Field Guide · technology

Also known as: libairspy, Airspy library

libairspy is the host-side C library that drives the Airspy R2 and Airspy Mini receivers, streaming their high-dynamic-range IQ over USB.1 It is the driver layer beneath every Airspy application, and its design reflects the Airspy’s positioning as a step up in ADC quality from the RTL-SDR while keeping the same simple callback-based host interface.

Airspy libairspy unpack + convert application packed 12-bit float / int16 IQ RX callback delivers one buffer per USB transfer
libairspy unpacks the Airspy's packed 12-bit samples on the host and converts them to the requested format, delivering IQ (or real) buffers to the application via an RX callback.

How it works

An application calls airspy_open, then configures tuning and rate through airspy_set_freq, airspy_set_samplerate (the R2 offers 10 and 2.5 MS/s; the Mini 10/6/3 MS/s), and a gain interface that can be driven either as linearized “sensitivity” values or as the individual LNA, mixer, and VGA stages. The Airspy’s real ADC samples at up to 20 MS/s and the front end mixes to a low-IF; the R2 then performs an internal quadrature/decimation step, so the host can request either real samples or complex IQ. To save USB bandwidth the device sends samples packed at 12 bits, and libairspy unpacks them and, if asked, converts to float32 or int16 — meaningful sample-format work done on the host rather than the wire.

Delivery uses an asynchronous callback: airspy_start_rx registers a function that libairspy invokes with an airspy_transfer struct — a pointer, a sample count, and the format — once per filled libusb buffer. As with its sibling libraries this is a control-inverting callback API: the library owns the streaming loop and pushes data, and the callback must consume each buffer quickly enough to avoid dropped samples. airspy_stop_rx and airspy_close tear the session down.

In practice

The library ships with tools such as airspy_rx (record IQ or real samples to a file), airspy_info, and calibration/SPI-flash utilities. It is wrapped by gr-osmosdr for GNU Radio and by a SoapyAirspy plugin for SoapySDR, so most applications use the Airspy through those common layers rather than calling libairspy directly. A separate libairspyhf library serves the Airspy HF+ family — different hardware, different API — so the two should not be confused.

Relevance to SDR

libairspy is how software reaches the Airspy’s main selling point: a genuinely higher-quality front end (12-bit ADC, better dynamic range and out-of-band rejection than an 8-bit dongle) at 10 MS/s of usable bandwidth. For an SDR developer it is also a clean example of a host library that does real DSP-adjacent work — bit-unpacking and format conversion — on the CPU before handing samples up, which shapes how much processing the application still has to do.

GopherTrunk works with Airspy captures, and the Airspy is directly relevant to GT’s documented DSP notes: several trunking captures GT has analyzed came from an Airspy running at its native 10 MS/s. As a pure-Go application GT does not link the C libairspy; it ingests IQ over network transports or from recorded files, then runs a rate-invariant decode chain that normalizes any capture rate to the per-protocol channel rate. The buffers it receives are exactly what an airspy_start_rx callback would deliver.

Sources

  1. airspyone_host repository — Airspy, the source for libairspy, the RX callback API, packed-sample handling, and the airspy_rx tool. ↩

See also