At higher FFT resolutions, one can inspect the quality of signals such as this digital transmission on 20m. This snapshot was taken running my experimental web sdr
View live SDR here : http://safaridx.com/web-sdr/ReleaseNotes.html
Digital mode on 20m (using 8k FFT) using websdr
When observing digital signals, where transmitter energy is evenly distributed across the 2.8KHz transmitter bandwidth, the effects of rapid path fading, as a result of ionospheric conditions between the transmitter and receiver stations, become very visible.
This received digital signal to the right clearly shows the effects of fading - which would have had less impact on SSB voice transmission (left) , but because the even energy distribution, clearly shows the differential signal strength, for frequencies across the 2.8KHz pass-band.
By comparison, the SSB signal shows very little signs of fading. This could be a result of the different origin of the SSB signal (and of course the fact the the energy distribution of SSB makes it less obvious for casual observation)
Rapid fading visible in the 2.8KHz digital signal. (Using websdr)