Delay line stores

In an acoustic delay line store ("memory"), bits are inserted sequentially into one end of a long metal wire as acoustic pulses, by means of a transducer (see below). These pulses propagate along the wire at the speed of sound, until they arrive at the other end where they are translated back into electrical pulses by another transducer. By making an electrical connection between both transducers, the loop is closed and a pattern of pulses, representing the information to be stored, circulates indefinitely (that is, as long as the power supply remains uninterrupted!).
Using an arrangement of logic gates, bits can be extracted from or added to the system. A clock pulse generator is used for defining the time slots during which read/write transfers can take place; a counter keeps track of the location of the bit patterns.
The storage capacity of a delay line is determined by the pulse propagation speed, the physical length of the line, and the duration of a single pulse. One system in our collection contains a coiled wire about 29 m long, made from a nickel-iron-titanium alloy which has a negligible temperature coefficient and a large bandwidth. The propagation velocity of torsional vibrations (see below) is about 2950 m/s. The total delay time is 9819 microseconds. Hence at the maximal clock frequency of 1 MHz, the storage capacity is about 10000 bits. The maximal clock frequency is determined, among other things, by the fact that a pulse is broadened and attenuated during its travel through the delay line. This effect sets an upper limit to the capacity of delay line stores.

Translating electric pulses into acoustic pulses is accomplished using the effect of magnetostriction: applying a magnetic field to a magnetostrictive material (such as nickel) causes a mechanical strain. The delay line material is itself not magnetostrictive. Torsional impulses are induced in it by means of a pair of nickel ribbons attached to the end of the wire (see the figure). The ribbons pass through a pair of coils which generate magnetic pulses in opposite directions for each incoming (electric) bit pulse. At the other end of the line, a similar arrangement is used to translate the arriving acoustic pulses back into electric pulses. On both ends, the delay line is terminated with neoprene damping pads in order to prevent pulses from being reflected back into the line.
Acoustic delay line stores as described here have been very popular in the early 60's, when core memory still was too expensive for price-critical applications like calculators. They were used both in large computer systems and in electronic desktop calculators. Early computers (EDVAC, UNIVAC) used mercury delay lines. Quartz and glass media have also been used where speed was more important than storage capacity.