Hellschreiber Clock

Step 12 The principle

The figure shows the principle behind displaying the matrix of dots making up the characters.

The sequence of rising tones make up a staircase waverform, which, repeated at definite intervals, form a sawtooth across the band of frequencies forming the character.

I had an earlier instructable, http://www.instructables.com/id/Oscilloscope-clock/ , on displaying the time on an oscilloscope. The principle is similiar, except that the earlier one used voltage levels and this one uses frequency.

The difference is that voltage levels are very difficult to display using the sound card, and almost every program which displays voltage levels does not display it in the mode which makes characters visible.

Each character is displayed as a sequence of columns seven pixels high. If the bottom most pixel has to be lit, the frequency corresponding to it is switched on for a brief duration of time. In case of the "oscilloscope clock", a particular voltage level is held for that time. If that pixel is to be dark, the tone is not made at all, or, a blanking level is sent instead.

Since these frequencies (or voltage levels) are sent sequentially, one after the other, they do not form a vertical line. They form a line that leans to the right.

It is possible to sent those bits in the reverse direction, and then the resulting characters will lean to the left. This looks unnatural, and so the present arrangement is preferred.

Another type of hellschreiber, which sends all tones at the same time, is capable of producing perfectly vertical characters. Since this requires producing all tones at the same time, without distortion, it is not possible to implement it in a simple way using a single microcontroller.