ka9q-radio is a software defined radio for Linux I've been working on for a few years. It is very different from most other amateur SDRs in several respects:
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Efficient multichannel reception. A single Raspberry Pi 4 can simultaneously demodulate, in real time, every NBFM channel on a VHF/UHF band (i.e., several hundred) with plenty of real time left over.
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All I/O (both signal and control/status) uses IP multicasting. This makes it easy for more than one module, on the same computer or on a LAN, to operate on the outputs of other modules, or for individual modules to be restarted without restarting everything else.
If you want a user-friendly, interactive, graphics-laden SDR with a simple learning curve, then ka9q-radio is NOT what you're looking for! (At least not yet.) Try one of the many excellent SDR programs already available like SDR#, Cubic SDR, gqrx, etc, or the standalone Kiwi SDR. This is my personal experiment in building a very different kind of SDR that runs as a component serving other applications.
The core components in ka9q-radio run as Linux 'daemons' (background programs) with little (or no) user interaction. Turnkey systems can be configured to, e.g., demodulate and record every FM channel on a band, or decode and relay digital messages (e.g., APRS, WSPR, Horus 4FSK, radiosondes). These programs are automatically launched by the (new) Linux standard system manager program 'systemd'.
The core component is the 'radio' module. It accepts a (multicast) raw I/Q stream from a front end module and executes a configured set of digital downconverters and simple demodulators for various linear and FM modes, including AM, SSB, CW and a raw IQ mode intended mainly for use by other programs.
Separate programs talk directly to several makes of SDR front end hardware and generates the I/Q stream for 'radio'. These programs currently include 'airspy' (Airspy R2), 'airspyhf' (Airspy HF+), 'rtlsdr' (generic RTL-SDR dongles), 'funcube' (AMSAT UK Funcube Pro+) and 'hackrf' (Great Scott Gadgets Hack RF One, receive only).
Two very rudimentary programs are provided for interactive use; 'monitor' listens to one or more demodulated audio streams and 'control' controls and displays the status of a selected receiver channel. It can also dynamically create and delete receiver channel instances. The 'control' program uses a flexible and extensible metadata protocol that could be (and I hope will be) implemented by much more sophisticated user interfaces. Various utilities are provided to record or play back signal streams, compress PCM audio into Opus, pipe a stream into digital demodulators, etc.
Although I've been running all this myself for several years, it is NOT yet ready for general use. A LOT of work still remains, especially documentation. But you're welcome to look at it, make comments and even try it out if you're feeling brave. I would especially like to hear from those interested in building it into their own SDR applications.
My big inspiration for the multichannel part of my project was this most excellent paper by Mark Borgerding: "Turning Overlap-Save into a Multiband Mixing, Downsampling Filter Bank". You probably won't understand how it works until you've read it:
https://www.iro.umontreal.ca/~mignotte/IFT3205/Documents/TipsAndTricks/MultibandFilterbank.pdf
Although there are other ways to build efficient multichannel receivers, most notably the polyphase filter bank, fast convolution is extraordinarily flexible. Each channel is independently tunable with its own sample rate and filter response curve. The only requirement is that the impulse response of the channel filters be shorter than the (configurable) overlap interval in the forward FFT.
Phil Karn, KA9Q karn@ka9q.net