Clip Fix and Repair, including fuzz boxes, DC offsets and faulty power supplies
Clipping is where a waveform is damaged from exceeding the maximum deflection, usually 0dB. When Audacity encounters clipping, it highlights the clipped passages with red vertical lines:
In simple clipping the top is lopped off the peak, giving you a flat line until the amplitude comes back within limits. When playing clipped files you hear a crackle, sounding like a speaker cone bottoming out (and for the very same reason). Incidentally, early Telarc (and some Delos) audiophile CDs had clipping built-in to the loud passages, encoded right onto the CD. Even at very low playback volume they sounded distorted… because the engineers apparently thought that made it SOUND loud?
You can reduce the volume of the clipped wave, but then you have a mesa instead of a mountain (and it still sounds clipped):
What you need is Audacity’s Clip Fix. Below is a useful setting for this feature, which not only starts repairing the clip at 95% of peak amplitude but also reduces the overall volume to make room for the repair.
What you end up with is this: the flattened tops are changed it to smooth arcs.
In most cases this reduces the distortion to an acceptable level. By the way, if you run Clip Fix on a wave that is NOT clipped, it simply reduces the volume by the amount you specified, without altering the waveform. Therefore it’s safe to highlight a broad passage that has clipping somewhere within it, and only the clipped waveform will be redrawn.
Like most other changes, Clip Fix will give you glitches if your highlighted section doesn’t start and end at zero-amplitude crossings.
Be aware though you might run into clipping that is not “simple clipping.”
Some older-design digital electronics “bounce” when they hit maximum deflection, going instantly from maximum positive voltage to maximum negative voltage (or vice versa), resulting in a very loud audible pop. In the example below, only one channel is bouncing, and only the negative rail is bouncing－probably due to a defective power supply in the battery-powered mixer.
This kind of clipping is a bear to remove. Audacity’s Clip Fix isn’t enough by itself. You will have to redraw the waveforms, either by hand, or using Audacity’s Repair tool. You’ll find Repair is very handy for removing almost any audible defect, but it is unfortunately limited to 128 samples at a time. At 44.1 samples per millisecond that’s only 2.9 milliseconds! That gets tedious fast, if you have a long waveform to repair, but sometimes it’s your only option (we’ll cover this more in the episode on noise reduction).
Audacity’s Repair tool takes any short duration anomaly－volume jumps from cutting and pasting, ticks & pops from records, microphone handling noise, dropped sticks, etc.－and makes it no longer audible as a non-musical artifact. It redraws the wave, but does so in such a way－by comparing to the 128 samples BEFORE your repair, and the 128 samples AFTER your repair－so that the repaired section blends right in with its surroundings. Because of this “invisible repair,” it is sometimes useful (and not harmful) to try several repairs in the general area until you hit the one you’re looking for, if it’s not apparent where the glitch is.
This leads to the final point in this episode: what exactly is noise? Some musicians intentionally utilize feedback (Jimi) or distortion (Link Wray), and some engineers intentionally clip the signal (Telarc). When listening to a passage of music you may have to make a judgment call as to what to take out… and what to leave in.
ROBERT
01.03.26