A mud hammer is driven by the flow of drilling fluid to generate pressure pulses. Timing and/or amplitude of the pulses are altered to encode data by applying electromagnetic forces to a movable member of the mud hammer. In an example embodiment the movable member carries one or more magnets and electromagnetic forces are applied to the movable member by one or more electromagnets. The mud hammer may also generate electrical power that may be applied to charge batteries and/or drive downhole electrical apparatus.
