A system, comprising: an external device, comprising: first modulation circuitry for producing from first data a first signal modulated with on-off keying (OOK) modulation, wherein the first modulated signal comprises logic '0' bits of a first pulse width and logic '1' bits of a second pulse width different from the first pulse width, wherein each bit further comprises either an ON state with a signal that varies with a first frequency or an OFF state, wherein a transition between adjacent bits in the first signal is marked by a change in the first modulated signal between the ON and OFF states； a coil (34) configured to wirelessly transmit the first modulated signal to the implantable medical device； and an implantable medical device (10), comprising a first telemetry receiver in the implantable medical device for demodulating the first modulated signal to recover the first data.
