DSP Noise Mitigation

DSP Noise Mitigation

Flex Radios provides access to built-in noise mitigation features, plus a client-side NR2 spectral noise reduction algorithm through AetherSDR.

Quick Reference

Feature	Full Name	Target Noise	Runs On
NB	Noise Blanker	Impulse noise (ignition, switching)	All radios
NR	Noise Reduction	General broadband noise	All radios
NR2	Spectral Noise Reduction	Complex noise, weak signals	Client (PC)
ANF	Auto Notch Filter	Single persistent tone	All radios
NRL	Noise Reduction Leaky	Random band noise, background hiss	All radios
NRS	Noise Reduction Speech	Background noise during voice	BigBend + Aurora
RNN	Recurrent Neural Network	Complex/varying noise (AI-based)	BigBend + Aurora
NRF	Noise Reduction Filter	Steady broadband noise	BigBend + Aurora
ANFL	Adaptive Notch Filter (Leaky)	Steady tones, power-line hum	All radios
ANFT	Adaptive Notch Filter (FFT)	Multiple narrowband tones (up to 5)	All radios
APF	Audio Peaking Filter	CW signal enhancement	All radios (CW mode only)
WNB	Wideband Noise Blanker	Wideband impulse noise	All radios

Radio-Side Filters (FPGA)

These run on the radio's FPGA — zero CPU cost to your PC. All use preset, optimized parameters with no user adjustments required — just toggle on/off.

NB — Noise Blanker

Blanks short impulse noise (ignition interference, switching power supplies). Has an adjustable level (0-100) in the DSP side panel.

NR — Noise Reduction

General-purpose noise reduction. Adjustable level (0-100). Good starting point for noisy bands.

ANF — Auto Notch Filter

Removes a single persistent tone from the passband. Adjustable level (0-100). Best for a dominant interfering carrier.

NRL — Noise Reduction Leaky

Purpose: Reduce random band noise and background hiss.

Adaptive time-domain "Leaky LMS" filter that preserves correlated signals (voice, CW) while removing uncorrelated noise. Best for daily operation when signals are buried in static. Avoid when signals are already clean — may introduce slight "watery" audio if unnecessary.

Available on all Flex Radios.

ANFL — Adaptive Notch Filter (Leaky)

Purpose: Remove steady tones or power-line hum.

An adaptive filter that detects and cancels correlated tone interference. Best for hum, steady carriers, or single persistent tones. Avoid when no hum/tone is present — could partially notch the desired signal.

Available on all Flex Radios.

ANFT — Adaptive Notch Filter (FFT)

Purpose: Suppress up to five narrowband tones in the passband (≥ –110 dB).

FFT-based spectral notch filter that identifies and removes stable tone noise. Best for power supply hum, transformer buzz, grounding-related tones, or spurs.

Available on all Flex Radios.

NRF — Noise Reduction Filter

Purpose: Reduce steady background noise using frequency-domain processing.

Spectral subtraction estimates and removes broadband noise from each frequency bin. Best for constant hiss, fan noise, or environmental hum.

Available on FLEX-8000 (BigBend) and Aurora series only.

NRS — Noise Reduction Speech

Purpose: Improve speech clarity by lowering noise most aggressively between words.

Spectral subtraction combined with Voice Activity Detection (VAD). Best for SSB/AM voice operation in consistent noise environments.

Available on FLEX-8000 (BigBend) and Aurora series only.

RNN — Recurrent Neural Network Suppression

Purpose: AI-based removal of complex or varying noise while keeping speech natural.

Deep-learning model that separates speech from noise across time and frequency. Best for mixed or changing noise sources — fans, equipment, electrical noise, band noise.

Available on FLEX-8000 (BigBend) and Aurora series only.

APF — Audio Peaking Filter

Narrow bandpass filter centered on the CW pitch frequency. Enhances CW signals by attenuating everything outside the peak. Adjustable level (0-100). Only available in CW mode.

WNB — Wideband Noise Blanker

Wideband impulse noise blanker. Controlled via the ANT sub-menu on the left sidebar. Has an adjustable level (0-100) and appears as a "WNB +NdB" indicator on the FFT display when active.

Client-Side Filter

NR2 — Spectral Noise Reduction

Purpose: Advanced noise reduction using the Ephraim-Malah MMSE Log-Spectral Amplitude estimator with OSMS noise floor tracking.

Contributed by @EI6JGB

Unlike the radio-side filters, NR2 runs on your PC's CPU. It processes the demodulated 24kHz audio using a 256-point FFT with 50% overlap, estimates the noise floor per frequency bin, and applies a spectral gain that suppresses noise while preserving speech.

How it differs from NR: The radio's NR uses a time-domain adaptive filter. NR2 uses frequency-domain spectral subtraction with decision-directed smoothing — a fundamentally different algorithm that can catch noise the radio's NR misses, especially on very weak signals.

Key features:

• Ephraim-Malah MMSE-LSA gain computation
• OSMS (Optimally Modified Log-Spectral Amplitude) noise floor tracking
• Speech presence probability weighting
• Anti-musical-noise temporal gain smoothing
• 1-second startup ramp for smooth convergence
• CPU cost: ~1-2% of one core

When to use:

• Weak signals where the radio's NR isn't enough
• Can be stacked with radio-side NR for maximum noise reduction
• Effective on CW and SSB
• Adapts automatically when changing bands (~1 second convergence)

Enabling NR2: The NR button in the RX applet cycles through three states: Off → NR → NR2 → Off. Also available in the VFO widget DSP panel and the left sidebar DSP sub-menu.

FFTW3: NR2 optionally uses the FFTW3 library for optimized FFTs. Falls back to a built-in radix-2 FFT if FFTW3 is not installed. See NR2 Noise Reduction for details.

Usage Guidelines

• Only enable a filter when its target noise type is present
• Some filters can be used together, but excessive stacking may change audio tone or introduce artifacts
• As a rule of thumb:
  • NRL → Random noise, background hiss
  • ANFL / ANFT → Tones, hum, spurs
  • NRF / NRS / RNN → Voice clarity enhancement (BigBend/Aurora only)
  • NR2 → Weak signal extraction, can stack with any radio-side filter
  • NB / WNB → Impulse noise (ignition, switching)
  • APF → CW signal peaking (CW mode only)

Mode-Dependent Availability

Not all DSP features are shown in every mode:

Mode	Available	Hidden
USB/LSB	NR, NB, ANF, NRL, NRS, RNN, NRF, ANFL, ANFT	APF
CW	NR, NB, APF, NRL, NRS, NRF	ANF, RNN, ANFL, ANFT
RTTY	NR, NB, NRL, NRS, RNN, NRF	ANF, ANFL, ANFT
DIGU/DIGL	NR, NB, NRL, NRS, RNN, NRF	ANF, ANFL, ANFT
FM/NFM	Hidden (OPT tab replaces DSP)	All