The time-constant length (attack/sustain/decay times) is the root of the original problem with ALC of course. Is there a way to store the averaged envelope and remember it in a variable used to set the gain, so you don't have to recalibrate every 15 seconds with each new transmission, or syllabically with SSB voice?
GFSK will work fine through a non-linear amplifier, but you lose the benefits of the new smooth transitions. The amplitude variations are only there during start, end and transitions. The old way used to look like this in the frequency domain. That startup splat is more than 2kHz wide. This is audio direct from WSJT-X to Spectrum Lab within the same PC.
I don't have an image of the new GFSK, but it was much cleaner when I checked. The FSK transitions in this are done at zero crossing, but even so, that represents a step change in slope and generates a spread of frequencies.
Neil
Hi Neil, As far as I know the new GFSK of FT8 and FT4 are still constant envelope, so they are tolerant to non-linear amplification (but don't read this as "immune to all sorts of terrible clipping and distortion"). See the bottom of page 4 in https://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf You raise a fair point that nowadays with SDR it is often enough to adjust the gain in an open-loop fashion, by monitoring the output power and changing the gain until an appropriate level is found. This can often be done once in a set an forget fashion. I do that for my QO-100 groundstation. However it's true that the gain of PAs can vary somewhat with temperature and frequency, so sometimes some sort of close-loop adjustment of gain (of an appropriately large time constant and everything to prevent distortion) would be better. Best, Daniel.
-- Neil <a href="http://g4dbn.uk/"><small>g4dbn.uk</small></a>
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