An echo cancellation reference signal is generated within a DTV transmitter by approximating a pair of successive repetitive PN sequences with 8VSB symbols arranged in sequences that are capable of being generated by ⅔ trellis coding. The trellis coding is controlled so that the two sequences of 8VSB symbols can be linearly combined to generate a repetitive PN sequence doubled in amplitude and superposed on a constant-value pedestal. After this linear combining is performed by a comb filter in a DTV receiver, the resulting signal is convolved with the PN sequence as known a priori at the DTV receiver. This is a correlation procedure which generates the impulse response of the channel through which the DTV signal was received. The channel impulse response (CIR) provides a basis for computing the weighting coefficients of adaptive channel-equalization filtering in the DTV receiver.