What modulation method is predominantly used by over-the-air DTV broadcast stations in the U.S.?

The predominant modulation method used by over-the-air digital television (DTV) broadcast stations in the U.S. is 8-VSB, which stands for 8-level Vestigial Sideband. This technique was specifically developed to transmit digital television signals efficiently over terrestrial broadcasting systems.

8-VSB is an efficient method for encoding and transmitting digital information, allowing for the effective use of bandwidth while minimizing the impact of multipath interference. It combines error correction with data transmission, making it well-suited for the varied conditions encountered in terrestrial broadcasting. This modulation approach allows broadcasters to deliver high-quality audio and video signals to viewers' homes.

In contrast, other modulation methods mentioned, such as QAM (Quadrature Amplitude Modulation), are generally used in cable television systems, not for over-the-air broadcasts. FM (Frequency Modulation) and AM (Amplitude Modulation) are primarily associated with radio broadcasting rather than television, with FM being used for analog audio and AM serving primarily for various forms of radio communications. Thus, 8-VSB is the correct choice as it is specifically tailored to meet the needs of over-the-air DTV broadcasting in the United States.