How to Remove Clipped Samples from a Digital Audio Signal

Unlike recording audio to an analog medium, the digital realm is far less tolerant of incoming audio signals above 0dB. When a recorded audio signal exceeds this level, digital systems essentially have no more bits available to render the incoming signal as a waveform. What generally results from this is often referred to as "clipping," and is expressed as a loud pop, crackle or similarly distorted audio sample. Although many companies offer plug-ins and filters that can remove such pops and crackles from digital audio, these tend to affect more than just the individual, clipped sample of the audio. If re-recording the audio at lower levels is not an option, manually removing the distorted audio sample is a relatively simple process.

Instructions

Difficulty: Moderately Easy

Things You’ll Need:

• Computer running either the Windows or Macintosh operating system
• Two-track audio editor or multi-track sequencer

1. Transfer the affected file to a computer. A quick search on http://www.versiontracker.com or http://www.download.com provides freeware and shareware audio editors.
2. Open the audio file in your editing or sequencing application.
3. Potentially clipped samples are often easy to identify. Recognize the point(s) at which the clipping occurs. Look for a spike in the waveform of the audio signal that peaks at the 0dB level (See Figure 1). To verify that you have accurately identified the clipped portion of the signal, highlight the specific area of the waveform, play it and listen for an audible pop or crackle in the audio.
4. Select only the individual sample that is clipped. Use the application's view menu or zoom option to magnify the individual sample where the clipping occurs as much as possible, and highlight only the clipped sample(s) (See Figure 2).
5. Use the gain or volume DSP (digital signal processing) function of your editor or sequencer to decrease the amplitude of the selected sample. Simply reducing the level of this particular portion of the audio signal to below 0dB may still result in a distorted sample, as the full signal was never accurately captured when the recording was initially made. If this is the case, then set the gain or volume adjustment as low as your application will allow (generally, anywhere within the -40dB range or lower will suffice; as an alternative, if your program provides gain reduction in terms of percentages, set the amplitude to 0%). Particularly if the segment of the waveform that clips occupies only a handful of samples, reducing or removing them altogether will be completely unnoticeable when playing back the audio signal in its entirety.
6. Repeat this procedure for all audible clipping within the waveform and enjoy your clip-free recording.

Tips & Warnings

• The number of samples that can be reduced or removed while still maintaining the integrity of the audio signal as a whole will depend on the fluctuations of the waveform itself.
• Be sure to back up or make a copy of the audio file prior to editing in case unintentional changes to the original are made.