User Guide¶

Tuning¶

Frequency Entry¶

Press Ctrl+Q to open the LO frequency entry dialog. Press Q to hear the current LO frequency, or O to hear the listening (demod) frequency. Press Ctrl+O to enter a listening frequency directly — AccessDR calculates the offset from the LO automatically.

Enter a frequency in one of these formats:

• MHz (most common): 98.1 or 98.1 MHz
• kHz: values between 30,000 and 30,000,000 are treated as kHz
• Hz: values above 30,000,000 are treated as Hz

Press Enter to confirm or Esc to cancel.

Step Tuning¶

Use Up and Down to tune by the current step size. Hold Shift to tune by 10x the step size.

Press S to cycle through step sizes:

Pause / Resume¶

Press Space to pause the radio — IQ capture and audio stop, but the last spectrum snapshot remains visible and navigable. Cursor movement, probe tone, peaks, zoom and all other spectrum features continue to work on the frozen data. Press Space again to resume reception. Press F2 while paused to fully stop the radio and release the device.

Band Presets¶

Open the Radio > Bands menu (Alt+R) to jump to a preset band. Each preset tunes to the band's centre frequency and sets the appropriate demodulation mode.

Available bands:

Demodulation Modes¶

Press M to enter mode selection, then press the mode letter:

Each mode has selectable filter bandwidths in the main window's BW dropdown.

WFM — Wideband FM¶

The standard mode for FM broadcast radio (87.5–108 MHz). Uses a 200 kHz channel and includes stereo decoding — AccessDR automatically detects and announces stereo or mono. This is the default mode and the best starting point for new users. Typical bandwidth: 150–200 kHz.

WFM includes several sub-features configurable via Options > WFM Settings (Ctrl+W):

• De-emphasis — compensates for the FM pre-emphasis curve. Set to Auto (detects your region), 50 us (Europe/Asia), or 75 us (Americas/South Korea). Wrong de-emphasis makes audio sound too bright or too dull.
• Stereo mode — Auto detects the 19 kHz pilot and blends between mono and stereo based on signal quality. Force Mono disables stereo decoding (quieter on weak signals). Force Stereo always decodes stereo regardless of signal quality.
• Hi-blend — automatically reduces treble on weak stereo signals to cut FM hiss. Disable if you prefer full-bandwidth audio regardless of signal strength.
• RDS decoding — extracts station name (PS), radio text (RT), and program type from the RDS subcarrier at 57 kHz. When enabled, the station name is announced automatically on tune. Press I to hear RDS info. Disable to save a small amount of CPU.

NFM — Narrowband FM¶

Used by most two-way radio systems: amateur (ham) radio repeaters, PMR446 walkie-talkies, marine VHF, NOAA weather radio, taxi and business radios, and public safety communications. NFM channels are much narrower than broadcast FM (12.5 or 25 kHz), so you will hear a single voice conversation rather than a music station. Set the step size to 12.5 or 25 kHz when scanning for NFM signals.

NFM includes CTCSS tone detection — when a repeater or radio system uses a sub-audible tone (67.0–254.1 Hz) for access control, AccessDR detects and reports the tone. Press I to hear the detected CTCSS tone along with signal information.

A CTCSS notch filter automatically removes the detected sub-audible tone from the audio output, eliminating the low-frequency hum that can be distracting on speakers or headphones with good bass response. The notch filter is enabled by default and can be toggled in Options > NFM Settings (Ctrl+Shift+N).

AM — Amplitude Modulation¶

Used for AM broadcast radio (530 kHz–1.71 MHz), aviation communications (108–137 MHz air band), and shortwave broadcasts. AM is simpler than FM and works well over long distances, which is why it is used for aircraft and international broadcasting. Typical bandwidth: 6–10 kHz.

USB — Upper Sideband¶

A form of single sideband (SSB) modulation used by amateur (ham) radio operators on HF frequencies above 10 MHz and on VHF/UHF. SSB transmits only one half of the AM signal, making it more power-efficient and spectrally compact. Conversations sound distorted if you are slightly off frequency — tune in 100 Hz or 10 Hz steps until voices sound natural. Typical bandwidth: 2.7 kHz.

LSB — Lower Sideband¶

The mirror image of USB, used by amateur radio operators on HF frequencies below 10 MHz (e.g. the 80m and 40m bands). The convention is: LSB below 10 MHz, USB above 10 MHz. Tuning technique is the same as USB. Typical bandwidth: 2.7 kHz.

CW — Continuous Wave (Morse Code)¶

Used for Morse code (CW) transmissions. CW uses an extremely narrow filter (250–500 Hz) centred on a single tone, so you hear individual dits and dahs clearly while rejecting nearby signals. Common on amateur HF bands, especially during contests and low-power (QRP) operation. Tune slowly in 10 Hz or 1 Hz steps to centre the tone.

DSB — Double Sideband¶

Similar to AM but without carrier filtering. Useful for receiving AM signals that don't quite fit the standard AM demodulator, or for experimenting. In practice, most users will use AM mode instead. Typical bandwidth: 6–10 kHz.

Volume, Mute, and Squelch¶

Volume and squelch sliders are also available in the main window. The mute toggle button is labelled "Mute (M)" in the UI.

The squelch silences audio when the signal drops below a threshold. This is useful for scanning or monitoring — you only hear audio when someone is transmitting. Set it just above the noise level so that only real signals open the squelch. A value of -80 dBm is a good starting point; raise it if you hear too much noise.

Signal Information¶

Press I at any time to hear a spoken status report including:

• Signal strength in dBFS with S-meter reading (S0–S9+30)
• Stereo or Mono (in WFM mode)
• RDS station name (in WFM mode, when available)
• CTCSS tone frequency (in NFM mode, when detected)
• Squelch state (open or closed)
• Mute state
• Demod offset and listening frequency (when a software VFO offset is active)

RF Settings¶

Open the RF Settings dialog with Ctrl+R to configure hardware parameters.

SDR Device¶

If you have multiple RTL-SDR dongles connected, select which one to use here. Most users will only have one device listed.

Sample Rate¶

The rate at which the SDR digitises radio signals, measured in samples per second. Higher rates capture a wider slice of spectrum but require more CPU. The default of 2,400 kHz (2.4 MSPS) is a good balance — it captures ±1.2 MHz around the tuned frequency and works well on most computers. Lower rates like 1,024 kHz reduce CPU usage but narrow the visible spectrum.

RF Gain¶

Controls how much the tuner amplifies the incoming signal before digitising it. When the radio is running, the dropdown shows the exact gain steps supported by your tuner hardware (e.g. 0.0 dB, 0.9 dB, 1.4 dB, ... up to 49.6 dB for the R820T tuner).

• Too low: weak signals disappear into the noise floor — you won't hear anything.
• Too high: strong signals overload the ADC, causing distortion and spurious signals that aren't really there.
• Starting point: 30–40 dB is usually good for FM broadcast. Lower gains (15–25 dB) work better for strong local signals. Higher gains (40+ dB) help with weak or distant signals.

When RTL AGC is enabled, the gain dropdown is disabled because gain is controlled automatically.

PPM Correction¶

Compensates for the frequency error in your dongle's crystal oscillator. Most RTL-SDR dongles are off by a few parts per million (PPM), which means the displayed frequency doesn't exactly match the real frequency. A typical value is between -50 and +50 PPM. If stations seem slightly off-frequency, adjust this value until they are centred. You can calibrate using a known signal like a local FM station.

RTL AGC¶

Enables the RTL2832U chip's automatic gain control. When turned on, the hardware adjusts gain dynamically to keep the signal level optimal. This is convenient for scanning across bands with very different signal strengths, but can introduce pumping (gain changes causing volume swings). For manual control and best results on a single frequency, leave AGC off and set the gain manually.

Offset Tuning¶

Eliminates the DC spike — a narrow spurious signal that appears at the exact centre of the spectrum. This spike is an artefact of the direct-conversion receiver architecture and is not a real signal. Offset tuning shifts the tuner's local oscillator slightly so the spike falls outside the passband. Enable this if you see a persistent signal at the centre frequency that doesn't go away when you change gain. Some tuners or drivers may not support this feature.

IF Bandwidth¶

Sets the hardware intermediate frequency (IF) filter bandwidth inside the R820T tuner. This is the analogue filter applied before the signal is digitised.

• Auto (default): AccessDR selects a suitable bandwidth based on the current demodulation mode — 300 kHz for WFM, 100 kHz for NFM/AM/SSB/CW.
• Manual values (250 kHz – 2 MHz): override the automatic selection. A narrower IF bandwidth rejects out-of-band interference and can improve reception of weak signals next to strong ones, but setting it too narrow will cut off the signal you are trying to receive.

In most cases, Auto is the best choice. Manual IF bandwidth is useful when you have a strong interfering signal near the frequency you are listening to.

Noise Blanker¶

The noise blanker suppresses short impulse noise (electrical interference, ignition noise, power line clicks) from the raw IQ signal before demodulation. Enable it in Options > RF Settings (Ctrl+R).

• Threshold controls sensitivity — a lower value blanks more aggressively. The default of 5.0 means any sample exceeding 5x the median signal magnitude is treated as an impulse. Raise the threshold if the blanker clips normal signal peaks; lower it if impulse noise persists.
• Blanked samples are replaced by linear interpolation from neighbouring clean samples, avoiding the clicks that hard zeroing would produce.

WFM Settings¶

Open the WFM Settings dialog with Ctrl+W to configure broadcast FM-specific parameters. Changes take effect immediately if you are currently receiving in WFM mode.

See the WFM mode description above for details on each setting.

Spectrum¶

The spectrum display shows signal power across the SDR's bandwidth. AccessDR provides multiple ways to explore the spectrum: sonification sweeps, an interactive cursor with probe tone, zoom, and peak detection. All spectrum features work on paused (frozen) data as well as live data.

Shortcuts¶

Waterfall Display¶

Below the line graph, a waterfall (spectrogram) scrolls vertically with the newest data at the top. Each horizontal line represents one FFT snapshot, with colour encoding signal power — brighter colours mean stronger signals. This makes it easy to spot intermittent transmissions, drifting signals, and patterns over time.

The waterfall uses colour-vision-deficiency-safe colour schemes. Open Options > Spectrum Settings (Ctrl+S) to choose between:

• Viridis — perceptually uniform with monotonic luminance, safe for all forms of colour blindness
• Magma — high-contrast dark-to-bright scheme
• Grayscale — pure black-to-white, universally accessible

Adjust the dB floor (brightness) and dB ceiling (contrast) to control which signals are visible. A lower floor reveals weaker signals; a narrower range between floor and ceiling increases contrast.

Sonification¶

Sonification converts the FFT spectrum into audio, letting you "hear" the spectrum:

• The spectrum is swept from left to right using stereo panning — left ear = low end, right ear = high end
• Pitch encodes signal power — stronger signals produce a higher pitch, weak signals a low pitch
• Signals below the noise floor are silent, so only real signals are audible

Press F5 for a snapshot sweep — a single left-to-right pass. Press Ctrl+F5 to toggle continuous sweep — repeats until you press Ctrl+F5 again. Sonification activates automatically when you start a sweep.

Zoom¶

Zoom narrows the visible and sonified range so closely-spaced signals are easier to distinguish. Press ++=++ to zoom in, ++-++ to zoom out, and Backspace to reset. Press G to hear the current range.

Zoom levels: 1x (full) → 2x → 4x → 8x → 16x → 32x. At 2.4 MSPS, 32x zoom gives a ~75 kHz span — still useful for NFM or AM signals.

The visual spectrum panel always shows the full bandwidth, with yellow dashed lines marking the zoom boundaries and dimmed regions outside the zoom.

Cursor and Probe Tone¶

The spectrum cursor lets you explore specific points in the spectrum interactively.

Probe tone: Hold Ctrl to hear a continuous tone at the cursor's position. Pitch encodes signal power (same mapping as sweep sonification) and stereo pan encodes frequency position (left = low, right = high). While Ctrl is held, press Left / Right to move through the spectrum. Release the arrows to stop moving — the tone continues. Release Ctrl to stop the tone entirely.

Stepping: Press Left / Right without Ctrl to step the cursor one position and hear the frequency, power, and S-meter reading announced via speech. Press T to re-announce the current cursor position at any time.

Peaks: Press F to hear the top spectrum peaks announced as frequency and power level. The number of peaks reported is configurable in the Spectrum Settings dialog (Ctrl+S).

You can also click on the spectrum panel with the mouse to jump the cursor to that position. The cursor respects zoom — when zoomed in, the range narrows to the visible spectrum.

Software VFO Offset¶

By default, the demodulator processes the signal at the hardware LO (centre frequency). Press Shift+C to enable "demod follows cursor" mode — moving the cursor also shifts the demodulator to listen at the cursor's frequency. This works like clicking on a waterfall in HDSDR or SDR++ to move a software VFO.

The offset is applied by a software mixer in the DSP chain. The hardware LO does not move, so the full spectrum remains visible and the probe tone works normally.

Press C to reset the cursor and offset to centre. Press Ctrl+T to retune the hardware LO to the cursor and clear the offset. The offset is clamped to ±120 kHz (half the baseband rate). A yellow dashed "D" marker appears on the spectrum panel to show where the demodulator is listening. Press I to hear the current offset and listening frequency.

Sonification Settings¶

Open the Spectrum Settings dialog with Ctrl+S to adjust:

• Weak signal pitch — the pitch for signals at the noise floor (default 200 Hz)
• Strong signal pitch — the pitch for the strongest signals (default 4000 Hz)
• Sweep speed — how long one full left-to-right sweep takes (default 5 seconds; slower speeds give more time to distinguish closely-spaced signals)

Scanner¶

The scanner automatically steps through a frequency range, pausing on active signals.

1. Open the Scanner dialog with Ctrl+N
2. Set the start frequency, stop frequency, and step size
3. Set the squelch threshold — signals above this level will be flagged
4. Start the scan

Scanner Controls¶

When a signal is found, the scanner pauses briefly so you can listen, then continues.

Bookmarks¶

Save and recall favourite frequencies:

1. Open the Bookmarks dialog with Ctrl+B
2. Save the current frequency and mode as a bookmark
3. Load a bookmark to tune to it instantly

Bookmarks are stored in a JSON file and persist across sessions.