Click Removal (under the Effect Menu) is designed to remove individual clicks on audio tracks and is especially suited to declicking recordings made from vinyl records, without damaging the rest of the audio. The tool works by looking for short, abrupt discontinuities (known as "spikes") in the waveform, typical of those produced by a click on a record. Click Removal then interpolates the samples either side of the click to reconstruct the waveform. However, it's common sense not to rely exclusively on digital sound processing to clean up noisy records. Where possible, have the records professionally cleaned, or use your own cleaning methods.
First of all, select the audio to which you want to apply Click Removal. You can select all of a track by clicking on its Track Panel (where the mute/solo buttons are), or Edit > Select > All or CTRL + A (CMD + A on a Mac) will always select all the audio on screen. You can select part of a track by clicking in the track and dragging to left or right with your mouse, or by holding down SHIFT and the left or right arrow keys.
To use Click Removal, move the "threshold" slider to adjust how sensitive the click detection is, and the "spike width" slider to adjust the length of spiked audio to be considered as a click. Then click the "Remove Clicks" button. Softer clicks may require you to move the threshold slider further to left. Broader clicks may require you to move the spike width slider further to right. Very soft and rapid light ticks that sound like static electricity and which are typical of vinyl (even though the pressing is often the cause rather than a static charge) won't be effectively removed with Click Removal. For this type of noise you should select an area of the vinyl that contains only the noise (no music or speech) and use the Noise Removal tool under the same Effect menu - see Noise Removal.
How it works
The algorithm consists of two parts. The first part, click detection, works by measuring the amplitude of the sound in each small piece of the selected sound, and comparing it to the average amplitude in surrounding pieces. Short pieces of sound that are much louder than the region before or after might be spikes. (The threshold slider bar sets the square of the required ratio between the amplitudes inside and outside the loud section). Pieces of extra-loud sound that are shorter than the max spike width (measured in milliseconds) are marked for removal. This technique works well for vinyl record pops and clicks because it catches oscillations (not just single spikes). Electronic spikes typically drive the speaker in just one direction, but LP pops are harder to find because they often contain oscillation as the stylus and cartridge settle back down after the initial hit of the dust mote, groove gouge, or whatever.
The second part of the algorithm, click removal, replaces the marked segment of sound. This part of the algorithm is pretty simple minded -- it just smoothly moves the speaker from the position before the pop to the position after the pop. This has the effect of creating a very short, hopefully not-very-noticeable gap in the track where before there was a loud click.
Silencing and Draw Tool
Sometimes an even better result can be obtained by zooming in (CTRL + 1) to near sample level and either silencing the click (Edit > Silence), or using the Draw Tool to smooth out the contours of the samples and so attenuate the click. Remember, the click will be visible as a "spike" in the waveform. Most discrete clicks up to five milliseconds long can actually be simply silenced without leaving an audible gap in the sound, although many spread wider that that. If the click is not suitable for silencing, try using the Draw Tool by clicking the pencil icon top left of the Audacity screen, or press F3 on your keyboard. The mouse cursor will change to a pencil while over the audio track. You may need to zoom in a little further to use the tool. Simply click in the track at the point you wish the sample to be redrawn to, and wait for the samples to be rejoined together. Alternatively, click in the area of track where the line of samples is not smooth and hold down ALT on your keyboard. The cursor will now change to a brush. Each mouse click will progressively redraw the samples in the area so as to smooth out their contour. When clicking has no further effect, the samples are as smoothly drawn as possible.
Silencing highly zoomed areas or redrawing samples can get tedious even over a relatively short stretch of audio. Here are a few other tricks you can try if Click Removal did not help as much as you hoped.
Analyse the area with Analyze > Plot Spectrum to see if any spikes are concentrated in particular frequencies and then use Equalization under the Effect menu to reduce the volume of those frequencies. You can do this more precisely with the Nyquist Notch Filter plugin: http://www.shellworld.net/~davidsky/notch.zip
To install new plugins, unzip them into the Plug-Ins folder inside the Audacity installation folder. On Windows computers, this is usually under "Program Files." On Mac OS X, it is usually under "Applications." The plugins will be available after restarting Audacity.
- First, select the area of track with the click and and hit the downward pointing arrow in the Track Popdown Menu, then "Split Stereo Track" followed by Edit > Split which moves the selected area into a new track underneath your original track.
- Now on the new track, use the same Track Popdown Menu, make both tracks mono and then select one of the channels and Effect > Invert.
When you export the result as a stereo track, the area you split out will be effectively mono (in the sense that the previous music signal in the left and right channels will be mixed into both channels), but the click should be sharply attenuated without harming the music too much.
Some users find the Hard Limiter an additional way of removing clicks. A Hard Limiter is a particularly strong dynamic compressor to reduce the difference between loud and soft in an area of audio. A Hard Limiter is included in the Audacity 1.2.6 Windows installer under the Effect Menu, or is available in the SWH plugins suite:
- Windows: http://audacity.sourceforge.net/beta/ladspa/ladspa-0.4.15.exe
- Mac: http://ardour.org/files/releases/swh-plugins-0.4.15.dmg
- Linux: http://plugin.org.uk/download.php
The Hard Limiter controls are:
- dB limit: the input signal level above which the input signal will be clipped.
- Wet level: controls the amount of the clipped signal fed to the output. It thus acts as a volume control applied after the limiter (just as if you applied the effect and then used the Amplify effect).
- Residue level: controls the amount of the clipped signal that will be mixed back in. By default the value is set to zero and so the clipped signal is discarded. By raising this level, some of the clipped signal will be restored, making the limit softer and retaining more of the peaks and troughs of the original waveform. Alternatively if you turn the residue full up and the Wet signal full down, you get only the pieces of waveform that are above the dB limit you chose.
A new and automatic "Repair" effect for very short sections of badly damaged audio up to 128 samples in length is included in Audacity 1.3.2 and later.