Which of the following is required when using FT8?

FT8 relies on strict 15-second transmit and receive cycles that must begin at the exact start of the interval. Because the protocol is time-synchronous, all stations communicating must have their computer clocks tightly synchronized, typically within one second of standard time (UTC). If your computer's clock is off by more than a second, your software will transmit at the wrong time, and you will miss the critical beginning or end of incoming transmissions, making decoding impossible. Options like hardware modems are incorrect because modern FT8 uses your computer's sound card, not specialized external modems.