Hi. My name is Jan Zurcher.

I’m a singer-songwriter living in a small town called Friday Harbor, San Juan Island, Washington

This lesson is for Week 4 of the Introduction to Music Production Course at Coursera.org.

The topic I have chosen to look at this week is…

Dynamic Processors are used to control the dynamic range of a recording

• to enhance the highs and/or lows of your tracks to make the recording more exciting or to emphasize a particular part of a song;

• to reduce the highs and/or the lows;

• to make the amplitude more consistent;

• to eliminate unwanted noise; or,

• to adjust the transients (loud sounds at the start of a sound)

Use of Dynamic Processors

Dynamic Range

Dynamic range is the difference between the loudest and the quietest sounds. You could say that the dynamic range is represented by the red line shown above.

However, if you tried to analyse the signal above by finding the average volume, you would end up with an average of 0 because the wave spends as much time about the 0 mark as below it.

For this reason, dynamic processors often use the RMS (Root Mean Squared) measured in dB with 0 as one point of reference and -70 dB as another to determine the average loudness of a signal.

When you record a sound in your DAW, you get a wave form that looks something like this:

Dynamic Range

A larger segment of the audio recording looks like this:

If this wave form were converted into an RMS curve, the result would look something like this:

Here you can see that the (uncompressed) range is from -54.0 dB to +4.5 dB (or a dynamic range of 58.5 dB).

Dynamic Range

A common way that you see the dynamic range represented in your DAW is on the fader meter where the scale ranges from +6 dB to around -70 dB (which is basically zero sound).

For another segment of the above audio signal captured in a screen shot, the loudest point was +1.8 dB and the lowest was -33.6 dB (or a dynamic range of 35.4 dB as depicted by the red arrow).

Dynamic Range

Common Types of Dynamic Processors

Compressor

Expander

Limiter Noise Gate

Dynamic range can be changed using any of a number of devices called Dynamic Processors including (but not limited to): Compressors, Expanders, Limiters and Noise Gates.

Shown here are screen shots of some of the built-in Dynamic Processors in Logic Express 9.0.

Key Components of Dynamic Processors

Although each of these dynamic processors creates changes in dynamic range a little differently, they have some common components and work on some common principles.

First, each of these is basically divided into two sections – one section analyses the input signal to find the shape of the sound envelop (the RMS discussed above). This is called the “side chain section” (or key section).

The other section contains the volume fader that is used to make adjustments in the audio level – either up or down as needed. There are also a number of other factors that can be adjusted to create the type of dynamic range that you want.

Common Elements of Dynamic Processors

These other adjustment factors include :

Threshold: The volume level at which the dynamic processor begins to affect the signal

Ratio: How much the input volume is changed as a proportion of output level over the threshold

Attack: How fast the processor begins to alter the signal once the threshold is reached.

Release How fast the processor stops altering the signal once the threshold is crossed back into the acceptable range

Let’s take a closer look at what each of these terms mean when it comes to processing an audio signal. In each case, the examples are assuming the use of a downward compressor.

Input Level (dB)

In this example, the threshold is set at -20 dB. This threshold can be adjusted to whatever input level you need for the effect you want. A threshold set at -60 dB will affect the entire signal. A threshold of 0 dB would not affect any of the signal.

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Threshold

For downward compression, when the input level of an audio signal goes above a specified point (called the threshold), gain reduction begins.

Threshold Level – Downward Compression

Input Level (dB)-60 -50 -40 -30 -20 -10

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Once you have told the compressor the input level at which it should start processing the signal (that is, the threshold level), you next need to let it know how much you want it to reduce the gain.

This is determined by the ratio. If you set the ratio to 1:1. Even if the threshold is crossed, there will be no gain reduction because the ratio of 1:1 indicates that the output level should equal the input level.

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Compression Ratio

Input Level (dB)-60 -50 -40 -30 -20 -10

If you select a ratio of 2:1, after the threshold is crossed, if the input level is 20 dB over the threshold level, the output level will be reduced so that it is only 10 dB over the threshold level.

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Reduction in output gain

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The threshold level and the ratio work together to establish how much reduction in the gain of the output signal there will be.

In this example, with a -30 dB threshold and a 2:1 ratio, the gain reduction is 15 dB.

In this example, with a -50 dB threshold and a 2:1 ratio, the gain reduction is 25 dB.

Reduction in gain Reduction

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For downward compression, as the ratio gets larger, the amount of gain reduction increases until finally, at a ratio of Infinity:1, the output level of the signal will be the same as the threshold level.

At that point, the compressor is acting just like a limiter – limiting the level of the output signal to the threshold level.

In this example, with a -40 dB threshold and a 4:1 ratio, the gain reduction is 30 dB.

In this example, with a -50 dB threshold and a 40:1 ratio, the gain reduction is 39 dB.

Reduction in gain Reduction

in gain

Once you have told the device the threshold level and the relative amount that you want the output signal to be reduced, the next step is to tell it how quickly you want it to react when it detects that the threshold has been crossed.

This is known as the Attack Phase which is how long it takes the processor to decrease the gain down to the level established by the ratio.

This attack time is measured in milliseconds (ms) and can be set to be very fast ( < 1 ms) or relatively slow ( > 100 ms).

Compressors in Logic Express 9.0 allow adjustments in attack from 0 ms to 200 ms in 0.5 ms intervals.

Attack Time

Fast attack results in the gain being reduced almost instantly as soon as the threshold is crossed. This may not be what you want because that transient information at the beginning of a sound adds significantly to the character of that sound. Reducing it right away can lead to a dull sound.

On the other hand, if the attack time is set too slow, the dynamic processor may act too late and be less effective.

Input Level

Output Level

Time (ms)

dB

Input Level

Output Level

Time (ms)

dB

Attack PhaseAttack Phase

Fast Attack Slow Attack

Threshold Threshold

Release Time

The final common function found in dynamic processors covered in this presentation is the Release Phase.

Just as you have to tell the device how fast to reduce the gain to the required level once the threshold is reached, you also need to tell it how fast to return to the input level once the threshold is crossed in the opposite direction.

The Release Time is measured in milliseconds (ms) and can be set to be fast ( < 5 ms) or relatively slow ( > 5,000 ms).

Compressors in Logic Express 9.0 allow adjustments in release from 5 ms to 5000 ms.

A fast release time means the signal returns to match the input level almost immediately after the threshold is crossed. You might want to use a lower release time if you want to make dynamic differences stand out.

On the other hand, if the release time is set to a larger number of milliseconds, the dynamic differences in the signal will be more smoothed out – that is a gradual return to the input level.

Input Level

Output Level

Time (ms)

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Input Level

Output Level

Time (ms)

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Release PhaseRelease Phase

Fast Release Slow Release

Threshold Threshold

Dynamic Processors - Summary

This concludes this brief overview of the concept of Dynamic Processors and the four common elements that these types of processors have in common – that is threshold, ratio, attack and release.

Although the details of the explanations were provided in the context of how these work in a compressor, these common elements operate in similar ways in other common types of Dynamic Compressors including Limiters, Expanders and Noise Gates.