IEEE Spectrum April, 2014 - 34

apR 2014

nORTh aMERiCan

synthesizing
storage

Joseph allen's inspiration was a 1960s
calculator that used delay-line memory [top]. His
acoustic delay line uses a collection of amplifier and
logic chips [second from top]. An oscilloscope displays
the bits 0011 flowing through the delay line [second
from bottom] and the result of inverting a bit [bottom].

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I soon had wiring spaghetti all over my
breadboard and realized, to my dismay,
that I had become utterly spoiled by modern microcontrollers like the Arduino and
the Basic Stamp. These let you do all the
messy logic in software, with a rich collection of plug-and-play daughterboards
and modules for popular hardware functions such as sensing or powering motors.
But here I was back to the fiddly and errorprone business of creating hardwired logic,
gate by gate.
Debugging the circuit raised another
problem: noise. I should have paid more
attention to the fact that in the video on
his blog, Allen demonstrates his circuit
in his basement. An acoustic delay-line
memory, by its very nature, emits a clicking
noise loud enough to be picked up across
a fair distance when it's working right and
an irritating buzz when it's not. Not wishing to drive my new neighbors (or myself)
to distraction, especially late at night, I limited the operation of the circuit to a minute
or so at a time. This made hunting down
wiring mistakes difficult, especially when,
say, I pressed a button to invert a bit and it
worked a jumper loose.
In the end, I decided to break with habit
and solder the circuit together on perfboard before getting it to work on the
breadboard. Wiring the circuit as carefully
as I could, I finally turned it on at an hour
during the weekend when a little extra
noise might be forgiven. I was able to
reliably modulate the audible signal passing over the delay line as I pressed the various bit-controlling buttons. I could see
bits flitting across the oscilloscope display, but I wasn't quite able to get the nice
sharp readout of a frame of 4 bits that Allen
has been able to demonstrate, so we've
used Allen's pictures here to demonstrate
the output. Nonetheless, as soon as I can
purchase a pair of earplugs and get some
more free time on a weekend, I intend to
use a few of the other leftover chips from
the Electronics Learning Lab and see if I
can add a built-in display for the memory,
eliminating the need for the oscilloscope.
-stephen cass

from top: JosepH allen; stepHen cass; JosepH allen (2)

that the medium used to support the sonic
pulses is air, not mercury. A loudspeaker
emits a series of chirps, with the presence
of a chirp indicating a 1. The chirps travel
through air until they are picked up by a
microphone. A chain of amplifiers turns
the chirps back into digital pulses, and
they are fed back into the speaker. Some
flip-flops and a synchronized counter allow individual bits to be inverted by pressing a button that corresponds to each bit;
the entire system can hold 4 bits (a quantity also known as a nybble). To view the
memory requires an oscilloscope tapped
into the circuit at two points, one to provide
a synchronizing trigger signal and the other
to read out the actual bits.
Fortunately, although I had thrown out
the actual Electronics Learning Lab, I had
kept the assortment of integrated circuits
that came with it. This collection includes
chips from the 4000 series, which is a
menagerie of logic chips such as flip-flops,
XOR gates, and pulse counters. The acoustic delay-line circuit uses seven chips:
TL272, LM339, and LM386 op-amps;
a 4001 quad NOR gate; a 4013 dual
flip-flop; a 4070 quad XOR gate; and a
4017 decade counter. Two potentiometers
set the threshold for passing audio signals
picked up by the microphone back into the
circuit and the frequency at which the circuit operates.
Having discarded the kit's breadboard,
I built the circuit using a regular breadboard, dipping into my own supply of components such as buttons to replace those
built into the kit. For a microphone, I used a
RadioShack monodynamic microphone and
ran about 7 meters of hookup wire to drive a
speaker: Because the circuit relies on there
being a detectable delay between when a
pulse of sound leaves the speaker and when
it is picked up by the microphone, the further
apart they can be placed, the better.
While the functional arrangement of
Allen's circuit is pretty straightforward, the
physical layout is a different story. Gates
on the same chip are often interconnected,
and two of the ICs are used simultaneously
in completely different parts of the circuit.

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Table of Contents for the Digital Edition of IEEE Spectrum April, 2014