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 (see the left side of the Figure 1). It turned out that the lamp brightness, and hence the supply voltage, was quite critical. Consequently, the potentiometers had to be adjusted in order to give reliable performance.
|Figure 1.||By using a pair of headphones at the output of this older laboratory instrument, the author was able
to adjust the quadrature encoder’s potentiometers into the middle of the working range.
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 (see the right side of the figure).
The procedure involved dimming the room lights, twirling the shaft, and listening 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.