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Spectrum Scope Audio IQ
RigControl Web can drive its spectrum waterfall from a radio's baseband I/Q output, captured through a stereo USB audio interface. This is the third spectrum source alongside Hamlib UDP (Icom) and FT-710 via USB — it's generic (any radio with a baseband I/Q output on a stereo jack should work), but it has only been tested against the Xiegu G90.
No FTDI drivers, udev rules, or special permissions are required — this source uses the same generic USB audio device enumeration as regular backend radio audio.
Unlike the FT4222 path (a dedicated USB-SPI chip reading digital spectrum data), this source is pure audio: the radio outputs a low-level analog I/Q signal on a stereo jack, a USB sound card digitizes it, and RigControl Web computes a live FFT from the captured audio directly in the server process — no radio-specific driver or child binary involved.
Span = sample rate. Because I/Q (complex) sampling covers a full ±sample_rate/2 band around the tuned frequency — unlike ordinary real-valued audio sampling, which only covers half that — the displayed span always equals whatever sample rate you select for the capture device (48 kHz capture → 48 kHz span, 96 kHz → 96 kHz, and so on).
But the radio's real bandwidth doesn't grow with sample rate. The G90's I/Q output is an analog reproduction of its own internal 48 kHz-sampling SDR core — its true usable bandwidth is fixed by the radio's own hardware, not by how fast you digitize it. Capturing at 96 kHz instead of 48 kHz mainly buys headroom against aliasing (and, per user reports with this radio, does reveal usable content beyond what 48 kHz capture alone can show, since 48 kHz sampling's Nyquist limit clips some of what the radio actually outputs) — but going well beyond 96 kHz on the capture side won't hand you more real spectrum from a radio whose own SDR core tops out around there. Treat higher sample rates as "avoids clipping the radio's real output," not as "the radio suddenly has more bandwidth."
- Connect a 3.5mm TRS cable from the radio's I/Q output port to the USB audio interface's Line-In jack.
- Do not use a Mic-In jack. Many mic inputs supply plug-in-power (a small bias voltage) to support electret microphones — feeding that back into the radio's low-level (~50–100mV) I/Q output is not something the port is designed to tolerate. Line-In jacks don't carry this bias voltage.
- Connect the USB audio interface to the computer running RigControl Web.
- Add the Spectrum Scope panel to your layout if it is not already visible (Add Panel in the layout toolbar).
- Click the gear icon in the Spectrum Scope panel header to open its settings.
- Under Spectrum Source, select Audio I/Q.
- Turn on Enable Spectrum Scope.
- Under Capture Device, select your USB audio interface. The list is populated from the same device enumeration used for the radio's backend audio settings.
- Under Sample Rate (Span), pick the rate you want to capture at. Higher rates need a capable adapter — if the device rejects a rate, an error will appear next to the status indicator.
- The Capture running indicator turns green once the app has successfully opened the audio device.
The waterfall should appear within a second or two.
If a signal you expect to see on one side of center consistently appears on the other side (or a known upper-sideband signal shows its image more strongly than the real signal), the left/right channels are swapped relative to what RigControl Web expects as I vs Q. Toggle Swap I/Q in the panel settings — this is the same fix HDSDR uses for the G90's own "Swap IQ" option, and has the identical effect here.
The Noise Floor and Ceiling sliders control display contrast, the same as the other two spectrum sources. There is no calibrated absolute reference for this path (the G90's I/Q output level isn't documented to a precise spec, and any adapter's input gain further changes the effective level), so the defaults are starting points — adjust them against your own setup until the waterfall shows good contrast between the noise floor and real signals.
| Symptom | Likely cause | Fix |
|---|---|---|
| "No I/Q capture device selected" | No device chosen yet | Pick a device under Capture Device |
| "Configured I/Q capture device not found" | The saved device isn't currently connected/enumerated | Reconnect the USB adapter, then reselect it in the dropdown |
| "Audio engine not ready" | The bundled audio engine failed to load at startup | Check the startup log for the underlying audio-engine load error (same engine used for regular radio audio) |
| "Failed to open I/Q capture device" | The device doesn't support the selected sample rate, or is in use by another application | Try a lower sample rate, or close other apps using the device |
| Waterfall is flat/empty | Cable on Mic-In instead of Line-In, radio's I/Q output not actually active, or wrong device selected | Recheck wiring; confirm the correct capture device is selected |
| Signal appears mirrored / on the wrong side of center | Left/right channels don't match RigControl Web's I/Q convention | Toggle Swap I/Q |
Start RigControl Web with --debug-spectrum for a detailed trace of the I/Q capture pipeline:
From the command line:
./RigControl-Web-<version>.AppImage --debug-spectrumIn development:
npm run dev -- --debug-spectrumOutput appears in the terminal where you launched the app.
This source is generic by design — any radio exposing a baseband I/Q signal on a stereo jack should work — but it has so far only actually been validated end-to-end against the Xiegu G90. The default Noise Floor/Ceiling range and Swap I/Q behavior are reasoned from that radio's real output levels, not a calibrated reference for I/Q audio generally. If you try this with a different radio, expect to retune the floor/ceiling sliders and confirm the Swap I/Q default for your own hardware.