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What happened?

Audio crackling on SSB when NR2 is enabled

Labels: bug, DSP, audio, SSB, windows

What happened

When demodulating SSB, enabling NR2 introduces persistent crackling artifacts in the audio stream. The artifacts resemble discontinuities (short bursts/pops) rather than broadband distortion and are immediately removed when NR2 is disabled.

The issue appears isolated to the NR2 processing path:

• NR (baseline LMS-style NR) → no artifacts
• NR4 → no artifacts
• NR2 → reproducible crackling

Behavior is consistent across versions v0.8.12 and v0.8.15.

Environment

• AetherSDR version: v0.8.12 and v0.8.15
• OS: Windows 11 Home 22H2 (Build 22621.4317)
• CPU: Intel Core i7-13700HX 2.10 GHz (13th Gen)
• RAM: 32 GB (31.7 GB usable)
• Runtime: Latest Visual C++ redistributable installed
• Linux: Not yet tested — will follow up

Suggested fix / investigation areas

The symptom (impulsive crackling) suggests temporal discontinuities rather than steady-state DSP artifacts. Likely areas to investigate:

1. Buffering / block boundary issues

• Check for frame boundary discontinuities in NR2 processing:
  • Missing overlap-add / overlap-save handling
  • State reset between blocks
• Verify continuity of internal state (e.g., noise estimate, filter taps)

2. Numerical instability / denormals

• NR2 may involve spectral-domain or adaptive estimation:
  • Check for NaNs / inf / denormals propagating into output
  • Sudden gain spikes or zeroing could produce audible “ticks”

3. Gain / scaling discontinuities

• Inspect:
  • Per-frame gain normalization
  • Log/exp or MMSE-style estimators producing abrupt gain changes
• Compare with NR4 smoothing behavior (likely better temporal smoothing)

4. Threading / real-time constraints

• Possible audio underruns masked as DSP artifacts
• NR2 may be more CPU-intensive:
  • Check if processing exceeds buffer deadline
  • Look for contention in DSP/audio thread

5. AGC interaction (ordering)

• If NR2 is applied post-AGC, feedback interaction may amplify artifacts
• Validate:
  • NR2 pre/post AGC placement
  • Consistency with NR / NR4 pipeline

6. Implementation divergence vs NR/NR4

• Since NR and NR4 are clean:
  • Compare:
    • Windowing strategy
    • FFT size / hop size
    • Smoothing constants
• Look specifically for missing temporal smoothing in NR2 gain function

Additional notes

• Issue is 100% reproducible on SSB voice.
• Not observed on other NR modes under identical conditions.
• Sounds like sample discontinuities or short buffer underruns, not typical NR “musical noise”.

What did you expect?

NR2 should reduce noise without introducing discontinuities or non-stationary artifacts beyond those typically associated with aggressive DSP noise reduction.

While some artifacts are expected in NR algorithms, they should manifest as smoothing/warbling—not impulsive crackling.

Steps to reproduce

1. Launch AetherSDR and connect to a radio.
2. Tune to a stable SSB signal (voice preferred for clarity).
3. Enable NR2.
4. Observe crackling artifacts in the audio.
5. Disable NR2 → artifacts immediately disappear.
6. Enable NR or NR4 → audio remains clean.

Radio model & firmware

Flex 6600 Firmwar 4.1.5

Linux distro & Qt version

No response