Removing a Single Click
by John M. Clement, PhD
Click removal is the most difficult cleaning task that needs to be done. Each program has a slightly different methods of detecting clicks and as a result clicks removed completely by one program may not be removed by another. Even the best programs sometimes leave in some audible clicks. Often a large click will only be muted, or turned into a soft low frequency pop. When this happens you can only get rid of it by manually cutting it out. The links in the text allow you to listen to the result.
Here is an example of a large very audible click in a few seconds of music.
Two programs failed to remove the click. First Sonic Foundry Noise Reduction on the vinyl setting turned the click into a very soft pop. Other settings on Noise Reduction could make the click less audible, but still left a low frequency pop. Audio cleaning lab did a better job of muting the click at maximum declicking or with marker declicking, but still left a soft audible pop. The spectral cleanser in ACL 10 did an even better job, but still left a soft pop. If none of these are satisfactory solutions, the only way to remove the click completely is to manually remove it.
This is very easy to do with ACL. First zoom in on the click by clicking the + button on the lower right. Then click on the scissors key and click on the region to the right of the large click as shown in the next picture.
The resulting cut is much wider than the click. In that region the two parts of the wave file fade in and out so as to make the cut inaudible. To cut out the click with minimal disturbance to the sound you need to narrow the fade in/out region of the cut. Click on the arrow key to restore the normal cursor. Then grab the box on the right of the cut and slide it to the left as shown below. Make the width of the cut about the same as or narrower than the width of the click. It should not be narrower than the visible waves.
Next slide the cut closer to the click by grabbing the bottom box as shown below and sliding it to the left. Be careful to leave some space between the click and the edge of the cut. If the cut is too close you will still have some audible click in the result.
Finally grab the right half of the wave and slide it until the cut is on the left of the click as shown below. Slide the cut a little bit past the click. The audible click is actually wider than you see, so you must cut out a little extra to completely remove it.
The final result completely removes the click and the transition is not audible. This should not be surprising as the record producers have been removing unwanted noises for years by manually cutting them out of the master tapes. When this is done carefully the listener can not really hear the transition.
This process may require repeated trials. By typing cntrl-Z you can easily back up and try it again. The main thing to keep in mind is that the click actually extends beyond the visible peak, so you may have to put the cut a bit farther to the right, and then slide the wave a bit farther to the left. Finally magnify the cut and to increase inaudibility you can line up the waves.
This particular method has problems with version 4.0 of ACL. If you have a short segment of wave to the left of the cut which is less than 4 seconds long, it is automatically moved to the far right of the entire wave. A partial workaround in V 4.0 is to always start manual cutting on the right hand side of the wave file, and then fix ticks moving to the left.