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Modern DTV broadcasting systems, such as DVB and the Chinese DTMB system, usually have multiple code rates for forward error correction to accommodate a variety of channel conditions. In these systems, each code rate corresponds to a special set of code words, which put a limit on rate granularity and thus cannot achieve seamless code rate adaptation. To solve this problem, we propose a known-bits padding scheme to generate multi-code-rate LDPC codes of arbitrary granularity. In our scheme, we take a standardized LDPC code of specific rate as mother code, then pad known-bits according to the targeted code rate and given shortening optimization algorithm. Simulation study shows that our proposed scheme can obtain satisfying performance in terms of code rate flexibility and decoding threshold.