A medical apparatus configured to neuromodulate tissue and/or record patient information is provided. The apparatus includes an external system to transmit transmission signalts, each signal having at least power or data, and an implantable system to receive the transmission signals. The data transfer between the external and implantable systems is asynchronous. The external system includes external antennas to transmit a transmission signal. The transmission signal is an amplitude modulated signal modulated by varying a load on the external antenna(s) that causes an impedance mismatch prior to amplifying the signal for transmission. An implantable device includes implantable antennas to receive the transmission signal. The implantable system comprises a receiver to receive the transmission signal from the implantable antenna(s), implantable transmission module(s) to transmit data to the external system, and a variable load connected to the implantable antenna(s). Data is transmitted by varying the load.
