Thursday, February 21, 2013

Clipping

A question popped up in a forum the other day about clipping and that got the blogging juices flowing. Here's the original question. (the names have been omitted to protect the innocent)

I've read on many forums that clipping is often used in order to get a hot master, but what kind of clipping is this? Are they referring to engineers pushing it hard on analogue gear, or over-compressing/limiting audio in which is being clipped as a result? Are they referring to soft-clipping plugins?

Audio amplifiers, from the tiny ones on chips in a mixer to the big ones on guitar stacks all operate on the same principle. You have a large reservoir of power and you use an audio signal to modulate it. Think of it as a dam holding back water and the audio signal is telling the sluice operator how much to let through at any given moment.

In a good system, the audio coming out looks just like the audio going in except bigger. There are a lot of factors that can cause this to not be true, or as true. Let's assume that good components are in use and we can expect a fairly true output.

If you keep turning up the level of the input, eventually the power supply just won't have enough energy to do what the input is asking it to do. Any requested output that exceeds this limit gets flat topped. The audio looks normal up to the point where the output runs out of gas but any peaks going in above that point are just leveled off at the maximum voltage.

The simple answer is that you just raise the input until you get there but different systems handle clipping in different ways. Solid state amplifiers (transistors) usually run out of gas in a fairly nasty way that adds all sorts of unpleasant harmonics to the signal. Tubes, and tape machines have a much smoother transition which lends a much more pleasing set of harmonics. Transformers saturate in a nice way too which is one of the reasons so many people like big, old analog consoles. Digital on the other hand is very cut and dried. Once you run out of numbers your signal is instantly flat topped, no transition, lots of unpleasantness.

There's a whole lot more to this that you need to understand before you set out to clip something to make it louder. There are a lot safer ways like compression and limiting with the application of make up gain to get your stuff loud. In fact about the only time I can think of where it's common to clip is with the amps driving the subs in a large format audio system. But even that is starting to become frowned upon due to the falling cost of amplification and the implications of the harmonics generated.

One last thing. If you're using plugins, many are designed to simulate analog devices which are capable of producing nice sounds when driven into saturation (clipping). In that case it's OK to clip because it's a simulation and not an actual digital clip.

Then another redditor jumped in with another doosie of a comment which I shall now share with you as well.

To add upon [that elegant explanation], psychoacoustics (the science of how sound is perceived) dictates that we are more sensitive in the midrange, about where the human voice sits. Those curves are essentially the inverse of our hearing sensitivity: they display the loudness needed for various frequencies to be perceived at an equal loudness, hence the name. If you have sounds outside of the most sensitive range you can turn them up, but eventually they start to fight each other. You can increase the perceived loudness of these sounds by having them generate harmonics in the range of our greatest sensitivity and we also tend to perceive the additional harmonics as part of the original signal because they are a type of correlated noise. One way of doing this is by clipping the signal: the way the circuit behaves when it clips includes the generation of harmonics, some types of circuits tend to generate higher-order harmonics (solid state) and some tend to generate lower-order harmonics (tubes). As an extreme example, this is what overdrive and distortion effects do to varying degrees and the effect can be measured as THD (total harmonic distortion). As a less extreme example, this is one aspect of what devices like the BBE Sonic Maximizer or the Aphex Aural Exciter do.

This of course is aside from the fact that if you're clipping a signal, the parts that aren't clipping are being brought up in level along with the rest of the signal.

Just one final note. The audio forums over on Reddit are some of the best around. The ones at r/audioengineering and r/livesound in particular are flame and troll free for the most part. Just people asking questions and exchanging ideas. You should check it out.
 

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