A Few Words on Receivers
If you decide to get into digital, you will need better equipment than a simple portable. While some portables will work with reasonably stronger signals, the more complex the signal (or conditions), the more a portable will fall short.
While the number of newer desktop receivers has plummeted, a sharp eyed listener can often find such receivers at hamfests. Do your homework - use the Receiver Reviews category to begin your research. Even though most of the reviews do not specifically refer to digital capabilities, keep an eye on tuning accuracy (10hz resolution is necessary for some modes), tools that enhance selectivity (to be able to isolate one signal out of crowded conditions) and stability (little or no drift). Those folks with a modern ham transceiver have an advantage - they often have a general coverage receiver built in.
Several years ago, Icom came out with their 'receivers in a box'. These were radios with no traditional front panel, and run entirely by computer. Often (and erroneously) called IF Receivers, these were the PCR-1000 and more recently the PCR-1500 and PCR-2500. Unfortunately their performance was often compromised by a tendency to overload in the presense of strong signals, and rather poor filtering that, at least in the case of the PCR-1000, could not be selected independently of the mode being received. Thus, these Icom radios would be useable for some of the more popular modes (SITOR-B, ALE, HFDL, etc.) but would likely fall short in working with more demanding modes such as STANAG.
About the same time, TenTec (now Dishtronix) came out with their RX-320 (and later, the RX-320D which added an output for DRM reception). Unlike the Icom models, the 320 had up to 34 different filters (dependent on the software being used to interface with the PC) that could be selected regardless of the mode used. The bandwidths ranged from 8 khz all the way down to 300 hz. While it did drift somewhat when cool, when warmed up this was far less of a problem. It had a decent front end on it, and didn't have nearly the tendency to overload as the Icoms did. In fact, Globe Wireless took this radio and modified it for a remote receiver project before the project was abandoned for unknown reasons. These radios found their way into the market. Dubbed the RX-321, this version eliminated the drift issues and made several other improvements. There were only a limited number of these made, so the price on the used market tends to be higher than on a RX-320. These models can handle many of the digital modes pretty easily.
Both the Icoms and RX320s have since been discontinued by their respective manufacturers (although it is possible to find them on the used market). Only the Bonito RadioJet remains in this market. It's too early to make any definitive statements about this radio (as of this writing, there are very few reviews) but early comments suggest that the software used to interface with this radio is a bit technical (perhaps too much so for some users) and is prone to crashing. To their credit, users have reported that Bonito has been actively working with its users to fix these and other issues.
The big push these days is in the development of Software Defined Radios (SDRs). Many are very capable at HF; one of their biggest advantages is being able to see a large swath of frequencies, all at one time. For modes like ALE, which show up for a few seconds and are sometimes gone, this is a big plus for capturing data quickly.
Pay close attention to the calibration of your receiver. Many of the more modern modes such as STANAG4285 and MIL-STD-188-110A/B require a high degree of frequency accuracy. Decoding can be affected or completely prevented with more than +/- 50 Hz of error. Some of the more sophisticated decoding programs can compensate for errors within this tolerance with some form of automating tuning tracking but even they cannot cope with greater frequency errors. SDRs, for all their technical wizardry are often the most prone to large frequency errors. Checking any receiver by tuning to WWV or a similar frequency standard and calibrating is strongly recommended.
There are some digital decoding programs that natively support SDRs (or with the installation of a few DLLs). This can make routing audio to the application easier, without the need of installing a separate app such as Virtual Audio Cable. See our SDRs and Digital Decoding article for a list of applications and which SDRs they support.
Unfortunately, many lower end communications receivers, as well as Software Defined Radios, may be overloaded by local MW and/or FM stations, causing noise and other artifacts in the HF spectrum. You may need to add some front-end filters - the very bottom of our HF Equipment and Accessories has a selection of these.