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
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
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
- Haley and Scott (eds): Analogue and Digital Computers. Newness
- Middelhoek, George and Dekker: Physics of Computer Memory Devices.
Academic Press 1976