Here's my analysis:
First generation radio was Marconi and the spark gap. Below 200 meters, huge antennas. Pre WW-2.
Second generation is exemplified by continuous wave and amplitude modulation. Spurred on by the Second World War, the technology leapfrogged with tubes and manufacturing which made radio gear smaller and more efficient (although hams could not operate during the war).
Third generation is represented by the introduction of Single Side Band, the transistor, and smaller rigs of the 60's. This also included economic boom-times which led to dramatic growth in our hobby.
Fourth generation (where I come in, licensed in 1977 as a teenager) is the late 70's to mid 80's. Smaller gear with much more transistor implementation, CW and SSB filtering using mechanical devices. Handheld VHF. Also, AMSAT (separate post for a future date).
Fifth generation is the 90's and early in this century, which included signal processing implementation and further miniaturization. Also, we saw the emergence of computers providing audio input to rigs for widepsread digital communications (PSK, for example).
At this point, you see that each generation builds upon the previous one; however, the differences and advances are becoming less and less from each previous generation. We're improving the way we do things, but we're not really CHANGING it. Roofing filters, high end rigs, second receivers.
This is all an improvement on how we LISTEN to the radio and TUNE the dial. Even if we use a computer to tune the radio or operate over the internet, it is still the same fundamental concept: listen, tune, listen. Tweak. Listen, tune, listen.
Now comes 6th generation radio. SDR. Mr Gerald Youngblood (K5SDR) in Austin Texas gets an idea and develops a prototype transceiver in 2004. It catches fire. Why? Partly timing, and let's give credit where it is due to genius on his part. But hams are looking to take operating to the NEXT level. DXCC? Got it, they say. Honor roll? Lot of them have it. Digital, CW, satellite, meteor scatter, earth-moon-earth. What's left? A PARADIGM SHIFT. (Side note, programs like MixW can give you a small taste of 6th generation when operating digital, but only with the spectrum available from the receiver...a couple of kHz. You can already hear what you can see, so it's neat but not really 6th gen).
Instead of LISTENING to a single frequency, imagine being able to SEE the entire spectrum. You could use PANADAPTER view to see everything. Here we see ALL signals from 6995 kHz to 7025 kHz SIMULTANEOUSLY! We are listening to a sub-segment which is indicated by the green vertical stripe, nut we can shift to any signal we want in a single mouse click.
You could use the WATERFALL view to see signals over time (time is vertical, frequency is horizontal. If you have never seen a waterfall view of frequency spectrum, it is 6th generation implementation!).
Here we see 60 seconds of history for the frequency segment 6995 kHz to 7025 kHz. The vertical green lines represent signals. The horizontal green lines represent jumps in the noise floor across the spectrum (there were thunderstorms coming in when this image was taken).
Notice how you can observe past history of station transmissions on the waterfall display. This is very helpful...you can tell if the band is open even if you don't hear anyone on your frequency, because you have a displayed record of the spectrum for 60 seconds! Also, yo can SEE where a station was transmitting even if you were not listening on that frequency at the time of the transmission. Maybe you missed a CQ. You can watch for the station to transmit again (this is a HUGE advantage in pileups, more on that in another post). You can also monitor beacons on 10 and 6 meters to see if the band is open, while simultaneously calling CQ or looking for other stations during brief openings!
You can combine them (I like this one best). While the panadaptor on the top may not show a signal instantaneously, there is a RECORD of it in the waterfall. Also, by watching the panadapter levels, you may see a signal pop up for just an instant and it does not display a differentiating color in the waterfall. These are the signals which DXers need to focus on...weak, in and out, the rare and distant DX who has not yet been pounced upon with a huge pileup.
This is just a brief sampling (pun intended, DSP lingo) of what 6th generation radio has to offer. Think about it for a minute. Tradtionally, you listen, tune the dial, listen some more. Maybe you get more efficient with DX clusters. But you're still using your ears in the traditional way. Why not use another part of your brain, ENGAGE it for the expansion of fun for your hobby?
SDR uses your EYES AND YOUR EARS! Now relying upon TWO sensory inputs. And the information from the eyes can augment what you hear. Or even show you that a signal is there when you cannot hear it (which means you tweak the receiver so you CAN hear it).