Analysis of information sources in references of the Wikipedia article "Tempest (codename)" in English language version.
All of the implants were quite sophisticated. Each implant had a magnetometer that converted the mechanical energy of key strokes into local magnetic disturbances. The electronics package in the implant responded to these disturbances, categorized the underlying data, and transmitted the results to a nearby listening post. Data were transmitted via radio frequency. The implant was enabled by remote control.[...] the movement of the bails determined which character had been typed because each character had a unique binary movement corresponding to the bails. The magnetic energy picked up by the sensors in the bar was converted into a digital electrical signal. The signals were compressed into a four-bit frequency select word. The bug was able to store up to eight four-bit characters. When the buffer was full, a transmitter in the bar sent the information out to Soviet sensors.
