In the early 1990's DSP chips finally became financially feasible and two DSP based, phase-shifted modulation modes became available to hams. Ham developers in the USA and Germany separately developed two HF digital modes that combined high data transfer speeds while remaining fairly robust and capable of maintaining point-to-point links over the HF radio spectrum.
The two modes were Clover and Pactor II. Implementation of the phase shift modulation was slightly, different, but these two modes were generally the same speed and claimed nearly equal robustness in the face of low S/N ratios.
I have always been a big fan of both of these modes and have used both since about 2003. These modes have be upgraded over the years, but the version two of these modes have always been very good for keyboard links.
Recently I was on the air with Pactor II with VE1XL. Conditions were poor with large fades in the path. Even the robust Pactor II was having some trouble moving data. It was still good enough for keyboard work, but the fades caused several error messages going back and forth as the data packets did not make it. Given that these were adverse conditions, I suggested to VE1XL that it would be a nice opportunity to test Clover and Pactor head to head.
We ended our Pactor link and switched to our DXP-38 Clover modems. These are the latest of the Hal Clover II modems and gave the mode the best chance at keeping a link.
We were able to establish the link, but we had trouble with the data blocks. Unlike Pactor, Clover uses what it calls Clover Control Blocks. These are sent at a modulation scheme that is the most robust it can muster. Unfortunately the CCB's modulation is not available for data packets. This proved to be the deal breaker on this link. Despite some error packets, the CCB's managed to get through, but as soon as the data packet tried to make it through, it was not able to get there.
Sadly eventually the link failed and we had to go back to Pactor.
So what shall we make of this? While I still find Clover more fun to operate due primarily to the fact that it has a more effective semi-duplex paradigm, and the ability to exchange station data on both ends of the link, it still needs more power to keep the S/N ratios up to the point where data can make it through. Pactor is a superior poor link mode, and this is a common consensus.