Minimal set of code to initialize sbitx hardware and connect to external SDR software, like quisk or SDRConsole.
• main() initializes the hardware and spawns the network thread (hpsdr_poll) and audio thread (sound_thread_start).
• The external SDR app discovers the radio via the UDP thread and sends a start stream command.
• The audio thread reads IF data from the audio chip at 48k samples per second, does a 24 kHz anti-aliasing filter and sends 48k samples per second to sound_process().
• sound_process() performs complex mixing to baseband, passes the arrays of I and Q data to hpsdr_send_iq() for hpsdr Protocol 1 over the network.
• The external SDR app handles all FFT processing, demodulation of various signal types, and audio routing
