Delay-line memory

Delay-line memory is a form of computer memory, mostly obsolete, that was used on some of the earliest digital computers, and is reappearing in the form of optical delay lines. 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. 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 bits (although the term "bit" was not in popular use at the time), with recirculation times measured in 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 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. 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.^[1] 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.