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Audacity
2.3.1
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Audacity
2.3.1
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A two-pass effect to remove background noise. More...
A two-pass effect to remove background noise.
The first pass is done over just noise. For each windowed sample of the sound, we take a FFT and then statistics are tabulated for each frequency band - specifically the maximum level achieved by at least (n) sampling windows in a row, for various values of (n).
During the noise removal phase, we start by setting a gain control for each frequency band such that if the sound has exceeded the previously-determined threshold, the gain is set to 0, otherwise the gain is set lower (e.g. -18 dB), to suppress the noise. Then frequency-smoothing is applied so that a single frequency is never suppressed or boosted in isolation, and then time-smoothing is applied so that the gain for each frequency band moves slowly. Lookahead is employed; this effect is not designed for real-time but if it were, there would be a significant delay.
The gain controls are applied to the complex FFT of the signal, and then the inverse FFT is applied, followed by a Hanning window; the output signal is then pieced together using overlap/add of half the window size.
1.8.6