On 07/12/2018 12:29, Richard Lamont wrote:
FT8 does not use Gaussian keying. Instead, the tone frequency is
switched instantaneously at a zero-crossing point. This produces a
wideband transient between symbols. It's comparable to frequency
modulating a carrier with a square wave.
There's also a wideband transient from the amplitude keying at the
beginning of the first symbol and the end of the last, where the tone
switches suddenly between zero and full amplitude. This is comparable to
a CW key click.
Hi Richard,
a couple of minor corrections to your statements.
The WSJT-X Tx audio synthesizer does not change frequency at
zero-crossings, it happens at exactly the time the protocol expects the
shift. The phase is continuous as far as it possible but you are correct
in that the shift is between two samples so there will be a small
wideband component at switching of symbols. Since there is only one tone
at any time there will be no IMD components.
The start of Tx is at full amplitude but the samples are synthesized
starting from the zero-crossing. The tail end of audio is ramped down to
zero amplitude over a few milliseconds.
73
Bill
G4WJS.
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