It's because of these horizontal bars, you can get the same effect with similar graphics: All video interfaces we use transmit data serially, line by line, pix

It's because of these horizontal bars, you can get the same effect with similar graphics: All video interfaces we use transmit data serially, line by line, pixel by pixel. The bars are 16px tall (black + white) so at 60Hz and 1920×1080px they'll produce a tone with fundamental frequency of 4.05kHz (60Hz × 1080 lines / 16 lines) and harmonics.