The human ear is very good at
detecting noise or missing cycles in a
waveform, and since we have two ears,
we can monitor two signals at once.
I recently repaired an older lab instrument
containing a quadrature encoder
made with an incandescent lamp, a slotted
disc, and two photocells (left side in
the figure). It turned out that the lamp
brightness, and hence the supply voltage,
was quite critical, and the potentiometers
had to be adjusted to give reliable
performance.
A digital oscilloscope was hard to use
as an output indicator because its display
tended to freeze every time there
was a sudden change - exactly what we
want a digital scope to do under normal
circumstances. An analog scope was not
available, so I decided to use headphones
as the output indicator (right
side in the figure). The procedure was to
dim the room lights, twirl the shaft, and
listen for smooth whines in both ears. It
took me only a couple of minutes to
bring each potentiometer into the middle
of the working range.
MICHAEL A. COVINGTON is
a senior research scientist in
the Microelectronics Laboratory
in the Artificial Intelligence
Center at the University
of Georgia.