Comparing this to Dachau suggests you’re feeling a massive amount of psychological pressure. Usually, when the mind is under extreme, prolonged stress, it can start to externalize internal thoughts as voices or 'beamed' messages. It might be worth talking to a professional about the distress this is causing you—they might have ways to help mute those signals.
Heterodyne systems don't let you pack more information into a lower-frequency signal, they go the other way.
EEG "spellers" c. 2000 required a cooperative participant who's actively engaged in a behavioural task: you attend to the letter/word you want to send and thereby produce a different response when it (vs the other letters) flash.
Implanted electrodes can do a lot better but it's still not something that will let someone "slurp" your thoughts out of you -- it'd be like subvocalizing them.