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Dynamic Audio Processing


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Dynamic Audio Processors are used to correct or control signal levels as well as to creatively shape a sound in its amplitude envelope.

 

The ones we shall look at are -

 

Compressor

Limiter

Expander

Noise Gate

Ducking

 

 

 

To begin with, i might as well explain a very important part of audio levels and how they are represented. This includes -

 

Dynamic Range

Signal To Noise Ratio

Clipping Point

Headroom

Nominal Level

Noise Floor

 

 

And to fully understand these, it is important to understand the Amplifying Factor (AF) and it's correspondence in decibels (dB). Here is how it works -

 

post-59-1216041094.jpg

 

For example, if you double the amplitude you actually increase it by 6dB.

 

 

 

Audio Levels

 

 

The Dynamic Range -

 

The Dynamic Range is the difference between the minimum (Noise Floor) and maximum (Clipping Point) amplitude a device can record. For example, if the maximum of a device is 10 dB and the Noise Floor is 3 dB then the dynamic range is 4.85 dB, since 10 dB−3 dB = 4.85 dB (remember that care must be taken when adding numbers in the decibel scale, as they do not equate like usual numbers).

 

 

 

Signal To Noise Ratio -

 

This is the difference between the Noise Floor and the normal Sound Pressure Level (about 100dBSPL or +4dBu for a professional studio system or -10dBu for home studio setup).

 

 

 

Clipping Point (+25dB) -

 

In the audio domain, clipping may be heard as general distortion or as pops. Because the clipped waveform has more area underneath it than the smaller unclipped waveform, the amplifier produces more power when it is clipping. This extra power can damage parts of the loudspeaker, including the woofer or the tweeter, by causing over-excursion, or by overheating the voice coil. However most loudspeakers are designed to handle signals like cymbal crashes that have even more high frequency weighting than amplifier clipping produces, so damage attributable to this characteristic is rare. You can see here, where the peak of the waveform is flat, where it should be a clean rounded signal.

 

 

 

 

Headroom -

 

The Headroom is the part of an audio system that allows for the signal to go over the Nominal Level before it reaches the Clipping Point, allowing for an additional increase in gain. You see examples of this on your DJ mixer at home on the LED display that shows the volume levels -

 

 

For example, if the compressor ratio is set at 2:1 then for every 2 dB it will be diminished to an output of 1dB - effectively halving the signal. If the ratio is set to a ratio of 10:1 then for every 10dB it will be diminished to an output of 1dB, which is known as Hard Compression and acts the same way as a Limiter.

 

 

 

Limiter

 

A Limiter is a type of compressor designed for a specific purpose — to limit the level of a signal to a certain threshold. Whereas a Compressor will begin smoothly reducing the gain above the threshold, a Limiter will almost completely prevent any additional gain above the threshold. A Limiter is like a compressor set to a very high compression ratio (at least 10:1, more commonly 20:1 or more). The graph below shows a limiting ratio of infinity to one, i.e. there is no gain at all above a the threshold.

 

 

 

 

 

Expander

 

Audio expansion is basically the opposite of audio compression, which means to expand the dynamic range of a signal.

 

Like compressors and limiters, an audio expander has an adjustable threshold and ratio. Whereas compression and limiting take effect whenever the signal goes above the threshold, expansion effects signal levels below the threshold.

 

Any signal below the threshold is expanded downwards by the specified ratio. For example, if the ratio is 2:1 and the signal drops 3dB below the threshold, the signal level will be reduced to 6dB below the threshold. The following graph illustrates two different expansion ratios — 2:1 and the more severe 10:1.

 

 

 

 

Noise Gate

 

A Noise Gate is essentially and audio switch to mute signals, which can be used to cut background noise picked up by open mics, noise from guitar amps, etc. The threshold here is the level at which the Gate closes, cutting off the signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is 'open'. If the signal falls below the threshold no signal is allowed to pass (or the signal is substantially attenuated): the gate is 'closed'.

 

 

 

 

Ducking

 

Ducking is an audio process of automatic compression, e.g. when the announcer's voice signal causes the level of music to be attenuated. For example, if you are DJing at a club and you want the crowd to hear your voice over the microphone, you will turn the volume down slightly and then back up again once you've finished talking. Ducking is a process that automatically activates when a signal source is present.

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thats good stuff mate, would it be handy to also add that a limiter is basically one extreme of a compresser, and a gate is one extreme of a limiter?

 

i might have worded that really badly, its just something i remember finding quite useful when i learnt this stuff

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thats good stuff mate, would it be handy to also add that a limiter is basically one extreme of a compresser, and a gate is one extreme of a limiter?

 

i might have worded that really badly, its just something i remember finding quite useful when i learnt this stuff

 

 

 

ye i think i mentioned that a Limiter is the extreme of a Compressor

where the hard compression is in effect limiting the signal at the threshold

 

but im not sure a Noise Gate is the extreme of a Limiter.

they act in similar ways but have different attributes

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Mastering = A good set of meters + a good set of ears.

Even better get a mastering engineer to do it. Mixing and mastering are very different skills. I think too many people think they can do it all. Obviously if you're a one man show on a budget you do wat you have to, but for professional work, involve professionals.

 

Also www.digido.com is one of the best places for mastering info. Bob Katz is one of the best.

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