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 recirculation time divided by the time to transmit one bit. 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.

Variously described as pioneered or invented by William Shockley for memory in 1942 while at Bell Labs, it was 100 times more cost effective than the previous electronic competitor of using vacuum tubes and about as rapid. Use of a delay line for a computer memory was soon further developed by J. Presper Eckert at Massachusetts Institute of Technology in 1944, and adapted for use in computers such as the EDVAC and the UNIVAC I starting in 1945.

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.