Delay line

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A delay line is a device which takes in a signal, and sends it out again after a short delay. They were the base technology for a common choice for early main memory.

They are now usually implemented with capacitor-inductor neworks; they appeared in DIP format shortly after that was introduced.

Delay line memory

The ones used in early computers were acoustic delay lines, which consisted of a transducer (speaker) which sent sound signals into a channel filled with a suitable material (mercury was often used), with another transducer (acting as a microphone) at the other end. (There are other ways to produce delay lines, but the ones used in computer memories generally used the acoustic approach.)

A string of bits could be stored in the delay line by taking its output, sending it through some electronics (including an amplifier) to 'clean up' the signal, and then directing it back to the start of the delay line for re-insertion.

Although they were cheap and simple, they had one large drawback; they were not random access. If the computer needed a value that had just been sent into the delay line, it had to wait until it emerged at the other end. This often required the use of optimum programming, to control the execution delays which resulted if instructions were not placed in the optimum positions in the delay line. This made alternatives such as Williams tube storage attractive, despite their issues.

Still, despite delay lines' inherent problems, several important and influential families of early computers used them:

Once core memory appeared, delay line memories rapidly disappeared.