 Punching
Up Your Audio: Part 2
More Compression Settings and Suggestions
by Jeff Earley
Almost all recordings can benefit from at least a little compression
in the final mix, but the effect can be, and indeed should be, subtle.
Last month, we discussed dynamics and examined a few of the variables
and controls found in a typical plug-in. Hopefully, you've taken some
time to insert your compressor plug-in effect and experiment with
it a little, especially on your dialog and sound effects tracks. In
your exploration, you may have noticed that there are quite a few
more advanced controls and parameters available that we didn't discuss
last time. This month, we'll take a look at some of these more advanced
controls and help you fine-tune exactly how your compressor processes
signals.
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| Figure 1 |
ATTACK AND RELEASE
Attack sets how fast the compressor responds to the signal once it
passes the threshold. Release controls how quickly the compressor
stops compressing once the signal falls below that threshold. These
two controls together can help smooth the effect or make it more responsive
to changes. Generally, you will want very fast attacks (e.g. 5ms)
and longer releases (e.g. 150ms). This depends on what you are trying
to compress, however. For example, you can use much shorter releases
on percussion than on vocals.
The Output control lets you set the level that will feed out of the
compressor and back into your mixer channel allowing you to make up
for the overall amount of gain reduction that results from the initial
compression. Some compressors have an Auto Gain Compensation (AGC)
feature that will automatically calculate this overall reduction in
gain and boost the output by that amount, allowing you to maintain
a stronger signal without complex manual settings (Figure 1).
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| Figure 2 |
Figure 3 |
KNEES
Hard Knee and Soft Knee refer to the way in which the compressor treats
the signal around the threshold. A Hard Knee (Figure 2) will apply
the full amount of compression to the signal the moment it crosses
the threshold, while a Soft Knee (Figure 3) might start at a 1:1 ratio
and gradually increase to the final ratio, resulting in a more gentle
effect. A Soft Knee setting is often useful at higher-ratio compression
settings (e.g. 15:1) for smoother results.
Figure 4
PEAK AND RMS
The Peak setting detects signal peaks and applies compression to them.
In contrast, the RMS (Root Mean Square) setting detects the average
level of the signal over time and applies compression based on this
average. A combination of Soft Knee and RMS (Figure 4) is useful for
gently shaping the dynamics of vocals, for example, while a Hard Knee
with Peak is more often used to control signal spikes and transients
to prevent overloads and distortion, perhaps on a percussion track.
Figure 5
CHAINED TOGETHER
If you need to boost your source track because the overall level is
too low, with occasional signal spikes, consider addressing each problem
in separate stages with a plug-in chain (Figure 5). You can use the
first compressor in the chain to control the spikes followed by another
compressor to gently decrease the dynamic range and bring up the overall
level.
Audio compression is extremely important for editors to understand,
since pros use it on every project. We've barely scratched the surface
here. Unfortunately, real-world audio fixes do not always fall into
neat and tidy presets. Understanding some of the science behind the
controls will help, but there's still no substitute for practical
experience.
Jeff Earley is Director of Digital Juice
Audio Products.
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