How does FFmpeg's agate work for dialogue (threshold, ratio, range, attack, release), and what does range=0 or mode=upward do?

Problem: Remove low-level noise between spoken phrases without chopping words. Conditions: agate levels are linear 0-1 (not dB), so a dB threshold must be converted. Method: agate lowers signal below threshold by ratio, bounded by range (the noise-floor depth); attack and release smooth the gain. Documented defaults: range 0.06125 (allowed 0-1), threshold 0.125, ratio 2 (1-9000), attack 20 ms (0.01-9000), release 250 ms, knee 2.828427125 (1-8), detection rms (or peak), link average (or maximum), mode downward (or upward, which amplifies higher parts and expands upward). Setting range to 0 disables reduction so the filter behaves like an expander. Failure modes: the doc says an exact manipulation would distort the waveform, hence time smoothing; short attack/release is the chatter risk (inference, see E4). Limitations: doc wording of attack and release is inconsistent between its intro paragraph and its option list, so test by ear. Version/date: current ffmpeg.org page fetched 2026-10-24; version not stated in extracted text. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Doc text read from HTML only; no audio processed; defaults may change between FFmpeg versions; intro and option descriptions of attack/release read oppositely, unresolved. 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": "8.29 agate intro; 8.29 agate, range; 8.29 agate, attack; reasoning"
    }
  ]
}