On Wed, Aug 3, 2022 at 06:03 AM, Reino Talarmo wrote:

Hi Martin,
There is an excellent description how FT8 works in the reference material nro
33 https://www.physics.princeton.edu/pulsar/K1JT/FT4_FT8_QEX.pdf. It is mainly
talking about decoding and protocol and only shortly how FSK modulation is
made very narrow band using GMSK to tame frequency shifts.

Hi Reino, I was already aware of the QEX document about FT4/8, but thanks for the link anyway!

As far as I can see, the modulation used by FT4/8 is GFSK, ie. Gaussian-filtered FSK, as opposed to GMSK, which I believe is a different beast altogether.

The QEX paper is well worth reading by anyone who wants to get a better understanding of how FT4/8 work at the modulation and decoding levels; with regard to the former the paper includes the following statements:

"FT4 and FT8 waveforms have constant amplitude except at the very beginning and end of a transmission. FT8 signals are ramped up gradually over a transition interval T/8, or 20 ms, at the beginning of the first sync symbol. The ramp function is a raised cosine ... The same taper is used in reverse to gradually ramp down the signal at the end of the transmission ... FT4 waveforms are similarly soft-keyed."

73
--
Martin G0HDB