A capability system is sort of like having lots and lots of those roles. But a key element is that within the system you don't have to be an administrator or have any kind of privileged-operation access to create a new capability, you just do it, and then you can pass it around (loosely equivalent to giving the role to others).
Capability systems include a way to _pass_ capabilities as part of the basic routine operations.
Some people have used it with Raspberry Pis; I don't own any myself. I tried a BeagleBone Black once and found that the CPU couldn't keep up. The CPU use will vary a lot depending on what sample rate you set and what demodulator you're using. Lots of room for improvement, too.
The thing that happened is RTL-SDR, which is _not_ the name of a product but refers to the _use_ of a mass-market TV receiver USB device that happened to have a raw IQ signal mode in the hardware (RTL2832U chip). This being so cheap ($15-20) lots and lots of people jumped on it and this created a lot of interest in SDR in general.
I don't have a good overview to recommend you, but for ‘what kind of signals…’ I do recommend grabbing a RTL-SDR device, and just reading about how to use it should give you lots of pointers into the rest of the SDR world.
A public demo instance is blocked on adding sufficient anti-DoS provisions to the server. One possibility I've considered is to make a simulation-only version packaged for Sandstorm, which would allow for a one-click trial experience — but not show any actual RF, so the internal simulator would need to be made much richer to actually show off the features.
It turns out that UI is hard! But the biggest not-crap thing from back when I wrote that (persistent waterfall) is not actually visible in a screenshot; it would need a video.
As the author, I'll say that there's a large amount of "ooh shiny bleeding edge" and "if it works for me it's fine" going on. I see that newer Safari has a _different_ failure mode and I should look into that… okay fixed https://github.com/kpreid/shinysdr/commit/67327273
The metagame (or "the meta") is the long-term game in which the moves are "I am going to play this character/deck and practice/use these strategies" -- decisions you make at the beginning, or before, play starts in the base game and the rounds are complete games of the base game.
Learning new information about the base game (or it being updated by the developers) changes what moves are best in the metagame — but as this knowledge propagates through the player base, the probability distribution of what-you-will-be-facing changes, which also changes the best choices of meta-moves.
For an example of why metagames are more than just knowledge about the base game, suppose that we have a fighting game with character A (or a CCG with a player-designed deck A) who is well-rounded and B who doesn't do so well in most cases but is good at beating A. Then even if the base game doesn't change at all and nobody learns a new trick, B is a good choice if and only if lots of other people are playing A — meaning you have a dynamical system.
A lively metagame keeps things interesting because players keep doing new (or dusting off old) things to defeat the current things, rather than sticking to what works — because "what works" changes. It avoids the problem of "X is best, so either you ignore other parts of the game or you are deliberately playing suboptimally".
I don't know what the limits on efficiency are, and the Wikipedia article mainly addresses AC-to-DC conversion rather than DC-to-AC, but I assume simply from the fact that they aren't used these days that they aren't an improvement on solid-state inverters.
The cable is the least likely part to be the source of the problem. The data lines are inside a shield, and in USB they are balanced (“D+” and “D-”). Both of these act to prevent radiation of the signals.
If a keyboard radiates it is likely to be either from the unshielded, unbalanced matrix wires going to the keyswitches, or leakage from the controller going onto the outside of the shield. (I think the latter could be reduced by using (more/bigger) decoupling capacitors, i.e. shorting out the RF.)
Coiling a wire does not generally make it a more effective antenna; it may or may not make it less effective depending on the circumstances. (The reason some antennas are coiled is to get the same length of conductor into a smaller space.)
Capability systems include a way to _pass_ capabilities as part of the basic routine operations.