete, that was used on some of the earliest [[Digital data|digital]] [[computer]]s, and is reappearing in the form of [[#Optical_delay_lines|optical delay lines]].  Like many modern forms of electronic computer memory, delay-line memory was a [[memory refresh|refreshable memory]], but as opposed to modern [[random-access memory]], delay-line memory was [[Sequential access|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 shaping|pulse shaper]] are connected between the output of the delay line and the input. These devices recirculate the signals from the output back into the input, creating a loop that maintains the signal as long as power is applied. The shaper ensures the pulses remain well-formed, removing any degradation due to losses in the medium.

The memory capacity equals the time to transmit one bit divided by the recirculation time. Early delay-line memory systems had capacities of a few thousand [[bit]]s (although the term "bit" was not in popular use at the time), with recirculation times measured in [[second|microseconds]]. To read or write a particular [[memory address]], it is necessary to wait for the signal representing its value to circulate through the delay line into the electronics. The [[Latency (engineering)|latency]] to read or write any particular address is thus time and address dependent, but no longer than the recirculation time.

Use of a delay line for a computer memory was invented by [[J.&nbsp;Presper Eckert]] in the mid-1940s for use in computers such as the [[EDVAC]] and the [[UNIVAC&nbsp;I]]. Eckert and [[John Mauchly]] applied for a patent for a delay-line memory system on October&nbsp;31, 1947; the patent was issued in 1953.<ref>{{US patent|2629827}}.</ref> This patent focused on mercury delay lines, but it also discussed delay lines made of [[Ladder topology|strings of inductors and capacitors]], [[magnetostrictive]] delay lines, and delay lines built using [[Drum memory|rotating disks]] to transfer data to a read head at one point on the circumference from a write head elsewhere around the circumference.