Subsystem Search

What a software-defined radio is

By the end of this chapter, you know what a software-defined radio is, and the words radio uses for where a transmission is: its frequency, its band and its radio channel. You know what the receiver plugged into your computer does and what the computer does, and why that lets Substation listen to a whole band at once.

Nothing here needs installing, and there is nothing to type.

Know what a software-defined radio is already, and have a receiver? Skip to Installing Substation, or to A first scan if Substation works with it already.

Your computer as a radio receiver

Software-defined radio lets a computer receive radio, through a small receiver plugged into a USB port. The receiver turns the signals its antenna picks up into numbers, and software on the computer does the rest: it tunes in to a transmission, filters out everything around it, and decodes it into sound.

An ordinary radio is built from physical circuits, each with one job: one tunes to a station, one filters out its neighbours, one decodes the sound from the signal, and one drives the speaker. To receive anything else, it needs other circuits. A broadcast FM radio cannot listen to an aircraft, which talks on frequencies the radio does not tune, in AM, which it cannot decode.

A software-defined radio, an SDR, keeps as little as it can in circuits. Its receiver takes in a whole range of frequencies at once and turns it into numbers, and software does everything else. Wikipedia's article on software-defined radio puts it as the parts a radio would conventionally build in analogue hardware, its mixers, filters, amplifiers, demodulators and detectors, done in software instead.

So one receiver hears anything within its range that software can decode, and listening to something new means changing the software, not the radio.

Frequencies, bands and radio channels

A frequency is how many times a radio wave cycles per second, counted in hertz (Hz): a kilohertz (kHz) is a thousand cycles per second, and a megahertz (MHz) a million. A broadcast FM station is somewhere between 87.5 and 108 MHz.

A band is a range of frequencies set aside for one use. The 2m amateur band, where this guide's first scan listens, runs from 144 to 146 MHz in the UK, and licensed radio amateurs talk there. Its name is its wavelength: about two metres.

A radio channel is one of the frequencies in a band that a transmission is sent on, each a fixed step from the next, which is its spacing. On the 2m band, Substation spaces them 12.5 kHz apart, so a transmission on 145.5 MHz and one on 145.5125 MHz are on neighbouring radio channels. Every band Substation ships is in its band library.

What the receiver does

The receiver is tuned to one frequency, the centre of the range it hands over, and measures the signal from its antenna hundreds of thousands or millions of times per second. Each measurement is an IQ sample: two numbers taken a quarter of a cycle apart, which between them give the signal's strength at that instant, its amplitude, and where it is in its cycle, its phase. From how the phase moves between one IQ sample and the next, the software tells a transmission just above the centre frequency from one just below it.

The more IQ samples per second, the wider the range of frequencies the receiver hands over, which is its bandwidth: a receiver taking two million per second hands over two megahertz. On an inexpensive receiver, that is room for a whole band.

What the computer does

Everything else happens in software, on the IQ samples the receiver sends over USB. Software can pick out any frequency inside the receiver's bandwidth, filter away everything around it, and decode the sound it carries, which radio calls demodulating.

How a transmission carries its sound is its modulation, and each needs decoding its own way. FM varies the signal's frequency with the sound, AM varies its strength, and single sideband is a narrower kind of AM. In Substation's band library, NFM is narrow FM, and USB and LSB are upper sideband and lower sideband, the two kinds of single sideband, nothing to do with a USB port.

Because software has the receiver's whole bandwidth to work on, it can do all of that for many frequencies at once.

That is what Substation does. A scanning receiver of the usual kind steps through frequencies one at a time and stops on one that is busy, missing whatever happens elsewhere while it listens. Substation watches every radio channel in a band at the same moment, and records each transmission to a file of its own, two at once if two radio channels are busy at once.

Reading more

The next chapter suggests a first receiver, and where to get one.