A vent valve apparatus for use with a system for supplying breathable gas pressurised above atmospheric pressure to a human or animal. The DC-to-AC converter includes a signal input port for externally connecting an environment sensing apparatus. The processing may comprise two or more data transformation techniques coordinated in time, concurrently executing autonomous sets of instructions, and provided in a manner preserving the sequence of processing and storing the processed data chunks. The reconstructed signal may be the same as the input signal, inverted from the input signal or level-shifted from the input signal.