Tutorial - Click and pop removal techniques - Audacity Development Manual

# Tutorial - Click and pop removal techniques

From Audacity Development Manual
There are a number of ways you can use Audacity to remove clicks and pops from your recording. First, without zooming in too far, visually inspect your recording for clicks - they will show up as abnormally tall (sticking up or down), very narrow (one or two pixels wide) vertical lines protruding from the waveform.

The primary tool in Audacity for dealing with clicks and pops is the Click Removal effect.

Clicks which do not get removed with Click Removal can be treated individually with other methods. These methods are only really useful if you have a relatively small number of clicks and pops to deal with; otherwise, these approaches will be too labor-intensive and time-consuming:

- Audacity's Repair effect can repair a short length of up to 128 samples by interpolating from the neighboring samples.
- For hard to spot clicks you may want to try Click removal using the Spectrogram view.
- For somewhat longer regions of audio, try:
  - Silence Audio The silenced sections must be short enough so as to not be audible.
  - Draw Tool The principle is to put samples back into line with their neighbors to smooth out the contours of the samples and so attenuate the click or pop.

## Audacity Click Removal

To use Audacity's Click Removal first select the audio from which you want clicks repaired. You can select all of a track by clicking on its Track Control Panel. Alternatively, you can select all the audio in the project by using Select > All or its shortcut Ctrl + A (⌘ + A on a Mac). If there are a lot of clicks it is possible they may be removed more effectively by selecting individual clicks or groups of clicks rather than the whole track. 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. Zooming right in and selecting only an individual spike may not work well - extend the selection a little either side of the immediate click or group of clicks to give the algorithm more idea of the undamaged audio it can use.

Then, choose Effect > 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 OK button. Softer clicks may require you to move the threshold slider further to left (but moving it too far to left may create a "broken up" effect with too much audio removed). For broader clicks, move the "Spike Width" slider further to right.

Preview the effect with these different settings to get the best results. Then, using the settings from your preview testing, use the Click Removal effect on selected regions of audio or on the whole project.

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) will not be effectively removed with Click Removal. For this type of noise, select an area of vinyl that contains only the noise (no music or speech) and use Effect > Noise Reduction.

For details of how Audacity's Click Removal works see this page in the Wiki.

Click Removal requires an audio selection of more than 4096 samples. This is about 93 milliseconds at 44100 Hz project rate as shown in Selection Toolbar.

## Repair of short sections

Audacity has a Repair effect which can be used to repair a short length of up to 128 samples long (for most audio, only a few thousandths of a second) by interpolating from the neighboring samples. You will need to zoom in to see the individual samples to use this effect.

Above this length, it becomes too hard to interpolate what should be going on in the section under repair. You will get an error message if you select too much audio to try and repair. In general, the shorter the section of audio you select to repair, the better the results will be.

The Repair effect is unusual because it requires there to be audio outside the selection region on at least one side of the section to be repaired. If the surrounding audio in the track is very short or non-existent there may not be enough information to make the interpolation, in which case an error message will be shown.

When using Repair it is often convenient to set Selection Toolbar bar to show hh:mm:ss + samples, then click the Length radio button to ensure you are only selecting up to 128 samples.

## Silencing and Draw Tool

Sometimes an even better result can be obtained by zooming in to sample level and either silencing the click using Edit > Remove Special > Silence Audio (or the Ctrl + L shortcut), or using 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 10 milliseconds long can actually be simply silenced or deleted without leaving an audible gap in the sound, although many clicks spread wider that that.

If the click is not suitable for silencing or deletion, enable Draw Tool by clicking the pencil icon in the Tools Toolbar, or press F3 on your keyboard. The mouse pointer will change to a pencil while over the audio track. You must zoom in until you can see the individual sample dots before you can use Draw Tool.

Click in the track at the point you wish a 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 pointer will now change to a brush (or spray can on Linux).

Some patience may be needed with this tool, but the principle is to put samples back into line with their neighbors so that a smooth contour is presented.

## Use Spectrogram view to identify clicks more easily

Click removal using the Spectrogram view is a workflow tutorial giving steps to remove hard-to-spot clicks using Audacity's Spectrogram view. In the default Waveform view, loud clicks often show up as easily seen spikes, but smaller, lower amplitude clicks can be very hard to find without zooming in to near sample level then scrolling the waveform to identify the exact location of the clicks. Considerable time can be saved by first using Spectrogram view which identifies clicky regions more readily.

## Other techniques

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.

### Spectrum analysis

Analyse the area with Analyze > Plot Spectrum to see if any spikes are concentrated in particular frequencies and then use Effect > Equalization to reduce the volume of those frequencies.

You can do this more precisely with the Effect > Notch Filter.

### Subtraction

Where the clicks sound equally loud in both left and right channels of a stereo recording (this often is not the case), but the music information in the channels is very different at that point, you can try to cancel out the click by making the affected section of track mono and inverting one of the channels:

- First, select the area of track that has the click, then click in the track name by the downward-pointing arrow to open the Track Dropdown Menu (right). Choose choose Edit > Clip Boundaries > Split New. This moves the selected area into a new track underneath your original track.
- Then click in the new track's in track name to open its Track Dropdown Menu and choose Split Stereo to Mono.
- Now select one of the channels by clicking where it says "Mono". In the menus at the top, choose 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.

### Repair of broader clicks

Click Removal may not work well with broader clicks of 10 milliseconds or longer. Here are two Nyquist plug-ins you can try. Installation instructions for Nyquist plug-ins are here:

- PopMute: heavily attenuates loud clicks and pops (and even hand claps or small bangs) to make them less obtrusive.
- EZ-Patch: really loud and wide glitches may still sound too bad even if moderated with "Pop Mute". In that case, try "EZ-Patch" which lets you repair the damage by selecting the glitch together with some undamaged audio on one side of the glitch, then smoothly replaces the glitch with the undamaged part of the selection.

## Alternative applications

It is also possible to use other applications than Audacity for click removal, while still using Audacity for the other editing tasks and to produce the final master. Export your audio track from Audacity as a lossless WAV or AIFF file, and also save the click-removed audio as a WAV or AIFF for import back into Audacity.

- ClickRepair: An excellent tool for removing clicks and pops is Brian Davies' ClickRepair. Once you have understood the settings you want to use, it is effectively an automated tool. It is not free, but a fourteen day free-trial is available. Many users report that it saves a lot of time and produces oustandingly good results.

You may find the default settings for ClickRepair remove a little too much signal. An alternative recommendation:

- DeClick = 30 (default is 50)
- Pitch Protection = "on" (default is "off") though leave this "off" for brass recordings
- Reverse = "on" (there is no processing penalty for this and it helps on percussive music)
- Method = Wavelet

- GoldWave: Though nominally not free it is a good, free-trial click remover as well as an alternative audio editor. Its click removal is an effect, just like in Audacity, and there is a "Smoother" effect for broad unwanted noises and an excellent "Noise Reduction" effect for steady noise.

The trial version limits you to a hundred or so commands per session, and a total number of several thousand commands before it expires, but if you export from Audacity as 32-bit WAV and just do Click Removal in it, you should be able to declick several hundred records for free.

- Gnome Wave Cleaner: Only for Linux users. Digital restoration of CD-quality audio files. Dehiss, declick and decrackle in a GUI environment. It can also automatically mark song boundaries if required.