Digital television (DTV) broadcasting using COFDM modulation is designed to modulate orthogonal frequency-division-multiplexed (OFDM) mid-band carriers with metadata including synchronization signals and transmission-mode signals. DTV signals modulate OFDM carriers occupying portions of the frequency spectrum of the transmission channel that extend in frequency both below and above these mid-band carriers. The OFDM midband carriers are capable of signaling when a new broadcast service is used that differs from the one disclosed. The signaling is provided by modulating the midband carriers with respective elements of signature sequences, each of which signature sequences is composed of Zadoff-Chu sequences and repetitive pseudo-random sequences scrambled by a Zadoff-Chu sequence.