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Tuner and Live
GP-SDR separates two frequencies:
- Hardware center is where the SDR's IQ passband is physically centered.
- Listen VFO is the software-demodulated frequency inside that passband.
Leaving Hardware center blank lets GP-SDR place the VFO away from the receiver's DC spike. You can center a HackRF at 450 MHz while listening at 452 MHz without retuning the hardware, provided 452 MHz remains inside the captured bandwidth. Clicking the spectrum or waterfall moves the software VFO when possible.
- Auto analyzes local IQ and selects AM, NFM, or WFM.
- AM is typical for civil aviation and broadcast AM.
- NFM is typical for land-mobile, GMRS, MURS, marine, and many conventional voice channels.
- WFM is used for broadcast FM audio.
- Bandwidth changes the software channel filter, not the SDR's entire capture width.
- Sample rate controls the instantaneous RF width and USB/CPU load.
Changes apply immediately.
- Gain is the general receiver setting used by supported devices.
- LNA and VGA expose HackRF gain stages.
- RF amp enables the HackRF input amplifier; use it sparingly.
- Antenna power controls supported bias-tee output. Enable it only for an antenna device designed for the receiver's voltage.
- AGC automatically adjusts audio level; it cannot repair RF overload.
If the display is solid with activity, turn the RF amplifier off and reduce gain before changing detection thresholds.
- DC removal suppresses the center spike common to zero-IF receivers.
- Q balance compensates I/Q amplitude mismatch.
- IQ phase compensates quadrature phase error.
- Swap I/Q corrects reversed spectral orientation on affected sources.
- PPM correction compensates oscillator frequency error.
- Apply saved calibration loads settings saved for the selected device.
Use Auto calibrate only on a strong, known reference. Review its result, then save it per device. Poor reference selection can make tuning worse.
The signal indicator compares channel energy with the measured noise floor. Squelch is a margin above that floor, not an absolute guarantee of speech.
- Enable Open squelch while diagnosing audio.
- Raise squelch until idle noise closes without clipping the start of calls.
- Voice noise reduction is a low-latency cleanup for speech.
- Strong removes more stationary noise but can make marginal speech sound processed; use Off when evaluating raw decoder or RF quality.
The spectrum shows current power over frequency; the waterfall adds time with newest samples at the top. Hover for frequency/power and click to move the VFO. Settings provides frame rate, quality/FFT detail, smoothing, peak hold, and waterfall history controls. Reduce these before lowering receiver sample rate when the interface is sluggish but audio is otherwise stable.
The top bar controls whole-app mute and volume. Live and decoder mixers add per-channel or per-talkgroup mute, solo, volume, pan, and priority ducking. If spectrum activity is present but audio is silent, check master mute, the selected row, solo state, squelch, mode, output device, and P25 JMBE status.