"Digital" digital radio
I am an amatuer radio enthusiast (advanced qual), not a pilot, so my comments are restricted to the underlying HF technology, not current flight applications.
HF is HF, as regards propagation. However, there are two distinct areas of digital innovation with the potential to affect long-distance, weak-signal comms in different ways.
Digital mode, in contrast to analog mode, means signals are converted from analog (voice) into digitized levels and transmited. The mode has the advantage of high fidelity, with the disadvantage that the signal can completely "drop out" under weak signal conditions.
Digital signal processing is something else again, where the signal, which may be transmitted as analog, is detected and filtered using digital processing. DSP is very useful for weak signal work, as the operator can (depending on equipment) create extremely narrow custom filters on-the-fly to exclude interference. (The Flex-Radio Software Defined Radio is probably the best of breed available in the amatuer category).