SDRplay RSP radios with OnAir: installing the SDRplay API

SDRplay's own driver is needed once, because its licence does not allow other programs to ship it.

Setup#

Install the free SDRplay API from sdrplay.com/api once, and restart OnAir.
Make sure the SDRplay API Service is running (the installer starts it).
Pick the RSP in the source list. A help card appears if an RSP is plugged in but the API is not installed (0.2.6).

Supported models: RSP1, RSP1A, RSP1B, RSP2, RSPduo and RSPdx (and RSPdx-R2).
The RSPs deliver up to 10 MHz of bandwidth (10.66 Msps). Wide TV channels should work; DAB, FM and narrow modes do.
The RSPduo opens on tuner 2 (fixed in 0.2.3).
An RSP-compatible radio that is not made by SDRplay (for example a Mirics MSi2500 "RSP1A clone") is not seen by the SDRplay API. Use SoapySDR's mirisdr module for it instead, see SoapySDR.
RSPs reach HF and LF directly, which suits the time-signal and shortwave modes.