C port of https://github.com/kosma/bakingbread because the python was too heavy to run on my Raspberry Pi 1. Compiled version runs on Raspberry Debian minimal.
gcc scopeclock.c -o scopeclock -lm
Connect your stereo sound card output to your oscilloscope in XY mode.
Use any player capable of reading raw audio in unsigned, 8-bit, stereo, 44100Hz sample rate mode. Linux example:
./scopeclock | aplay -f U8 -c 2 -r 44100
Any other sample rate can work as well, 48000 is sometimes better.
If the clock appears square, the soundcard is clipping. Reduce the volume with for instance alsamixer.
Sound card output is band-limited. The implications, amongst others, are that it's really hard to just draw in a goto(x,y) manner. I used some crude tricks to prevent the dot from swinging wildly around the screen; alas, they are not perfect and may cause flickering as a result. See this presentation to learn enough DSP basics to know how band-limiting affects signals (especially square waves).