DYNAMICS: A TUTORIAL

the basics of compression, limiting, gating and expansion.

(Redacted from various sources including Electronic Musician!)

Part One:

No matter how you do it,

controlling the dynamic range of any recording is an

important part of creating a useful product.

Anyone familiar with the recording process knows the difficulties of taking sound from the real world and turning it into sound from speakers. That in itself is not a big feat, but creating the illusion of big sound takes some understanding of the limitations of recording and playback systems, as well as an understanding of how the ears and brain process what they hear.

The real world has a much larger dynamic range than most practical recording systems. At first, this may seem to be a problem, until you consider that human hearing system has a limited dynamic range as well. The brain and ears work together to control the size and amount of information that you take in.

A Listening Test

Try sitting in a quiet room for 15 minutes and listen carefully to the sounds around you. Your brain will turn up the gain on your ears to the threshold of hearing. In an anechoic chamber, you can hear the sound of the blood moving through your ears and the sound of your heart beating. If a sudden, loud noise occurs, the brain immediately decreases the gain. In an extremely loud environment, the muscles around the ear canal will constrict to limit the incoming sound. Exposure to extreme sounds can cause the equivalent of a >charley-horse in the muscles of the ear canal. Inner-ear scar tissue can result in permanent threshold shift and ringing in the ears.

(Eh, what’s that?)

The point of all this is that it is quite possible to make good-sounding recordings within the limitations of practical recording and playback systems and create the illusion of wide dynamics. Limiters, compressors and other forms of dynamic control are the tools in this process.

Basic Types of Dynamic Processing

In the world of dynamics control, there are dozens, if not hundreds, of devices available They range from sophisticated, multi-task units to the blunt instrument of tape saturation, converter saturation or amplifier clipping. An understanding of the effect of each of them gives you a large tool box to choose from, when its time to control the dynamics for any given audio context.

There are several different classifications of dynamic processing which are essential to know and understand. The various manufacturers provide different types of interfaces to the know processes which range from simple boxes (such as the RNC - Really Nice Compressor) to software plug-ins for digital recording programs such as Sound Forge.

A basic setup usually leads to fine tuning and adjustment. There are few magic settings that work for every dynamic situation, but there are guidelines and general methods that are useful to know and use. More on these in part two. For one view of basic settings visit these links about dynamics processing at Home Recording - gates and compressors and startup dynamics settings.

Limiter: as the name implies, a device that limits the level of a signal which passes through it.

Above the threshold, it doesn’t let things get much louder. Limiters generally have a rather large compression ratio, usually 8:1 or greater. What this means is a difference between the change in input level and the change in output level above the threshold. An 8dB increase In input signal will result in a 1dB increase in output level.

Compressor: a device that reduces the dynamic range of a signal.

Compressors typically use moderate compression ratios, like 2:1. This means that a 2dB increase in input results in a 1dB increase in output. Compressors are usually operated well above their threshold point, to reduce the overall changes in dynamic range without squashing it down to the point that the VU meter just sits on '0'. Of course, that may be just what you're looking for.

Gate: a device that passes no signal until a threshold is reached.

Above the threshold, it passes the signal without changing the dynamics of it.

Expander: A device which increases the volume of sounds below a given threshold.

These units are often combined with limiters to set a maximum peak in the amplitude of the signal produced. The utility is in bringing up the level of small sounds and making them more equal in power to louder transients.

Compander: A device which combines the action of both the compressor and the expander.

A very useful mastering device. The compander allows for simultaneous control of dynamic range for both loud and soft sounds.

New Orleans-based engineer/producer Keith Keller offers this analogy: imagine the signal as yourself on a trampoline -the higher you go, the louder it gets. A limiter is like having a hard ceiling above you. The threshold is the height of the ceiling. A compressor is like a ceiling made of pillows. An expander is like having a trampoline net which, beyond a certain point (the threshold), gets stretchy. A gate is a hole that opens up in the floor that the signal disappears into.

The basic mechanism of compressors and limiters is a gain reduction circuit. This circuit automatically adjusts the gain in response to the size of the incoming signal. How much gain reduction, and at what level and for how long, is determined by the control section (or side chain). Every manufacturer has its own idea of what those parameters should be.

The earliest forms of compressors were called automatic mixers or 'lazy-man's mixers'. They took a sample of the output level, rectified it and applied the resulting negative voltage to the input amplifier . The change in bias on the input stage reduces the gain. This priciple is the scheme used in most tube-type limiters and compressors. It is a voltage-contol amplifier or VCA.

(end of Part One)