Digital data in self-timing reference is free from error due to irregular data spacing because of variations in speed and/or direction of scan in manual or machine applications with a pulse modulation of retrospective nature. Initially reference data manifestations are established and thereafter digital data are established partly on the basis of preceding manifestations of the data. In a binary data translating system, for example, a pair of reference pulses are spaced apart by a given interval. A binary unit is thereafter manifested by a pulse following at the same or similar interval and a binary zero is manifested by a pulse following at a differing interval. Each manifestation of a binary number thereafter depends on the interval between preceding pulses. A principle advantage of retrospective pulse modulation lies in demodulation. Large variations in the spacing and relatively larger variations in the scanning speed are accommodated readily. Magnetic tape and like records can not only be addressed at conventional high speeds in searching and at conventional low speeds later used in reproducing but, also can be searched continuously as the change is made between those speeds. Adaptive rate communications are particularly enhanced by the principle. Optical scanning of bar codes is improved by differing the spacing of uniform width bars or with bars of differing widths and differing spacing. These arrangements are applicable to railroad car and like object identifying, label data processing, human identification, card data processing, graphic display data probing systems and many other uses. Synchronous and asynchronous capability permits input to almost any digital data processing system.
Retrospective pulse modulation including bar coding and apparatus therefor