Why an SDR Shows a Waterfall but No Audio

[Pavel]

Short answer: A waterfall proves that samples reach the display; it does not prove the correct demodulator, channel position, filter width, squelch or audio output is selected. Work from the visible signal through the demodulator to the operating-system audio device in that order.

Before you start​

• Use one strong, known local signal for the first test. FM broadcast is convenient in many countries because the expected modulation and audio are easy to recognize.
• Write down the receiver model, antenna, operating system, application and sample rate. If the result changes, this record shows which variable actually changed.

Step-by-step method​

1. Step 1: Click directly on a known signal and confirm the active VFO or channel filter surrounds it. In software with several VFOs, make sure the audible channel—not only the center frequency marker—is selected.
2. Step 2: Choose the correct mode: WFM for broadcast FM, NFM for narrow voice, AM for AM broadcast or airband, USB or LSB for single-sideband. Set a filter width appropriate for that mode rather than leaving a previous narrow filter active.
3. Step 3: Open squelch completely and disable mute. Watch any audio-level meter while the signal is present; movement there with silence at the speakers usually means the wrong sink, muted application or operating-system routing.
4. Step 4: Select a real audio device, check per-application volume in Windows and test with headphones or another output. If using a decoder, first prove normal speaker audio before inserting a virtual audio cable.

Concrete example​

For an FM broadcast signal, use WFM and roughly 150 to 200 kHz bandwidth. For a narrow FM voice channel, try NFM with roughly 10 to 15 kHz. For AM airband voice, use AM around 6 to 10 kHz. These are starting ranges, not fixed rules for every service.

How to judge the result​

The fault is localized when the RF signal is inside the channel filter, the demodulator audio meter responds and the chosen output device plays sound. If the audio meter is flat, remain in the VFO, mode, filter and squelch layer instead of reinstalling USB drivers.

What to record​

• A useful first result is repeatable: the same station appears at the same frequency after restarting the application.
• Judge reception by signal-to-noise ratio and intelligible audio, not by how bright the entire waterfall looks.
• Change only one control at a time and wait several seconds before deciding whether it helped.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Increasing RF gain cannot repair a muted audio sink or the wrong demodulation mode; it may only overload the receiver and add more visible signals.

Final check​

Restart the receiver application and repeat the known-signal test with the saved settings. A beginner baseline is complete only when the same signal, mode and audio result can be reproduced without guessing.