De-Clipper

[Craig Maier]

De-Clipper


The De-Clipper is provided to reduce the distortion which results from an over-driven or clipped signal. When viewed in the time domain via the Source display window, this problem often looks like the signal has a “crew cut” or is “maxed-out.” The De-Clipper is equipped with two modes of interpolation, one having adjustable strength while the other uses a complex frequency domain method of interpolation. The “Adjustable Strength” mode is most useful on signals having a fairly high level of coherence while the “non Adjustable Strength” mode is best suited to signals having a higher level of randomness in their makeup. The “non Adjustable Strength” routine requires a high level of CPU and thus is quite slow, whereas the “Adjustable Strength” mode is much faster. Experimentation is the best way to find the optimal mode to apply to your particular clipped file. We recommend starting with the Adjustable Strength mode first, since it contains the fastest running replacement algorithm.



The De-Clipper
The De-Clipper can be used to repair signals, which were either clipped by digital or analog mechanisms. It performs its magic by detecting signals with very low or zero values of slope (user adjustable from 0 - 0.5) above a settable threshold amplitude value. When this condition is detected, the routine mathematically interpolates a new signal and replaces the zero slope portion of the bad waveform with one containing curvature. This results in decreased distortion. If the material being de-clipped has been directly clipped by the digital recording process (in other words, the signal is clipped at full scale output as indicated on the destination window), then you must first decrease the overall gain of the .wav file by 6 dB (- 6 dB) before applying the De-Clipper. This sort of de-clipping can be accomplished with very low values of slope. If the signal was clipped previous to the transfer to the digital domain by an overloaded analog amplifier, the signal can be de-clipped by raising the slope control until the “total clips fixed” display starts incrementing. The following is a listing of controls available on the De-Clipper:

• Adjustable Strength Checkbox

This checkbox determines which of two different replacement algorithms are used by your De-Clipper. Check this box when dealing with audio signals which are fairly coherent in nature and un-check this box for signals which are more random (or stochastic) in general nature.

• Threshold

The threshold setting determines the amplitude above which signals will be applied to the de-clipper detector. Lower values imply a more aggressive response. The range for this control is from 10 to 100 on a relative and normalized percentage scale.

• Clip Level

The clipped level of your signal is measured and displayed on this bar graph and is useful for setting the “Threshold” control. Close alignment of the “Threshold” control with the “Clip Level” while watching the “Total Clips Fixed” display will allow you to find the best “Threshold” setting. The “Total Clips Fixed” indicator will begin to increment when the proper threshold is found. Do not overdrive the system by setting the “Threshold” control to an excessively low setting as this may introduce distortion into the process.

• Slope

The Slope control determines the “flatness” of the clipped waveform that will be interpreted as a “clipped” event. Low numbers like zero, imply a perfectly flat line as will be found in digital clipping. Higher numbers represent slight slopes associated with analog clipping. The range for this control is from 0.000 to 0.500 with 0 representing zero slope and 0.500 representing a 45 degree slope.

• Strength

The Strength control affects the curvature of the applied interpolated waveform. Adjust this for the best sounding (or looking) replacement waveform. The range for this is from 1 to 5, with 5 being the most aggressive.

• Total Clips Fixed:

This is a numerical display indicating how many “clips” were detected and fixed by the De-Clipper. It is useful when using “preview” mode to assure that the routine is detecting the clipping while adjusting the various controls.

• Bypass

This bypasses the De-Clipper so that you can preview and hear the before and after results of the de-clipping process in an instant.
De-Clipper Operating Procedure (Tutorial)


File Preparation: Before de-clipping a .wav file, it is necessary to reduce its amplitude by 6 dB before applying the following procedure. To reduce the gain of the .wav file, use the Gain Change feature found under the Edit menu. The reason for this step is to provide the de-clipper algorithm enough headroom to interpolate the clipped peaks of the .wav file.

1. Set the Threshold control to its maximum value (all the way up).
2. Set the Slope to a value close to zero for digital clipping, but higher for analog clipping.
3. Preview the filter and watch the “Total Clips Fixed” display.
4. Reduce the level of the Threshold until it begins to show increments on the Total Clips Fixed display.
5. Adjust the Strength for a minimization of distortion as heard in preview mode.
6. Run the Filter.
7. Done

Note: You can look at the results produced by the De-Clipper after you have run the Filter. You will notice that the flat-topping has been replaced with smooth rounded waveforms if the controls have been set properly.