Pro audio signal processor buying guide
What is a signal processor and why would I want one?
In the 1950's, I'd take the family television's vacuum tubes down to Willow Grove Radio and TV Repair, check them with the giant tester machine, buy new replacement tubes, and reassemble the repaired television, so my mom and dad could enjoy their precious, respectively, Dean Martin and Red Skelton shows. In the 1960's, I studied radio and electronics at the Franklin Institute in Philadelphia. After college, in the early 70's, I joined a rock 'n roll band as the soundman, learning how to operate the electronics that make music sound good. Then, I worked in a music store in Austin manufacturing, installing, repairing, and operating sound systems and components for recording studios, nightclubs, and touring bands. I moved back to Charlottesville permanently in 1984 and opened a little demo recording studio. I also attempted to put to practical use the creative writing degree I had picked up along the way. In 2006, I finally came to my senses and got this job at Crutchfield where they actually pay me to ramble on, rant, and explain the things I love about music, electronics, and getting good sound.
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An external equalizer, like the dbx 1215 dual 15-band graphic equalizer, lets you adjust specific frequencies in your music
Simply put, a signal processor is any electronic device that controls or changes the audio signal in some way in order to improve the sound.
The infinite number of different combinations of tone, loudness, rhythm, and timbre in the creation and reproduction of music often makes it necessary to add to or subtract something from an individual sound or group of sounds in order to make the whole song sound right.
Consider a vocal microphone in a live music situation, for example. The microphone picks up much more than just the voice. The other parts of the ensemble's music may also get picked up and muddy the sound — bass does this a lot. One solution is to turn down the bass tone control on that microphone channel, which will clean up the sound considerably.
Here are brief descriptions of the different types of signal processors we'll cover in this article:
- Processing the tone of a signal, even if it's just the treble and bass, is the job of an equalizer.
- Dynamic processing, controlling the relative loudness and quietness of signals, gets accomplished by compressors and limiters (to keep the levels down) and noise gates and expanders (to make sounds louder).
- You add presence and ambience to music with reverb and echo or digital delay processors, or add bass harmony or swirling psychedelic sound effects with a modulation processor.
- And many pros use speaker and PA management devices to perform a few of these and other processing functions all automatically and at the same time, to ensure the sound's quality and the equipment's safety.
Which types of processors you need depends on how much fine-tuning control you want over your music. Most mixers have some control capabilities, and that's enough for some people. But if there's a problem you just can't seem to correct with the mixer, then consider adding one of more of these processors. Outboard signal processors often feature multiple effects capabilities, so you can control more than just one aspect of your music from a single device.
The dbx 215s dual 15-band graphic equalizer lets you instantly adjust any of the frequency bands
Equalization is a process in which the volumes of separate, narrow ranges of frequencies in a signal get adjusted, boosted or cut, so the resulting program sounds balanced, or equalized, with all the frequencies, from low to high, playing at their best volume for each instrument's timbre and tone, and the room's acoustics. You can think of multi-band EQ as tone control on steroids — instead of the high, mid, and low tone controls of a mixer's standard 3-band EQ, an outboard graphic equalizer can offer, for instance, 31 bands of control over the audio spectrum.
You need an outboard equalizer if you want finer, more detailed tone adjustments than just the treble, midrange, and bass control you get from your mixer. Here are common situations where you need an equalizer:
- Individual instrument tone – One reason to use an equalizer is to adjust the tone of an individual microphone or instrument channel to sound its best. We already mentioned how a bass control can clean up a vocal track. You can also add some oomph to a kick drum and sizzle to a high hat with some EQ adjustments on your mixer.
- Tone of a PA in a room – Another function of an equalizer is to match the sound to the space it's played in, compensating for room acoustic problems like the peaks and nulls created by reflections and absorptions. A high degree of tone control is necessary in order for a PA system to sound good in a difficult room or if it has to play in a different room every night.
- Onstage feedback control – Using an equalizer is crucial when running stage monitors. The fine control over separate frequencies lets you dial in vocal sound that cuts through the rumble and roar of a band on stage. You prevent acoustic feedback — that horrible ringing that occurs when a mic picks up its own sound from a speaker — by using an equalizer to turn down just the notes that start to ring.
There are two kinds of equalizers: "graphic" and "parametric." A parametric equalizer allows you to adjust all three parameters of equalization of each of its bands. You can select a band's center frequency (what note you're trying to adjust), the width of the adjustable band (how many notes will be effected, also called "Q"), and the volume of that band (boost or cut). This means you can dial in a wide, gentle EQ curve to bring out a vocal, for instance, or you could create a sharp, narrow notch in the sound to eliminate a specific noise.
A graphic equalizer is easier to use, with a control for each of its preset bands that graphically shows you at a glance what its frequency response is by its position on the front panel. Graphic equalizers generally come in 15-band or 31-band models and feature a vertical slider to control the level of each band. With a 15-band EQ you can fine-tune a PA or monitor system to sound tonally right and prevent most feedback issues. The finer resolution of a 31-band equalizer gives you twice ability to focus in on a problem and solve it without affecting the rest of the sound.
A dynamic processor controls the relative loudness and quietness of the signals going through it to clean up and smooth out rough and overly dynamic music, or to just turn the volume up or down automatically. You adjust a dynamic processor to automatically turn down a signal with a compressor or limiter, and to turn it up with an expander or noise gate.
A dynamic processor, like the dbx 1066, has separate controls for all three functions, as shown in this close up of one of its two channels
A limiter acts like an automatic volume control — the incoming signal level never gets bigger than a set threshold level. No matter how loud the input gets above that threshold, the limiter's output will remain at that level and no higher. This is great for protecting speakers, and limiters are often featured in PA amplifiers and powered speakers for just that reason.
A limiter has three controls:
- The "threshold" control, which tells the limiter at what input level to take effect,
- An "attack" control that adjusts how fast the limiter comes on,
- And a "release" to adjust how long it will remain on after the signal falls below the threshold level.
These controls allow you to set your limiter to sound natural and not so noticeable and mechanical.
Take a limiter, add a "ratio" control, and now you have a compressor.
The ratio control adjusts the amount of compression the device will exert over the signal. A compressor lets you control how much the sound can exceed the threshold, either a lot (very little compression), just a little (tight compression), or not at all (like a limiter).
A limiter is actually a compressor with a "hard" or "brick wall" ratio of infinity-to-one. It would take an infinite increase of input volume to produce 1 dB of output gain. Any compressor with a compression ratio of 20:1 or higher is considered a brick wall limiter (actually, most techs consider any compression ratio of 10:1 or higher as limiting). Lower than that, and we enter the world of compression.
Another feature you may want for your compressor/limiter is a "sidechain" connection, which gives the unit the ability to accept a controlling or triggering signal to make it automatically cut in. For instance, a microphone can be set to automatically turn down the music when the DJ talks — this is called "ducking." Another example is a specialized "de-essing" function that uses an EQ in the sidechain of a compressor to find and eliminate unwanted high frequency sibilance, the hissing of s's.
Noise Gates and Expanders
Just as a compressor/limiter acts like an automatic volume control to turn down a signal, a noise gate/expander turns it up.
- With a noise gate, a microphone will be completely off until its signal reaches a threshold. Then the noise gate turns the mic completely on.
- With an expander, the mic starts off turned down some, but not completely off, until the threshold is reached. Then the mic is turned on full.
Noise gates and expanders provide a versatile and practical way to control the sound in live, multi-microphone setups, so the sound of one instrument won't get picked up by another instrument's microphone when the second instrument isn't playing. This cleans up the overall sound, keeping each channel clear of the other channels' content. It also saves the soundman some time, for instance, by automatically turning a vocal mic down when the singer isn't singing so the drums won't sound through that mic.
Noise gate/expanders are almost always used on individual microphone or line input channels, so your mixer will have to have insertion jacks (I/O) on its inputs in order for you to use them. As on compressor/limiters, attack and release controls let you adjust a gate/expander to sound natural and not hear it "pump" on and off. A ratio or depth control turns a hard noise gate into a soft expander, to attain even more natural-sounding dynamic control.
Many dynamic processors include both gate/expanders and compressor/limiters in the same package so they'll work together. The signal goes through the noise gate first, so it gets turned on when necessary, and then on to the compressor for its dynamic sculpting. Some noise gate/expanders feature sidechain or "key" circuitry so they can be controlled remotely to engage or disengage in coordination with other effects.
The Lexicon MX200 multi-effects processor lets you add various audio effects to your music
An effects processor adds something to the sound that recreates a sense of space or distorts the signal in some way for a special audio effect. Here are some common types of effects:
A reverberator or reverb unit creates sound that imitates the sonic reverberations of a room, the sounds that remain after an original sound stops. It comes in many flavors, imitating different size and reflective spaces, different kinds of reverb generation, optimized for drums or vocals, or artificially cut to produce snappy percussive sounds, for example. Typically, a multi-effects processor offers a preset selection of different reverbs and has a few controls that can modify each effect's tempo, decay, depth, or some other variable.
Reverb is non-uniform and chaotic, like natural room reverberation. A digital delay reproduces an exact duplicate of a sound and plays it back as an echo delayed by almost any time you set. A digital delay unit can then manipulate that echo to repeat with a gradual decay, for instance, or play back with a totally different tone. "Slap back" echo, a favorite in rockabilly recordings, consists of one or two very quick and crisp echoes, emulating the sound of a gymnasium, that impart excitement to up-tempo songs.
Another delay effect is called "chorusing" or "doubling." A chorus echo makes a single voice sound like a whole chorus of voices by delaying and slightly detuning a multitude of delayed signals. Doubling makes a soloist sound like a duet. Another kind of digital delay device is a "tapped" delay, which lets you "tap" a signal at multiple times after its initial attack for some very natural-sounding echo effects.
Harmonizing and pitch correction
Not only can you chorus a voice or instrument, you can create 3-part harmony, for instance, from a single voice with a "harmonizer." You can also add bass parts pitched below the original. These processors often also feature pitch correction, which can create an on-key version of a less-than-perfectly sung performance.
Modulation and distortion effects
And finally, there're effects processors that make unique and strange sounds that are completely unknown in nature. "Tremolo" or "Leslie" effects quickly and repeatedly raise and lower the pitch of a sound, giving it an unworldly, science fiction feel. "Flanging" is a distortion effect that seems to make a sound flare and swirl around in space. And "wah wah" aptly describes what such a device does to sound, like flapping a mute on and off in front of a trumpet.
Speaker and PA management systems
Management systems like the dbx DriveRack 260 are all-in-one processors that give you many audio tools in one box
A speaker or PA management system is a signal processor, connected between the mixer and the amplifiers of a PA system, that combines the features of many of the processors previously discussed all in one box, in order to keep the final mix sounding good and the speakers safe from damage. These features include compression, limiting, and equalization all dedicated to matching the music to a venue. A PA management device could also provide the crossover and signal-routing function for a large multi-speaker PA system.
Some PA management devices include automatic feedback suppression, which senses whenever a note starts to ring out that way and turns it down. A very handy feature to look for is a built-in RTA (real time analyzer) and auto-EQ to measure and automatically adjust your system's frequency response to attain the best-sounding curve possible, even if it plays in a different room every night. If the venue is large, you may want to think about including a time alignment/delay feature, so speakers in the rear can be coordinated with the front speakers and sound clear, not echo-y.
Signal processors provide solutions to specific problems
Each kind of processor mentioned in this article is designed to control overly fluctuating sound or add something that was missing — in other words, a sound processor is only needed when there's something to fix.
Unfortunately, it's all too common for recorded or live music to sound dry and devoid of physical and emotional context because the microphones were so close to the sources of their sounds they didn't pick up the ambiance or excitement of the performance. Effects processors restore those feelings while shaping the sound to fit both the equipment and the space.
If you need help choosing a signal processor or deciding whether you need a focused, single-effect device or a multi-effect processor, contact our advisors. They will be happy to discuss your system, what you intend to do with it, and help you determine which kind of processors (if any) you need.