From Wikipedia:

Joe Taylor, K1JT, announced on June 29, 2017 the availability of a new mode in the WSJT-X software, FT8.[13] FT8 stands for “Franke-Taylor design, 8-FSK modulation” and was created by Joe Taylor, K1JT and Steve Franke, K9AN. It is described as being designed for “multi-hop Es where signals may be weak and fading, openings may be short, and you want fast completion of reliable, confirmable QSO’s”.

According to J. Taylor, the important characteristics of FT8 are:

  • T/R sequence length: 15 s
  • Message length: 75 bits + 12-bit CRC
  • FEC code: LDPC(174,87)
  • Modulation: 8-FSK, keying rate = tone spacing = 6.25 Hz
  • Waveform: Continuous phase, constant envelope
  • Occupied bandwidth: 50 Hz
  • Synchronization: three 7×7 Costas arrays (start, middle, end of Tx)
  • Transmission duration: 79*1920/12000 = 12.64 s
  • Decoding threshold: -20 dB (perhaps -24 dB with a priori decoding, TBD)
  • Operational behavior: similar to HF usage of JT9, JT65
  • Multi-decoder: finds and decodes all FT8 signals in passband
  • Auto-sequencing after manual start of QSO

Compared to the so called slow modes (JT9, JT65, QRA64), FT8 is a few dB less sensitive but allows completion of QSOs four times faster. Bandwidth is greater than JT9, but about 1/4 of JT65A and less than 1/2 QRA64. Compared with the fast modes (JT9E-H), FT8 is significantly more sensitive, has much smaller bandwidth, uses the vertical waterfall, and offers multi-decoding over the full displayed passband. Features not yet implemented include signal subtraction, two-pass decoding, and use of a priori (already known) information as it accumulates during a QSO.”[14]