The present invention concerns a system for transmitting a plurality of modes of digital television signals within the same transmission channel where one transmission mode is more robust than another mode. The present invention also concerns a system for receiving and decoding such signals. More specifically, an aspect of the present invention involves a method and an apparatus for utilizing a proper length of preamble data for the improvement of reception. Furthermore, another aspect of the present invention involves a method and an apparatus for inserting a preamble into a proper place in a transmitted data stream relative to the filed synchronization data. Another aspect of the present invention involves a method and an apparatus for decoding trellis-coded data, using the predetermined preamble data.