gnu radio
    i'm a newly licensed amatuer radio operator, ZL2RTM
    what is radio?
        terminology, baseband, passband, heterodyne, inphase and quadrature, sdr
        a baseband signal encodes the information you want to send, and is modulated to RF
        on the recieving end, this is demodulated to get the baseband back
        heterodyning is ... weird
            if you add together signals with different frequencies (x and y), you get a signal, with those frequencies in it.
            something interesting happens if you pass this though a "non-linear device"
                (non-linear devices don't follow the "superposisiton principle", f(x) + f(y) != f(x + y))
                these non-linear devices can be as simple as a single diode
            you get new frequency content, at x + y and x - y
            if you mix a 5000hz baseband signal with a 20MHz reference signal, you get new peaks at 19.995MHz and 20.005MHz
            this is AM radio, amplitude modulation.
            likewise, if you mix a modulated signal with a reference signal, you get the baseband signal back, as long as the reference signal is in phase.
            what happens when the signal is out of phase?
        inphase and quadrature
            any sine wave of arbitrary amplitude and phase can be represented as the sum of 2 sine waves of fixed phase, 90 degrees (or a quarter of a wave) apart
            this makes it very easy to control the phase of an output signal using only amplitude modulation techniques
            in fact, any modulated signal of a given bandwidth can be broken down into inphase and quadrature amplitudes of the same bandwidth
            this is the basis of SDR, how they can send and receive any arbitrary signal up to a given bandwidth and frequency
    what is gnu radio?
        gnu radio is a signal processing software development tool, based on individual blocks that each implement a signal processing operation
        gnu radio companion is the graphical interface for building flow graphs
        it also provides inputs and outputs, reading and writing to the console, files, the network, and SDR devices.
    building some demos
        WBFM rx
        NBFM rx/tx
        TV rx/tx