DSP Visualisations
What it is
A running effort to make the digital signal processing behind SDR visible: IQ constellations, spectra, and the way real signals actually behave, through small tools and explanatory writing. It includes a simple IQ visualisation plugin for SDRconnect, for people who want a quick feel for IQ without the GNU Radio learning curve, alongside step-by-step setup notes.
Signal Explorer
The newest piece, and the one to try first: a radio simulator that runs in a browser, currently in beta. Pick a mode and it generates the signal from the theory up, showing the audio going in, I and Q against time, the constellation, the spectrum and a waterfall, all from the same samples so every view is the same instant. Sliders for tuning error, noise, gain and fading, and you can turn the sound on to hear both the audio going in and what comes back out of the receiver.
It covers the analogue modes (AM, FM, SSB in both sidebands, double sideband) and the digital ones (CW, RTTY, FT8, BPSK through 8PSK, 16-QAM and 64-QAM). Each mode carries the sample rate, tuning and filtering of the flowgraph that made the matching capture, and says on the page where each number came from. Where a parameter is a standard value rather than a measured one, it says that too.
Nothing in it was fitted to make a picture look right. The modulators are checked against closed-form results instead: FM sideband amplitudes against the Bessel functions, with the carrier vanishing at the zeros of J₀ exactly as it does on a bench; AM sidebands at half the modulation index; single sideband suppression; and a round trip in which the audio put in comes back out of a detector as the same waveform.
Why I built it
I care as much about why things work as how, and IQ and DSP concepts click far faster when you can see them move. These pieces are aimed at demystifying the theory rather than just listing procedures.
Current status & goals
- Setup guide for IQ visualisation with an SDRplay, using GNU Radio and Radioconda, plus a lightweight SDRconnect plugin as a quick-start path.
- Explorations of FM and repeater IQ.
- The material became a full talk, Making the invisible visible, given to CDARC in May 2026 and invited to the RSGB Convention in October 2026, and a RadCom article, Seeing signals: I and Q from first principles, in September
- A purpose-built Raspberry Pi device plots live IQ and demodulates the same signal to audio for demonstrations.
Key links
My code
- sdrconnect-iq-visualiser, the SDRconnect IQ visualisation plugin
- sdrplay-gnuradio-examples, GNU Radio flow graphs for SDRplay devices
Elsewhere on this site
- Setup for IQ visualisation and digital signal processing
- Talks & Publications, covering the IQ talk and article
- Raspberry Pi SDR, the demonstration device
Tools used
- SDRplay, the RSP receivers
- GNU Radio, the DSP environment
- Radioconda, the easy path to a working GNU Radio install
