Special Handling Case Study: Fixing a Broken Record

There are some recording problems that can be corrected using automated software tools. Then there are other problems that require human detection and intervention to fix. Here we examine one such case in detail to introduce the techniques of matching and  zero crossing edits.

Figure 1. Broken record effects produce a comb pattern.

Consider the recording in Figure 1. (Click on any of the figures to see it full-size.) The comb pattern is the hallmark of a "broken record," a permanent defect in the groove wall of an LP that causes a turntable stylus to skip back to the previous groove, endlessly repeating the same 1.8 seconds of audio. In this case, the phenomenon went on for over five minutes before a little vibration on the recording room floor jostled the stylus just enough to get past the defect and continue.

You can listen to the affected passage here.

Because automated tools see nothing unusual about this audio, one would think the recording is ruined. But a hands-on matching technique can be employed to save it. By matching, I mean the sound wave just before the unwanted audio section is matched to the wave just after, cutting out the sound in between to effect a smooth transition.

Matching is one example of a more general class of technique called zero crossing edits. It is a technique that doesn't require anything more than a good eye and the ability to zoom and cut.

Because you can't remove much sound without an audible skip, matching is best used to remove artifacts no larger than a fraction of a second -- stubborn clicks or pops that somewhow eluded removal by automated tools. Despite the length of the sound that needs to be cut here, the full set of repetitions is the equivalent of one long stubborn pop, making it a good candidate for removal by matching.

Figure 2. Extraneous sound removed in the problem area.

To make the match, we'll have to move the two halves of the wave a lot closer together. Step one is to select and cut all the sound in between the first repeat and the resolution point (Figure 1). Now it is much easier to see the two halves of the wave that need to be matched (Figure 2). But the two spikes delimiting the problem area are still too far apart to zoom in close enough to perform the match.


Figure 3. Zoom and cut to find comforming zero crossings to match.

Repeat the process of zoom and cut until you can have both audio spikes in the field of view and still see the waveform in fine detail (Figure 3). The entire field of view here is around three hundreths of a second. Notice how spikes that appear instantaneous in real time have shape and duration at this scale.

To make a match, first look for zero crossings, where the digital sample values transition from positive to negative (downward) or negative to positive (upward). By choosing to join two audio sections at a point where there is no sound, there will be no audible hiccup at the junction. To the left of (before) an obstruction, you want to get a zero crossing as close as possible to the obstruction. To the right (after) you want a zero crossing as close as possible to where normal audio resumes.

Two zero crossings must be conforming in order to match, either both downward or both upward. In addition, two zero crossings are a "good" match if the slopes of the wave to the left and right of the resulting junction point are roughly equal (the closer the better).

Figure 4. A good match has been achieved.

Two viable candidates have been identified in Figure 3, and the area to cut has been selected. After making the cut, you can see the match is quite close and affects less than one thousandth of a second of audio (Figure 4).

Figure 5. No evidence of a broken record in the output.

Back at normal magnification, there is no visible evidence of the original problem (Figure 5). Judge for yourself the quality of the audio match here.


In conclusion: 1) manual edits are necessary when automated tools can't address a problem completely; 2) make the edits at zero crossings in the waveform whenever possible to prevent audible artifacts of your work. Use the before track if you'd like to practice matching.


© 2012 Thomas G. Dennehy. All rights reserved.