Recording with an RTL-SDR: audio, IQ and playing IQ files
There are two kinds of recording. Audio saves what you hear, as a normal WAV file. IQ saves the raw capture: the whole slice of spectrum, every signal in it, so you can play it back later and tune around in it as if it were live. Both does the two at once. Choose under Recording → Record, then press the record button in the VFO bar or the R key.
Where the files go
Press Choose folder under Recording and pick a folder once; every recording is then written straight into it. Without a folder, audio recordings are kept in memory and land in Downloads when they stop, up to a 200 MB limit. IQ captures always need a folder, because they are large. Folder picking needs a Chromium browser.
The File name field sets a pattern; the frequency, mode and UTC time are filled in when a recording starts. The app never overwrites a file: if the name is taken, it writes “name (2)” and so on.
Formats and sizes
| Format | What it is | Good for |
|---|---|---|
| Audio · WAV 16-bit | Small, plays anywhere | Keeping a broadcast or a QSO |
| Audio · WAV 32-bit float | Full dynamic range | Editing or decoding later |
| IQ · WAV u8 | The samples exactly as the stick sends them | The smallest IQ file; stops at 4 GiB |
| IQ · WAV 16-bit | Signed 16-bit IQ | Programs that want 16-bit; stops at 4 GiB |
| IQ · RF64 | WAV without the 4 GiB limit | Long captures |
| IQ · SigMF | Data file plus a metadata file | Sharing and other SDR tools |
IQ adds up fast. At 2.4 MS/s in u8 the stick produces about 4.8 MB every second, close to 17 GB an hour; the Recording panel shows the rate for your settings. Lower the sample rate if you only need a narrow band. Changing the sample rate stops an IQ capture, and a scan that hops the capture cannot run during one, because an IQ file keeps a single centre frequency.
Recording only what matters
- Only while squelch is open skips the silence and writes one file per transmission. Ideal for a quiet channel.
- Stop after ends each recording on its own after the time you set.
- Schedule plans a recording for later: a start time, a duration and the VFO it records. The app tunes that VFO first. The tab must stay open, and the stick connected, for it to run.
Finished recordings appear under Recordings in the Output bar, where you can play audio files back.
Opening an IQ file
An IQ file works like a stick that cannot retune. Open one with Open an IQ file on the connect card, from Recording settings, or with Open … as source on a finished IQ recording. You can also drop a file onto the connect card. The file is read in the browser and never uploaded. This also works where WebUSB does not, for example on an iPhone.
The app reads WAV and RF64 IQ (8-bit, 16-bit or 32-bit float, two channels), SigMF, and raw files by extension (.bin as 8-bit unsigned, .raw and .iq as 32-bit float), from 48 kS/s to 3.2 MS/s. It needs the centre frequency and the sample rate. SigMF and the app’s own recordings state both. For other files it guesses from the name (GQRX-style names work) and asks you to confirm; a guessed centre is marked, so check it against the spectrum.
While a file plays, a transport bar appears: play and pause, a position slider, loop, and speeds of 0.5×, 1×, 2× and 4× (you hear audio only at 1×). K plays and pauses, J and L jump 10 s back and forward. You can add VFOs, change modes, run the scanner over the recorded band and start decoders on the file, but you cannot tune outside the band it recorded. If the centre frequency was wrong, correct it under Source; the VFOs stay on the signals they are on.