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.