Harvard biphase
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Harvard biphase is a magnetic run length code for encoding magnetic tape.[1] It is one of the formats employed in forming the digital bits of logic one and logic zero, along with non-return-to-zero (NRZ) and bipolar-return-to-zero (RZ) formats.[2] Each bit in the Harvard biphase format undergoes change at its trailing edge and this transpires either from high to zero or zero to high independently of its value.[2]
FDR
Harvard biphase has previously been used for digital flight data recorder (FDR) where 12-bit words per second are recorded onto magnetic tape using Harvard biphase code.[3] The data are encoded in frames and each of these contains a snapshot of the avionics system in the aircraft.[4] For Harvard biphase, a phase transition in the middle of the bit cell indicates that the bit is 1. No transition indicates that the bit is 0. There is also a phase transition at the start of each bit cell.[5] The ARINC 573 serves as a standard for FDRs that feature continuous data stream encoded in Harvard biphase.[6]
