Click removal using the Spectrogram view

From Audacity Development Manual

Jump to: navigation, search
This is a workflow Tutorial which gives steps to remove hard-to-spot clicks using Audacity's Spectrogram view.

In the example the user can hear some clicks but cannot spot them with the default Waveform view despite zooming in quite far. In Waveform view loud clicks often show up as easily seen spikes but smaller, lower amplitude clicks can be very hard to spot without changing to Spectrogram view.

Clicks heard on playback

The user has identified a region where there are some clicks by listening closely on his/her headphones, and has zoomed in to try to spot them but it's very hard to identify them visually in the default waveform view.

Clicky example waveform view.png

Spectrogram view

So we change to the Spectrogram view (use the dropdown menu from the downward-pointing black triangle in the Track Control Panel). This gives us a better chance of seeing clicks because Spectrogram view picks up the greater energy in the click compared to the surrounding audio.

Clicky example spectrogram view.png

Clicks are pink

In the Spectrogram view the clicks will normally show up as vertical pink (or possibly vertical red) lines. Note that the Spectrogram view has identified two regions of clicks between 55.70 and 55.80 seconds that do not look like clicks in Waveform view.

Because we want to zoom in on these regions later, we will not select from 55.70 to 55.80 seconds but for now, just select the second clicky region. To select this region, click and drag with your mouse and then label the region by using CTRL+B (on a Mac CMD+B ) and typing a label name.

Clicky example spectrogram view click labelled.png

Back to Waveform

Change back to Waveform view as it is easier to work with to make the repairs.

Clicky example waveform view click labelled.png

Zoom in to clicks and delete or repair

Now zoom in to see the clicks in the selection more clearly (spot the little vertical spikes in the waveform).

Select a little more than the exact part of the selection that contains clicks by clicking and dragging with your mouse:

Clicky example waveform view click labelled and zoomed red arrows.png

Use Edit > Find Zero Crossings or its Z shortcut.

Now use the Delete key or the CTRL+K keyboard shortcut (on a Mac CMD+K ) to delete the selection, resulting in the image below:

Clicky example waveform view clicks deleted.png

When you used Find Zero Crossings this tried to move the selection edges a very small distance to a place where the audio sample dots were at zero amplitude (exactly on the center line). This helps to avoid a click at the new join after deleting the selection, but a residual click is still possible. Therefore the next step coming up will be to listen to the audio after the deletion.

Deletion removes a very small portion of the audio; you may instead prefer to retain the original audio length, not least because removing longer selections may be audible. As an alternative to deletion, zoom in much closer to each click and then for each click, use either of the following techniques:
  • carefully use Draw Tool to smooth the waveform manually
  • select a very small region (up to 128 samples in length) that includes both a click and some undamaged audio, then use the Repair effect.

Test the click deletion

Now test the result (this is easier using headphones). Zoom out and play from a little before the point where the selection was deleted. Note that we deliberately did not yet remove the label of the previously clicky region as this helps us to identify the location as we play back for testing.

Clicky example waveform view click repaired for test.png

If you are happy with the audible results then there is no further work to be done on this particular click.

If a discontinuity is heard at the join

If a residual click is heard after deletion despite using Find Zero Crossings, the click will probably be in one of the channels in a stereo track.

The zoomed in example below shows a very obvious visual discontinuity in both channels of the stereo track resulting from not using Find Zero Crossings before deletion.

Any residual clicks heard must be repaired. You could use Draw Tool as above, but we will repair the discontinuity shown below automatically using the Repair effect.

Select the discontinuity

Select the waveform around the discontinuity; a maximum of 128 samples (the maximum length for the Repair effect).

Clicky example waveform view discontinuity selected.png
Note that we have changed the selection display from End to Length in Selection Toolbar so that "Length" conveniently shows the number of samples in the selection.

Repair the discontinuity

Now use Effect > Repair to fix the discontinuity; note the resulting smooth curve.

Clicky example waveform view discontinuity repaired.png

Final test of the repaired region

As a final step you should audibly test all repairs in the region chosen in Spectrogram view, again preferably on headphones. Zoom out sufficiently and start playback a little earlier, so you can play a reasonable amount of audio either side of the label (which we've still retained to identify the repaired location):

Clicky example waveform view click repaired for final testing.png

Once you are happy with the results, the label can be deleted if required (see this page).

Undo your repairs if you are unhappy with the result

If you are unhappy with your results you can use Edit > Undo repeatedly to revert back to your original unedited audio. Alternatively you may use the View > History... command to choose or compare several previous editing states.

Deal with the remaining clicks

Now we have to repeat the process of finding the exact clicks within the first clicky region identified by Spectrogram, repair those clicks and audition the repairs, then likewise with any further clicks in our audio. If you have been making a lot of repairs you may want to take a break and come back to listen to the whole repaired track a little later.

Views

Donate securely by PayPal, using your credit card or PayPal account!

Useful External Links