Enhanced digital broad cast signals are transmitted on a digital radio broadcast signal by allocating spectral resources of the digital radio broadcast signal to simultaneously serve existing receivers (which can decode default content but not new content from the enhanced digital broadcast signal) and new receivers (which can decode default content and can decode new digital content from the enhanced digital broadcast signal) by modulating a first plurality of reference subcarriers with a first signal constellation, and modulating a second plurality of reference subcarriers with a second modified signal constellation such that a receiver processes the digital radio broadcast signal to produce a first coherent reference signal estimate for the first plurality of reference subcarriers to extract the default content for playback and to produce a second coherent reference signal estimate for the second plurality of reference subcarriers to disregard the new content.