In this paper a new enhanced coding scheme of ATSC DTV standard is presented. The proposed coding scheme is a modified trellis codes of enhanced 8-VSB for robust bit-stream mode. By computing the distance spectrum, we derive the theoretical upper bound on the first event error probability of the proposed trellis codes. Based on the derivation, the proposed coding scheme has slightly better performance on AWGN channel than the conventional enhanced S-VSB coding scheme at high signal-to-noise ratio (SNR). We also show that the proposed coding scheme has better performance than any existing coding schemes of ATSC DTV standard by using the Monte Carlo simulations.