Spectral Selection

Contents

 * 1) Making a spectral selection
 * 2) Adjusting a spectral selection
 * 3) Toggling a spectral selection on or off
 * 4) Spectral editing
 * 5) Example
 * 6) Limitations

Making a spectral selection
Here is a track with a few seconds of speech shown in waveform view:
 * [[Image:Spectral_01.png|link=]]

To define a time range change the track to Spectral Selection view, hover at a vertical position that you want to be the approximate center frequency to act on then click and drag a selection horizontally. A horizontal line appears beside the I-Beam mouse pointer that defines the center frequency.

Drag vertically (with or without continuing to drag horizontally) to define the bandwidth (range of frequencies) to be acted on. A "box" containing a combined frequency and time range is now drawn in a yellowish tint as shown below:
 * [[Image:Spectral_02.png|link=]]

A spectral selection may begin or end either at 0 Hz or at the maximum allowable frequency for the track (the Nyquist frequency), in which case no center frequency will be shown. In the illustration below the frequency selection goes from 0 Hz to about 5000 Hz:
 * [[Image:Spectral_02a.png|link=]]

Adjusting a spectral selection
When you hover the mouse pointer over the center frequency line it changes to a double triangle to show that the center frequency can be dragged. When you click and drag to move the center frequency and the current frequency range to a new position, the center frequency will snap to frequency peaks. To move the center frequency smoothly (without snapping to frequency peaks) hold down the key then left click and drag.
 * [[Image:Spectral_03.png|link=]]

To adjust the bandwidth of the selection hover the mouse over the upper or lower boundary until the pointer changes to a double triangle then left click and drag. This moves the upper and lower frequency boundaries so they remain equally spaced around the center frequency (thus not changing the center frequency).
 * [[Image:Spectral_03a.png|link=]]

To adjust the upper or lower boundary while holding the opposite boundary constant hold down the key then hover the mouse near the upper or lower boundary until the pointer changes to a single triangle then click and drag (this will change the center frequency).
 * [[Image:Spectral_03b.png|link=]]

Toggling the spectral selection on and off
Press the shortcut key to change from the last selected spectral selection (one that has at least the lower or upper boundary defined) to one where the lower boundary is 0 Hz and the upper boundary is the Nyquist frequency of the track. In this case there is no longer a spectral selection (all frequencies are selected which is the same as making a time selection in waveform view). Press the key again to restore the previous upper and lower boundaries. This command works whether in a Spectral Selection view or not.

Spectral edit multi tool
Use this to greatly attenuate or remove part of the frequency range in the spectral selection, for example to repair damaged audio. Depending on how the track is displayed and what spectral selection has been made, this effect will do one of the following:
 * When the track is not in Spectral Selection or Spectral Selection log(f) view an error message will be displayed.
 * When the spectral selection has a center frequency and an upper and lower boundary this effect performs as a notch filter with the center frequency defined by the center frequency of the spectral selection and the width defined by the upper and lower boundaries of the spectral selection.
 * When the spectral selection begins at 0 Hz this effect performs a high pass filter with a rolloff of 12 dB/octave and with the cutoff frequency defined by the upper boundary of the spectral selection.
 * When the spectral selection ends at the Nyquist frequency of the track this effect performs a low pass filter with a rolloff of 12 dB/octave and with the cutoff frequency defined by the lower boundary of the spectral selection.
 * When the spectral selection begins at zero Hz and ends at the Nyquist frequency the error message "Please select frequencies" will be displayed.

You can see some specific examples of this effect on the Spectral edit multi tool page.

Spectral edit parametric EQ

 * [[Image:Spectral edit parametric EQ.png|link=]]

Use this like an Equalizer to modify the tone quality, boosting or reducing the amplitude of the selected frequency band using the Gain control. As with an Equalizer, frequencies outside the selected band may be boosted or reduced, but less so than those within the frequency band.

Depending on how the track is displayed and what spectral selection has been made, this effect will do one of the following:
 * When the track is not in Spectral Selection or Spectral Selection log(f) view an error message will be displayed.
 * When the spectral selection has a center frequency and an upper and lower boundary this effect performs a band cut or band boost according to the value entered in the "Gain (dB)" control. The center frequency is defined by the center frequency of the spectral selection, the bandwidth is defined by the upper and lower boundaries of the spectral selection.
 * When either the upper or lower frequency boundary is undefined, an error message will be displayed to that effect. If the lower frequency boundary is 0 Hz there is also no center frequency or bandwidth and thus the effect will do nothing.

You can see some specific examples of this effect on the Spectral edit parametric EQ page.

Spectral edit shelves

 * [[Image:Spectral edit shelves.png|link=]]

A shelf filter curve is flat at both ends rather than flat only at the frequencies being passed. Use these filters when you don't want to reduce the lowest bass or highest treble frequencies in the spectral selection as much as you would with a standard high pass or low pass filter, or if you actually want to boost either the low or high frequencies respectively. Depending on how the track is displayed and what spectral selection has been made, this effect will do one of the following:
 * When the track is not in Spectral Selection or Spectral Selection log(f) view an error message will be displayed.
 * When the spectral selection begins at 0 Hz or the start is undefined, this effect applies a low frequency shelving filter with the half-gain frequency (the mid-point of the gain section of the curve) defined by the upper boundary of the spectral selection. The gain control sets the amount of low-frequency boost or cut. This is similar to adjusting the bass control on a stereo.
 * When the spectral selection ends at the Nyquist frequency of the track or the end is undefined, this effect applies a high frequency shelving filter with the half-gain frequency defined by the lower frequency of the spectral selection. The gain control sets the amount of high-frequency boost or cut. This is similar to adjusting the treble control on a stereo.
 * When the spectral selection has a center frequency and upper and lower frequency boundaries this effect applies both low- and high-frequency shelving filters. In this case the half-gain frequency of the low shelving filter is defined by the lower frequency boundary of the spectral selection and the half-gain frequency of the high shelving filter is defined by the upper frequency boundary of the spectral selection. The gain control sets the amount of boost or cut between the two frequency boundaries.
 * When the spectral selection begins at zero Hz and ends at the Nyquist frequency or if the start and end of the selection are undefined, the error message "Please select frequencies" will be displayed.

You can see some specific examples of this effect on the Spectral edit shelves page.

Limitations
The permitted upper frequency of the spectral selection is determined by the Project Rate and not by the track sample rate. Therefore if a project includes a track that has a lower sample rate than the project rate, Spectral Selection Toolbar could display an "illegal" frequency selection above the Nyquist frequency for the track at the lower project rate. Audacity will disregard this "illegal" selection when running Spectral edit effects.