A computer’s graphics card can generate simple signals in the VHF band without additional hardware. [Efe]’s browser tool creates electromagnetic leakage on HDMI or DisplayPort lines by changing the pixel clocks of the graphics hardware. The frequency is calculated by multiplying the horizontal and vertical resolution by the refresh rate, taking the timings in the display standard into account. In the CEA-861 standard, a 1080p image at 60 Hz corresponds to values of 2200 and 1125 with the required blanking intervals and produces a 148.500 MHz signal. By increasing voltage changes with black-and-white pixel patterns, simple Morse messages can be sent through flicker on the screen. The tool can be tested in a browser; to receive the signal, a radio antenna must be brought close to the monitor. Although monitors, cables and connections are designed to prevent leakage, weak signals can still occur. It is noted that intentional transmissions should not be made in unlicensed bands.
Why it matters
This study shows that display hardware used to generate images can also create electromagnetic traces that can be received in the surrounding area. The fact that no additional hardware is required demonstrates that the effect is not limited to specialized transmission devices and that the browser, display, cable and connections must be evaluated together. However, the fact that the signal can be received with a radio antenna brought close to the monitor shows that the method’s range and operating conditions are limited; the text does not explain how far the signal can travel reliably or in which environments. The warning against intentional transmissions in unlicensed bands also serves as a reminder that the boundary between technical curiosity and radio-use regulations must be observed when testing the technique.