CAM8: Declip

Eliminating clipping from overloaded audio

The Problem

Clipping occurs when a piece of equipment or a medium carrying a signal is unable to handle a high level presented at its input, and will usually be heard as a harsh distortion that increases in intensity as the clipping becomes heavier. In all cases of clipping, it is the portions of the waveform near its extremities that are affected, while portions of the waveform closer to zero are unaffected. Examples of clipping include:

• Digital clipping
  The signal contains many false samples. Viewing the audio waveform would reveal 'flat tops', at which points the genuine signal is destroyed and replaced by false samples at the maximum amplitude.
  
  
• Clipping of analogue equipment and media
  There are many different types of analogue clipping, and these will usually be less obvious than digital clipping. This means that the hard edges visible in the former case are replaced by a more rounded profile.

The Solution

Declip is a genuine declipping algorithm that allows you to identify and remove most instances of clipping in a single real-time pass. It does so by presenting a visual indication of the density of sample values in the signal. If the Signal Analysis window displays 'hard' vertical edges, this demonstrates that the signal has been clipped at that sample value. It is then simple to remove the offending samples and reconstruct the signal. If you wish to inspect a region of the display more closely, you can use zoom to increase the resolution, allowing you to see finer detail within the signal. Furthermore, like the CEDAR Cambridge Spectrum Analyser, Declip offers multiple display modes that help you to analyse the audio correctly.

Two processing modes are provided, and these allow you to choose between an algorithm optimised for light clipping, and one optimised for heavy amounts of clipping. This ensures that you remove the problem without causing damage to the genuine signal.

Since the amplitude of a declipped signal can be significantly greater than that of the incoming signal, CEDAR Cambridge provides Gain controls in the dedicated Gain Module and in the Dither Module.