A spectrum filter is a general purpose audio filter similar to Parametric EQ, but with much greater control. Instead of using individual bands, the entire frequency spectrum is controlled using Shape Controls to adjust the gain. This allows many kinds of filters to be designed, such as lowpass, highpass, bandpass, bandstop, notch, peak, comb, and more. Filtering is performed in the frequency domain using Fast Fourier Transforms (FFTs).
A spectral analysis window displays a shape line and several other controls. The X and Y coordinates are updated when you click-and-drag a shape point. The X coordinate is the frequency in Hertz and the Y coordinate is the magnitude in decibels. The time of the spectral analysis shown is given in the Time box. Moving the time scroll bar, located below the analysis window, changes the graph to show the spectral analysis of a different part of the sound. Each channels is shown in a different colour.
Master gain sets the overall gain of the filter, which is equivalent to shifting the entire shape up or down.
Initially the shape line is horizontal at 0dB, which means that no changes in gain are made at any frequency. Alter the shape line up or down to increase or decrease the gain at a particular frequency. In technical terms, the shape line represents the frequency response function.
| Spectrum Filter Settings | ||
|---|---|---|
| Group | Setting | Description |
| Graph range | Min (dB) | Sets the lower range of the y axis of the graph. |
| Max (dB) | Sets the upper range of the y axis of the graph. Setting Min to -5 and Max to 5, for example, sets the graph to show a narrower range between -5dB to 5dB, allowing shape points to be set more precisely with the mouse within that range. | |
| Settings | FFT size | Sets the detail of the spectral analysis and the resolution of the filter. Higher settings provide a higher resolution, allowing the filter to follow the shape more accurately, with sharper cutoffs. When processing high sampling rate files, such as 88kHz or 192kHz, the FFT size must be set higher for the filter to follow the shape. Using too high a value may cause overshoot and oscillations in the gain (Gibbs phenomenon) |
| Overlap | Sets the amount of audio data to overlap from one calculation to the next. The lowest value gives the fastest processing and generally works well. | |
See Also: Equalizer, Noise Reduction, Parametric EQ, Presets