SigDigger: The Machine That Reads The Air

Free, open-source and built like an instrument rather than an ordinary app, SigDigger turns mysterious radio traffic into something you can see, replay and interrogate.

The room is silent, but the spectrum is crowded. A bright line appears on the waterfall display, holds for half a second, splits into several narrow streaks and disappears. To an ordinary receiver, it is little more than a chirp. Perhaps a squawk. Perhaps interference. To SigDigger, it is evidence.

Most software-defined radio applications are designed to help you find a signal and listen to it. SigDigger begins where that process ends. It is for the moment when you have tuned into something unfamiliar and want to know what it is, how it behaves and what may be hidden inside it. This is not simply another digital radio dial. It is a laboratory for the invisible.

From listening to investigating

SigDigger describes itself as a free digital signal analyser for examining unknown radio signals. It works with a wide variety of software-defined radio hardware through SoapySDR and can inspect common digital modulation methods such as ASK, FSK and PSK; systems that encode information by changing a signal’s amplitude, frequency or phase. It can also demodulate analogue voice, examine burst transmissions and decode analogue video in real time. That distinction matters. A conventional SDR receiver is chiefly concerned with tuning, filtering and listening. SigDigger is interested in structure.

Is the transmission continuous or divided into bursts? How long is each symbol? Is there a hidden subcarrier? Does the signal repeat? Is its apparent frequency drifting because it came from a moving satellite? What happens when the recording is slowed down, enlarged or inspected away from the pressure of real-time reception? SigDigger gives you somewhere to begin answering those questions.

It is not interested in giving you a prettier tuning dial. It wants to show you what the signal is made of.

The software can operate on live radio data or previously recorded samples. It supports baseband recording, individual channel recording, waveform inspection, burst detection, symbol visualisation, autocorrelation analysis and blind parameter estimation. The latter is particularly useful when dealing with an unknown transmission: rather than requiring every characteristic to be entered in advance, the software can help estimate properties of the signal from the available data.

You can also listen to ordinary AM, FM, lower-sideband and upper-sideband voice channels. That makes SigDigger useful as a receiver, but listening remains only one tool in a much larger cabinet.

Familiar controls, different machinery

At first glance, SigDigger may remind experienced users of Gqrx. That is deliberate. Its creator concluded that Gqrx had one of the clearest interfaces in the SDR world and adopted a similar layout to reduce the learning curve. The spectrum plotter itself began as a modified version of the Gqrx component, extended with features such as configurable waterfall palettes and an OpenGL rendering backend. Underneath, however, it is a different machine.

SigDigger does not depend on GNU Radio. Its signal-processing work is handled by Sigutils and Suscan, two related projects developed for digital signal processing and real-time signal analysis. Suscan is designed to distribute work across multiple processor cores, while SigDigger provides the Qt-based graphical interface sitting above it.

That separation gives the project a distinct personality. SigDigger feels less like a collection of graphical radio blocks and more like a dedicated analysis console. The interface is recognisable, but the engineering beneath it has been shaped around signal inspection.

The OpenGL-enhanced waterfall is central to the experience. A waterfall is a continuously scrolling history of the radio spectrum: frequency runs from left to right, time moves downward and brightness represents signal strength. It allows a transmission to be seen as a shape rather than heard as a sound. Bursts become marks. Drifting carriers become diagonal lines. Repeating packets form patterns. With SigDigger’s time controls and replay mode, those fleeting patterns can be revisited rather than lost.

A serious collection of instruments

The feature list moves quickly beyond everyday radio reception. There is an interactive panoramic spectrum for exploring wider sections of radio bandwidth. There is Doppler analysis, along with frequency correction based on satellite orbital data stored in TLE files. There is subcarrier inspection, transition analysis and a spectrum integrator aimed partly at radio-astronomy work. The program can broadcast received samples and demodulated symbols over UDP, making it possible to pass data into other tools or across a network. It can even attempt to infer raw file parameters from the filename when opening recorded data.

Analogue television demodulation is included as well. This is not a fashionable checkbox added to fill space; it demonstrates the breadth of the inspection engine. A signal may contain voice, pictures, telemetry or an undocumented digital protocol. SigDigger tries to provide a common environment in which each can at least be approached.

The waveform inspection window is especially valuable. Spectrum displays show where energy exists, but waveforms reveal what the signal is doing over time. A short transmission that looks like a single flash in the waterfall may contain a carefully timed preamble, repeated data blocks or a sequence of changing symbol rates. Capturing and enlarging that event can turn an apparent burst of noise into an organised transmission. This is the pleasure of SigDigger: the gradual conversion of mystery into detail.

Built to become more specialised

No generic analyser can understand every radio protocol by itself. Weather satellites, remote controls, telemetry devices and experimental transmitters may all use different framing, error correction and data formats. SigDigger addresses that problem through plugins.

Plugin support arrived with version 0.3.0, allowing external modules to add interface components, side panels, specialised tabs and new inspectors. The project’s release notes cite an APT decoder as an early example; the sort of specialised component that can turn a general radio-analysis environment into a tool for working with a particular satellite transmission format.

This may prove to be the project’s most important architectural decision. The core application can concentrate on capturing, displaying and measuring signals, while specialised plugins handle the peculiarities of individual systems. In plain terms, SigDigger does not need to become bloated with every decoder imaginable. It can remain the workbench while plugins provide the purpose-built attachments.

The honest rough edges

SigDigger is powerful, but it is not frictionless. A source build requires three companion projects: Sigutils, Suscan and SuWidgets. The current repository documentation recommends a self-contained AppImage or the BLSD build script for GNU/Linux users who would rather avoid manually compiling the complete stack.

Platform support is also less tidy than the headline suggests. The project website lists GNU/Linux, macOS and Windows builds, while the current repository guidance presents GNU/Linux as the easiest route and warns that reliably packaging all Windows dependencies remains troublesome. Development builds have appeared for Windows and GNU/Linux, but this is still specialist open-source software rather than a perfectly polished commercial installer.

Documentation remains a work in progress, and some planned capabilities, such as OFDM inspection, DSSS inspection, Python integration and more advanced decoding facilities, are still aspirations rather than finished features. None of this should be hidden. SigDigger expects some patience. It may require reading, experimenting and occasionally compiling software. That is the price of entering a workshop rather than opening a sealed consumer appliance.

Older release, living project

The formal version history can initially create the wrong impression. Version 0.3.0, the latest tagged stable release shown by GitHub, dates from 5 July 2022. A newer development build was published on 8 April 2025, and the main branch recorded further work as recently as 11 February 2026. Recent changes have included plugin improvements, waveform behaviour, data export and a move to the LGPL licence.

So this is not abandoned software preserved behind glass. Development is deliberate rather than rapid, with much of the attention going into architecture, reliability and specialist extensions. The repository has also accumulated roughly 2,800 stars and almost 300 forks—substantial interest for a technical signal-analysis application that makes few concessions to mass-market simplicity.

The verdict

SigDigger is not the first program to install when all you want is to hear a local broadcast station. There are simpler receivers for that. It is the program to reach for when an unexplained signal appears and listening is no longer enough.

Its spectrum and waveform tools encourage careful observation. Its recording and replay facilities let you work without haste. Its inspectors provide a route into unfamiliar modulation. Its plugin system acknowledges that the most interesting signals usually demand specialised treatment.

The installation process could be smoother. The documentation could be fuller. Cross-platform support could be more consistent. These are real shortcomings, not charming eccentricities. But once it is running, SigDigger offers something uncommon: a focused, serious and surprisingly deep environment for people who look at a crowded radio spectrum and see questions waiting to be answered.

SigDigger does not remove the mystery from radio. It gives you the instruments to pursue it. The project is hosted on GitHub.