How does FFmpeg's afftdn denoiser work (noise floor, reduction, profile capture), and what artefact controls does it have?

Problem: remove steady noise (fan, hiss) from narration in a scripted pipeline. Conditions: FFmpeg with the afftdn filter (FFmpeg filter docs, fetched 23 Oct 2026). Method: afftdn denoises with an FFT. Key options: noise_reduction nr in dB (0.01-97, default 12); noise_floor nf in dB (-80 to -20, default -50); noise_type (white, vinyl, shellac, custom); track_noise tn (adapts the noise floor over time); output_mode (output, noise, input) where 'noise' passes only the removed noise so you can listen to what is being taken out; sample_noise sn start/stop captures a noise profile from a stretch of the input (docs example captures the first 0.4 s: asendcmd=0.0 afftdn sn start,asendcmd=0.4 afftdn sn stop,afftdn=nr=20:nf=-40); gain_smooth gs (0-50, default 0) smooths gains across frequency 'useful to reduce random music noise artefacts'. Docs examples use nr=10, nf=-40. Use (inference): record 1-2 s of room tone at the start of each take for the profile, start with nr near 6-12 dB and raise only while listening to output_mode=noise for speech content leaking into the removed signal. Failure modes: watery or 'musical noise' artefacts at high nr; speech dulled; a wrong nf makes the filter treat speech as noise. Limitations: docs give option ranges but no quality guidance; I did not run FFmpeg. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Starting values and workflow are my inference; the docs give option semantics only. Tool version of the docs page is current-release, undated. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://ffmpeg.org/ffmpeg-filters.html",
      "note": "8.23 afftdn; 8.23 afftdn options; 8.23 afftdn options; 8.23 afftdn options"
    }
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}