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Signal Processing For Stage:

Guitar Center ProfessionalNov. 4, 2016

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The proper use of signal processing in many settings can be confusing even for experienced engineers. To be clear, there are creative uses of signal processing with effects like reverbs, delays and more, but we won't be getting into that here. Instead, we're taking a look at one type of signal processing that's necessary for a positive audience experience (and the safety of your sound system)—dynamics processing.

Getting Dynamic With Compressors

The easiest way to understand what a compressor does is to imagine a tiny little audio engineer who lives in a box. His or her sole job is very clear: to keep the perceived volume (loudness or quietness) within a certain range. Our imaginary little engineer has incredibly fast fingers that are always on a fader, and can react faster than any real human to adjust the signal up or down, keeping it within a range that sounds good for the overall performance.

Let's look at a specific example. In a rock band, not all sounds are created equally. A bass player can be thumping along and then decide to start slapping in the chorus, creating a much higher perceived volume. A singer can be practically whispering throughout a verse, and then start screaming his way through the bridge. You get the idea. Music itself is dynamic, and we agree that using a full dynamic range is a good practice. However, we've all been through the unpleasant experience of hearing a poorly mixed band, where the drums overwhelm the rest of the music, or the vocals can't be heard at all over a feisty guitar player. What compression does is help even out the volumes of these individual sources so that the live sound experience is a little bit closer to the smoothness of recorded music.

Compressors are the key to creating this professional live sound presentation. They are particularly important for sources that have the highest levels, such as kick drums and bass. They are also crucial for sources with the widest range of dynamics, like the human voice. Please be aware that a compressor is not just a limiter, which is a device that stops audio signals from going above a certain range. Compressors also can raise the level of quieter sources so they, too, can be heard clearly.

Finally, from a practical point of view for live sound, compressors can be crucial to protecting the rest of your live sound system. A sudden burst of a very loud signal can damage everything from your console or mixing surface to your entire main PA and stage monitoring system, as well as having a detrimental affect on the audience's ears. It's very important to use compression for this reason, if no other.

Under Control

One interesting thing about dynamics processing: if a high-quality compressor is set correctly, you shouldn't hear it working. It will just do its job, and everyone will appreciate its work (whether they notice it or not). Let's take a look at what kind of controls are offered on a typical compressor, and how they work together.

"Threshold" allows you to tell the compressor the level at which it can start reducing the amplitude of a signal. "Ratio" lets you set the gain reduction with a ratio. Example: a 3:1 ratio means that if the signal is 3dB over the threshold you set earlier, the output will be 1dB over the threshold. "Attack and Release" lets you control how quickly the gain reduction starts and stops working. "Knee" allows you to choose how the compressor responds to signals that cross the threshold. Finally, "Output Gain" or "Makeup Gain" takes the compressed signal and boosts it so you have the right amount of volume to fit into the mix.

Plug It In, Plug It In

For a long time, live sound was an area where only hardware devices were trusted for their essential purposes. However, like most other areas of professional audio, software-based plug-ins have made their foray into live sound in recent years. Technology, such as Avid's VENUE live sound systems, has allowed performers and engineers to take advantage of the software-based signal processing that they use in studios while playing live. Companies, such as Waves, McDSP and others, have created software-based dynamics processing plug-ins that work exceptionally well.


Can't I just have a single compressor that covers the entire mix?

While it's important to have a compressor available for the complete mix as needed, each source is different in terms of its need for compression. Some instruments and vocalists might not need any compression at all. Others (kick drum, bass, often vocals) need their own types of compressors, each with their own settings, for a professional stage sound presentation. Also, different players tend to play … differently. Some hit the drums or strings harder; others don't. It's a wise plan to have compression available based on the style of the player, rather than an overall plan based only on the types of sound sources.

Why did everything sound terrible one time when I used a compressor?

As we said above, compression is perhaps the most misunderstood tool in the audio engineer's arsenal. Two things can cause a compressor to do more harm than good: the use of a poorly made, cheap compressor which doesn't have enough quality in its parts or design to meet your needs, or (more likely) having an inexperienced engineer who doesn't understand how to best set a compressor for each source and/or the mix itself. The most common way to misuse a compressor is to overuse it. Be conservative with your approach to compression and it will be your friend.

Can software-based dynamics processors really be trusted for live sound applications?

Live sound—both touring and fixed installations—was one of the last holdouts for use of software-based processing, especially for needs like compression. However, over the last decade or so, plug-ins have proven themselves in the largest venues and most high-profile tours. It's safe to say that if you want to build a system that uses either software exclusively or in combination with hardware gear, you can trust these tools on your most important live sound jobs.

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