Hi, This discussion seems to have branched in two different directions. One is drift and the other is absolute frequency. Please tell me if I'm way off on this, speculations follow: From WRPS perspective the absolute frequency the due to uncalibrated crystals and sample rates should not matter, given it does not take it outside of the allowed frequency window. It should still be a fixed frequency with a slight offset, indistinguishable with intended TX offset, but looking as a basically straight line from the decoders perspective. Perfecting sound card sample rates and oscillator calibration in the transmitter and receiver does not affect this. WEFAX/SSTV decoding is not the same, and the image slanting does not work the same way in WSPR or PSK.
Ignoring atmospheric effects, drift is one or more unstable clock signals in the system, making the received frequency wander around. This does make it more difficult to decode in WSPR as the 4 tones i nthe beginning is not the same as in the end, with other overlapping signals and nonlinear drifts.
To sum it up, frequency stability is more important than absolute frequency calibration in WSPR, JT65, JT9, FT8 etc.
Daniel Ekman SA2KNG