Signal Processing Topics:
» Compression - The Ins & Outs
» Reverb - When is enough, enough?
» Understand Exciters
» When Should I Use a Gate?
» How Does Feedback Reduction Work?
» EQ types explained
» Understanding PA Management

From compression and limiting to EQ and reverb, Signal Processing offers you the opportunity to create your own signature sound. This Sweetwater Buying Guide includes information that can help you choose Signal Processing for your needs. Since there's so much to consider when purchasing Signal Processing, don't hesitate to call us at 1-800-222-4700 for more information.

Compression - The Ins & Outs.

Compression is perhaps one of the most high profile audio tools in recording, yet it's often surrounded by a great deal of mystery. Beyond its capacity to perform some very necessary functions in recording, there have emerged a number of live sound usages as well. Naturally, being used for music, the functions of engineering and creativity tend to overlap, but for our purposes, a basic understanding of compression is necessary before we can delve into the more creative aspects.

A perfect compressor is an amplifier where the input/output ratio is constant: So when using a 2:1 compressor, increasing the input by 2 dB gives a corresponding output increase of 1dB, or one half the output versus the input, hence, the 2:1 ratio. Similarly, a ratio of 4:1 means that an increase of 4dB results in an output increase of 1dB. These figures are called compression ratio. The higher the compression ratio, the smaller the dynamic range. At compression ratios of 20:1 or infinite, the compressor acts as a limiter.

Compressor Control Explanations

	Threshold The compressor threshold sets the level where compression begins. When an input signal rises above the threshold setting, it becomes "eligible" for compression. Basically, the lower you set the threshold, the more input signal becomes compressed. (If you have a ratio setting of greater than 1:1.)
	Ratio Ratio sets the compression slope. This is defined as the output level versus the input level. As stated above, with a ratio of 2:1, any signals above the threshold setting will be compressed at a ratio of 2:1. This simply means that for every 1dB of level increase, the output will increase 0.5dB, thus producing a gain reduction of 0.5dB. As you increase the ratio, the compressor gradually becomes a limiter. For example, if you have the threshold set at 0dB and the ratio turned fully up, the compressor becomes a limiter at 0dB. This means that the signal will be limited to an output of 0dB regardless of the level of the input signal.
	Attack Attack sets the speed at which the compressor acts on the input signal. A slow attack time allows the beginning envelope of a signal (commonly referred to as the transient) to pass through the compressor uncompressed, whereas a fast attack time almost immediately subjects the signal to the ratio and threshold settings.
	Release Release sets the length of time the compressor takes to return the gain reduction back to normal (or 0) after compression. Very short release times can produce a choppy or jittery sound, especially in low frequency instruments such as bass guitar. The longer the release time, the less obvious is the compression. Very long release times can result in an over compressed sound often referred to as "squashing" the sound.

Hard Knee vs. Soft Knee
The terms, "hard knee" and "soft knee" refer to compression curves. The "knee" refers to the rate at which gain reduction occurs. With hard knee compression, the gain reduction occurs instantly when the input signal reaches the threshold. With soft knee compression, the onset of compression occurs gradually after the signal exceeds the threshold, which produces a more musical result (according to some people).

Compressor Side Chain
This is an insert jack found on some compressors that interrupts the signal that the compressor is using to determine the amount of gain reduction. When no connector is inserted into this jack, the input signal goes directly into the compressors control circuitry. When a connector is inserted into this jack the signal path is broken. The side chain input functions similarly to an "effects send" on a console. Using a tip-ring-sleeve connector, the input signal can be processed by an equalizer for example to reduce sibilance (de-essing) and then fed back to the compressor. The way this works with vocals, is that the equalizer is set to boost the offending frequency (around 4kHz), which will cause that particular frequency to pass the threshold causing the compressor to reduce gain only when that particular frequency is present, allowing the rest of the vocal track to pass through un- affected, thus keeping the dynamic range of the track and de-emphasizing the offending hiss. Since the signal entering the side chain is not heard, the processed signal does not affect the music.

Amounts of compression
In rock music, it is possible to use considerable amounts of compression (10dB) and still cause the effect to be slight. In essence, there can be no rule as to the correct amount of compression for any particular program material. This is where compression becomes a creative effect for the producer. For example, we discussed problems that occur when using very short release times. However, a short release when used with a very slow attack on drums will allow the transient to pass through uncompressed emphasizing the attack, while the pumping effect of the short release contributes to musical drive.

TC Electronic M-One XL

Reverb - When is Enough, Enough?

After equalization and compression, the most frequent effect live sound engineers rely on is reverb. In order to use this powerful tool to its optimum level, it’s important to understand some basic concepts.

Reverberation is the continued sound that exists in a room after the source of the sound has stopped. We've all heard it when doing something like clapping our hands (or bouncing a basketball) in a large enclosed space like a gym. All rooms have some reverberation, even though we may not always notice it. The characteristics of the reverberation are a big part of the subjective quality of the sound of any room in which we are located. Reverb is sometimes mistakenly called echo (which is an entirely different phenomenon).

Our brains derive a great deal of information about our surroundings from the sound of a room and its reverberation. Consequently you need to make careful choices when you enhance a room’s natural reverberation with electronically generated reverb. Here are some tips to help you keep reverb from muddying up your live sound:

Alesis NanoVerb

Use short reverb decay times for up-tempo songs. Try Room, Gate or Early Reflections programs with a decay time of up to 1.5 seconds. A longer reverb will result in a loss of clarity. Longer reverbs might work well for slow tempo compositions like ballads and ambient mixes. Try a plate or hall setting for the best choice. Even the economy-priced Alesis NanoVerb offers a variety of plate reverbs.

Using EQ on reverb can help control the effect by restricting its bandwidth, to avoid muddying up your sound. "Natural" reverberation has a fairly low bandwidth, so if you're trying to keep the sensation that you are playing in a real room, use a high-frequency bandwidth of no more than 8kHz. On bass instruments (kick drum, bass, synth or guitar), for example, try bass roll-off using filters or EQ at around 80-100Hz. This stops them from competing with low-frequency reverberation decay. Remember that when you apply reverb it will affect the listener's perception of the whole mix, not just the instrument you are treating. Unless it's being used as a creative effect in a sparse mix, long reverbs do not work well on bass sounds.

Eventide Eclipse

Reverb has a spatial effect on the sound as the listener perceives it, by placing voices or instruments near (less reverb) or farther away (more reverb). Make sure that you are creating the right environment for your sound within the context of the mix and the room. Some mix engineers put loads of reverb on the vocals, yet the voices are still at the front of the mix. Achieve this by using pre-delay, typically 45-120mS, to distance the dry sound from the effect. Vocal reverbs with a full bandwidth will emphasize sibilance, so be careful about too much high-frequency signal! The Eventide Eclipse provides reverb with full EQ control over individual channels, plus other effects.

Understanding Exciters

BBE 882 Sonic Maximizer

An Exciter (or enhancer) adds a particular type of sparkle to sounds, particularly voices. It appears to create brightness from sounds that were "flat." The exciter works by picking off the higher frequency part of the sound, compressing and dynamically altering it, filtering off the original sound and remixing the resultant harmonics back with the signal. It adds high frequency sparkle, making singing voices or instruments sound more present and exciting without adding the hiss and phase problems you get from simply boosting high frequencies with an equalizer. Used properly, it can create beautiful sounds. Overused it can be horrible.

When Should I Use a Gate?

Drawmer DS201

Noise gates are a type of compressor (or expander) often used to automatically mute the signal from a temporarily unused mic or other input source. When one mic’s signal level drops below a certain point, that mic’s input is cut ("attenuated") fairly quickly to eliminate the background noise it contributes to the mix. The number of open mics in your system reduces the total available gain before feedback, so keeping the input path as clean as possible is important. Gates are often used on drum mics to prevent bleed from other nearby drums and other instruments, and they are sometimes used on noisy sources like guitar amps so when the desired audio signal stops the noise is automatically muted. By adjusting the "gate" level (the point at which the signal is reduced or cut off) you can achieve a natural-sounding mix without excessive noise and bleed.

Gates can also clean up vocal mixes by (for example) taking background vocalists’ mics and letting the gate attenuate them during those parts of the song that are "silent." This can eliminate a lot of track mutes and un-mutes while controlling extra open mics onstage. A well-known and often used noise gate is the dbx 1066.

How Does Feedback Reduction Work?

Sabine FBX1200

Feedback is the bane of almost all PA systems. It can take a great performance and turn it into a painful and embarrassing experience for the performer, audience, and the sound operator. Up until a few years ago there was little that could be done electronically about feedback except rudimentary efforts using EQ. With the advent of Digital Signal Processing (DSP), automatic feedback elimination has been made possible. Unfortunately, many of these earlier products did not maintain the sonic integrity of the audio signal because they required wide notch filters to suppress feedback. A new generation of PA management systems such as the DriveRack 260 by dbx, employs an algorithm that solves this problem by using precision frequency detection with adaptive filter bandwidth to activate the minimum number of very narrow notch filters, which will stop the feedback with minimal degradation of the audio signal.

How Acoustic Feedback Occurs
Acoustic feedback occurs in a sound re-enforcement system when the signal output from the speaker is picked up by the microphone and amplified, creating a feedback loop. What results is an audible "squealing" or "howling" of the system. Historically (before automatic feedback elimination), feedback was removed manually using a 1/3 octave graphic or parametric EQ. When feedback occurred, the sound engineer would guess where the feedback was located, and pull down a fader to decrease the gain at that frequency. This method unnecessarily cuts out large portions of the spectrum, and a common complaint of traditional feedback eliminators is that the feedback filters also cut out large portions of the spectrum, which ultimately degrades the audio quality of the system. Unfortunately, another problem with either manual or most automatic feedback eliminators is that by the time the frequency feeding back is detected and action is taken, it's already too late.

EQ Types Explained

An equalizer is an audio device that has the ability to boost and/or cut the energy (amplitude) in specified frequency ranges by employing one or more filter circuits. There are many different types of EQ's in use today in many widely varying applications, but they fundamentally all do the same thing.

Samson S-Curve 215

Graphic EQ: A graphic equalizer is configured to provide a graphic display of the EQ settings. Years ago equalizers were all rotary-knob based. When units began to arrive on the scenes that could EQ 15, 30 or even 45 bands (frequencies), it became difficult to see what was going on at a glance. So equalizers that used sliders instead of knobs were developed and quickly won the favor of engineers due to their improved ergonomics. Engineers liked how easy it was to see the overall EQ curve at a glance, but they also just liked using sliders more than knobs. The ONLY thing that makes an EQ "graphic" is its capability of displaying the curve at a glance. There are graphic EQs that have the same features as "parametric" EQs (see below), but most graphic EQs, however, only give you control of a pre-selected set of frequencies at pre-selected bandwidths. The Alesis DEQ 230 is a digital graphic EQ with 31 bands.

PreSonus EQ3B

Parametric EQ: A Parametric Equalizer has several "parameters" for control of various filters that can be applied to audio frequencies. Parametric equalizers are most widely used in situations where very fine control over the audio signal is desired. In order for an equalizer to be parametric it must at least have control over gain, "Q" (the bandwidth of frequencies being boosted/cut), and frequency. In most cases each of these controls are on rotary potentiometers. Some equalizers have selectable frequencies that can be adjusted, but no Q control. These are known as quasi-parametric or sweepable equalizers.

Understanding PA Management

PA management in short, is handling every detail of setting up and controlling a PA system for live performance. It’s the job of the mix engineer to make sure that there is optimal sound and no surprises. This includes mic choice and placement, monitor mixing including EQ, notch filters, crossovers, and compressors for stage wedges and stereo EQ and compression for in-ear monitors. Setting gain structure and controlling the mix and output to the mains requires tuning the system for room characteristics along with EQ, compression, limiting, and effects processing. All of these subjects require a great deal more space to discuss than available here, but a good starting point for a more in depth discussion is the PA Bible published by Electro Voice in 1979.

dbx DriveRack 260

One of the nastiest live sound surprises is feedback. There’s nothing worse than an obnoxious squeal during an intimate vocal passage, causing the sound engineer to miss a cue while he frantically searches out the offending frequency. Generally, racks of equipment must be used from compressor/limiters, to multi-band graphic EQ, crossovers, delays to time-align monitors and house speakers, along with RTA (Real Time Analysis) gear to tune the room to avoid such surprises while creating optimal sound conditions. Of course, in live performance situations, nothing remains constant, and as any experienced sound engineer will tell you, always expect surprises. Fortunately a technology to manage these aspects of sound reinforcement that was once only available to high-end touring systems is now available to everyone with units like the DriveRack PA and DriveRack 260. Now, the functions of racks full of gear can be had at a very affordable price. These units are exceptional for bands on the local club circuit. A Wizard function allows easy setup of parameters such as automatic RTA, EQ, and feedback elimination. A DriveRack will allow you to save your settings so that when you come back to a club you’ve previously played, the setup can be recalled with just the touch of a button.

dbx DriveRack PA

At Sweetwater, we believe that there is no substitute for knowledge for getting the most out of your gear and to avoid the pitfalls only learned after years of experience. That's why when we find a unit like the DriveRack, which combines years of professional live sound experience and technology in a single rack space we can't help but get excited. Nothing makes us happier than being able to put that experience and technology in your hands to enhance your musical experience. To find out more about the DriveRack PA and DriveRack 260,or any of the other products mentioned in this guide, contact your Sweetwater Sales Engineer today at 1-800-222-4700.