Hellschreiber Clock

Step 2 Time as a sequence of frequencies

This screenshot shows the capture from the clock sending a sequential seconds information. Actually this is bogus, since each set of digits takes several seconds to be generated and so the displays cover a larger interval of time than the three seconds suggested.

The pattern of dots seen above the line of digits is due to the harmonics of the tones: the microcontroller generates tones by switching a port line to the supply or ground, and the resulting rectangular wave has many harmonics. Since this is directly fed to the sound card, the display will show all these harmonics along with the desired fundamental frequency.

Since arranging for a pure sine wave is difficult, the difference between the maximum and minimum frequencies used for display has thus to be arranged to be less than an octave. In other words, the maximum frequency has to be less than twice that of the minimum frequency.