IQ Recordings / 4 Users


Is it possible to make IQ recordings using the KiwiSDR?  Also what is the limiting factor on 4 users, could more be possible with more bandwidth or in future better hardware than the current Beaglebones?




  • I've had some requests for IQ output (essentially data after down-conversion but before demodulation). This if not available at present but is certainly possible. I want this myself for implementing specialty demodulators / decoders. I assume some people want to run the IQ over a local network into traditional SDR applications (SDR#, …) Recording IQ samples for post-processing analysis is also a very interesting possibility.

    The real question is what bandwidth you need. If you are looking for audio bandwidth IQ (< 12 kHz) then this is pretty easy. It's the same amount of data that now traverses the SPI link from the FPGA to the Beagle. The only difference is this data would be output to the Ethernet connection directly after FIR filtering.

    For an entire sub-band, or more, you need a single DDC pushing as much IQ across the SPI as possible. Right now running the 4 channels takes about 5.4 Mb/s. I ran a test once that moved about 30 Mb/s on the SPI but it's not certain you could reliably move IQ data this fast. If you assume 24-bit IQ data then 30 Mb/s would equal about 625 kHz of spectrum. If 16-bit IQ gives you enough dynamic range then the bandwidth expands to about 950 kHz.

    The 4 user limit is influenced by a number of factors. They are mostly latency related: the Beagle has to react fast enough to service the audio FPGA buffers so samples are not dropped. You would think this would be relatively easy. But the Beagle is very busy running lots of FFTs for the various demodulation and GPS tasks.
  • A general comment re. 24-bit vs. 16-bit IQ: I've done comparisons of Winradio Excalibur Pro 24-bit IQ and Perseus/NetSDR/CloudIQ 16-bit IQ. Very unscientifically speaking: You won't hear more with 24-bit IQs when DX-ing. Even very weak signals near strong ones do not seem to resolve better. While 24-bit IQ by nature does produce a better dynamic range, the question is if 16-bits IQ is adequate or not. I believe it is. Another factor to consider when recording IQ is that 24-bit files eat up your hard drive a lot faster than 16-bit files.
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