Whether it's used for cleaning up a problem track, enhancing an instrument sound, or smoothing out a master recording, a flexible and versatile equalizer is an indispensable part of any musician's toolbox. Although hardware EQs are still the preferred tool for a lot of engineers, producers, and musicians, a rapidly increasing number of them rely on EQ plug-ins. Given the power of today's computers, sometimes enhanced with DSP cards, that makes a lot of sense both technologically and financially.
For starters, you can make as many simultaneous instantiations of the same plug-in as your computer will allow. In a complex mix, that could be the equivalent of having a roomful of expensive analog devices. Automation is another huge advantage the digital realm provides; we take for granted the ability to automate virtually any parameter of any plug-in processor in a mix and to save complex setups for later recall, but only the very top-end digitally controlled analog systems can come close to that level of control.
As with hardware EQs, prices for EQ plug-ins vary widely. In this article, I'll take a look at whether you get what you pay for and try to ascertain whether some of the less expensive programs provide superior value. But whichever EQ plug-ins you buy — and you may want several of them — be sure you choose them for the right reasons.
THE RIGHT TOOL FOR THE JOB
As is so often the case with gear, the key with hardware or software equalizers is to find products that not only are affordable and sound good but are appropriate to your application. EQ can be used for corrective processing, such as rolling off low-frequency rumble; for tuning a control room; for mastering; or for creative processing, like making a drum sound unnaturally bright. This means that you probably should have more than one type of EQ in your bag of tricks, and you should not be afraid to use the various types in creative ways.
Equalizers that are used for tracking and mixing are generally referred to as program EQs, but it's a mistake to think that slapping that label on the EQ defines its sole purpose. For instance, let's say you are processing a track and want to slightly enhance its overall sound, as well as boosting or attenuating particular frequency bands. In that case, you might want an EQ that is designed to color the sound in a desirable way. But if you are trying to control a problem frequency without otherwise altering the timbre, you probably want a more transparent EQ that does not impose a sonic signature.
The same applies to mastering EQs. Most of the time, given a good mix to start with, you want to make very precise adjustments, and you don't want to color the overall mix. In that case, a clinical, precise, transparent EQ is probably the right choice. It will not sweeten the sound, but it can solve your problem. However, sometimes you are mastering a mix that could use some more “wow” factor — that extra sonic sweetening that makes it sound just right — and adding a bit of color is one way to accomplish that. In that case, you don't want a transparent EQ, and you might decide to master with an EQ that was designed for program use. There may be guidelines, but there are no hard-and-fast rules.
THE INSIDE STORY
Digital EQs work by applying complex mathematical processes (algorithms) to the incoming stream of numbers that represents a digital audio signal. One popular type of filtering algorithm is called finite impulse response (FIR). Some FIR filters are designed such that they do not smear the phase within the signal, which means that material with sharp transients, such as snare drums, can pass through the filters and still maintain the relative time positions of their complex series of frequencies. These filters are said to have a linear phase response; in other words, they eliminate phase distortion. In addition, they require a good deal of computer horsepower and RAM, and that means you'll get fewer instantiations on a given system. Note that not all FIR filters offer linear phase response, but the two FIR-based plug-ins we tested do.
As I worked with both FIR-based plug-ins, I discovered a few drawbacks for use in multitrack applications: in addition to being CPU-intensive, FIR EQs generate significant latency as an inherent part of their design. The latency is enough to throw an equalized bass part completely out of time relative to the unequalized drum tracks. You can certainly compensate for that by using plug-ins like Digidesign's Time Adjuster to delay the unequalized tracks so that everything is late together, and some programs, such as Steinberg's Nuendo, can automatically make this adjustment. That helps tremendously at mixdown, but for overdubs, latency remains a problem. However, this style of plug-in is usually chosen for applications such as mastering, where the latency becomes irrelevant. Again, it's just a matter of choosing the right tool for each job.
Another primary digital filtering algorithm is known as infinite impulse response (IIR). IIR filters require less computation and exhibit less latency than FIR filters but have their own drawbacks. These filters feed some of the filter's output back into the input in a process known as recursion. Recycling some output data is a good substitute for using more of the incoming data, and many digital EQs use this approach. Unfortunately, the trade-off is that IIR filters cause phase distortion, which can smear transients and cause problems with the stereo image. Until recently, most EQ plug-ins used IIR algorithms for efficiency's sake, but advances in computing horsepower are causing more FIR-based plug-ins to enter the market.
Instead of FIR- or IIR-based algorithms, one of the plug-ins in this group uses Fast Fourier Transform (FFT) algorithms, which analyze the signal and then use that information to set the filters. It exhibited the same kind of latency characteristics as the FIR plug-ins.
THINNING THE RANKS
One of the biggest challenges in creating this article was determining which products to test. My goal was to be as fair and impartial as possible while representing a good cross section of manufacturers and design approaches. I decided on a strict set of guidelines. First, to appeal to the greatest number of readers, all of the plug-ins had to run under Mac OS X and Windows or be offered in multiple formats, such as TDM, VST, and AU. That eliminated McDSP's FilterBank, which has received high praise in the Pro Tools community but is only available for Pro Tools on the Mac.
Each plug-in had to be a commercially available product (not shareware or freeware), and it had to be shipping or in late beta as of this writing. That eliminated a few more programs such as the new plug-in line from Universal Audio. I further narrowed the field by allowing only one product per manufacturer. If more than one was available, I chose the newest product. That's why, for instance, we evaluated Waves' Linear Phase EQ rather than its well-established Renaissance EQ or classic Q10.
When the dust finally settled, I ended up with an interesting and varied collection of seven EQ plug-ins from famous and not-so-famous developers, offering products ranging from less than $70 up to $850. The list came down to Anwida Soft's GEQ31V, Bomb Factory's Pultec EQP-1A, Elemental Audio Systems' Firium, Focusrite's d2, INA-GRM's GRM Tools Equalize ST, Sony's Oxford OXF-R3, and Waves' Linear Phase EQ.
THE LISTENING TESTS
All of the listening tests were done at my studio, Perceptive Sound Design, in Mill Valley, California, by a panel of pro recording engineers and musicians (see the sidebar “The Panel”). The professionally designed space is acoustically treated and big enough to minimize volumetric frequency aberrations. The signal chain began with a Pro Tools HD system running Pro Tools 6.0.1 on a dual-processor 867 MHz Mac G4 under OS X 10.2.5 and through a Digidesign 192 I/O. The signal then went to the monitoring section of a Digidesign Control|24 and directly to a pair of Genelec 1031A monitors. VST-based plug-ins were auditioned from BIAS Peak via the Digi Core Audio driver and through the same signal chain.
We listened to a wide range of source tracks — acoustic guitar, kick drum, snare drum, saxophone, female vocals, and full mixes — through each EQ, making notes and discussing the results as we went. To eliminate phase complications and to widen the listening area, all recordings were mono. Each panelist had hands-on experience, adjusting the knobs and sliders as the rest listened.
As with all aspects of music, taste in EQ is highly subjective. Some panelists clearly liked certain things more than others did. Overall, however, the panelists were in general agreement and expressed similar impressions. Here's what we found.
Anwida Soft is an Italian company that has been working in digital audio for eight years. Its inexpensive ($69) GEQ31V graphic EQ is available in VST 2.0 format for the Mac and PC. The plug-in emulates a standard ⅓-octave, 31-band analog graphic equalizer, down to the realistic look and feel of its front panel (see Fig. 1). It supports up to 24-bit, 96 kHz mono or stereo audio and has a simple set of controls consisting of 31 boost-cut sliders, a button to select 6 or 12 dB operation, and an output-gain knob.
Unfortunately, this plug-in received low marks for its sound quality. The listening panel described it as “harsh, edgy, woofy, digital-sounding,” and generally “not that musical.” On the other hand, GEQ31V was designed to provide transparent, colorless, accurate processing, not to add character to the sound. Its 64-bit internal processing path is designed to create as distortion-free an output signal as possible. The panel noted that it sounded better in cut mode than in boost mode.
In addition, the interface was rather cumbersome to work with: you can grab and move only one band at a time, and there's no easy way to reset the bands to zero. We concluded that this plug-in would be strongest in the realm of corrective processing and room tuning. Keep in mind that fixed-bandwidth EQs typically sound different than parametric EQs, in part because of their different design philosophies. Despite its shortcomings, however, GEQ31V's simple interface and low price make it a viable choice as a back-to-basics plug-in for musicians on a budget who want a different flavor than their audio-editing software's stock EQ provides.
The original Pultec hardware equalizer is the stuff recording legends are made of. It's a tube-based passive equalizer long out of production that now fetches thousands of dollars on the Internet. The EQP-1A is not meant to be clean or clinical; it has a sweet, thick, colored sound, capable of hyping the lows or the highs.
Bomb Factory's plug-in emulation of the EQP-1A (high-definition version $399; 48 kHz or lower $249) attempts to capture the sound, look, and quirks of the venerated hardware machine. The front-panel interface has been beautifully rendered, right down to the faceplate color and type fonts (see Fig. 2). As with the real unit, there has been no attempt here to make a clean, flexible, jack-of-all-trades processor. Instead, EQP-1A has a particular sound that is highly useful for vintage emulation.
EQP-1A has a rather unconventional system of controls. The low-frequency selection knob identifies a target frequency that the separate low-frequency boost and cut knobs work from. That's right, you can boost and cut the same low frequency at the same time. For the high end, a high-frequency selection and boost knob is provided along with a high-frequency bandwidth (Q) knob. A separate high-frequency attenuation knob has its own separate high-frequency selection knob. Confused? Once you start working with the various knobs, it makes some sense, but initially it's far from intuitive.
EQP-1A was fun and colorful in some applications, not as useful in others. Some panelists liked the sound more than others, but everyone agreed that it was warm with a huge low end. Some listeners liked the warmth that the high band applied to the signals, others found it dulling. Everyone was surprised that it sounded pretty good on full mixes of music, although it would not normally be anyone's first choice for that application. (According to Bomb Factory, the original Pultec EQ was actually designed for equalizing full program material for mastering and broadcast, and it's still used that way in some circles.) The opinion was unanimous that this EQ was highly musical and well suited to creating colorful tone shaping for a variety of instruments in rock and pop styles.
Anwida SoftGEQ31V31-band ⅓-octave graphicVST$69Bomb FactoryPultec EQP-1A4-band parametric hybridAudioSuite, MAS, RTAS, TDM$399 (high definition); $249 (48 kHz or lower)Elemental Audio SystemsFiriumlinear phase stereo masteringAU, VST$99Focusrited26-band parametricAudioSuite, RTAS, TDM$595 (bundled with d3)INA-GRMGRM Tools Equalize ST31-band ⅓-octave graphicAudioSuite, HTDM, RTAS, VST$549 (bundled with 3 other plug-ins)SonyOxford OXF-R37-band parametricAudioSuite, RTAS, TC PowerCore, TDM$850WavesLinear Phase EQlinear phase 6-band parametricAudioSuite, DX, MAS, RTAS, TDM, VSTpart of Master's Bundle: $900 (native); $1,800 (TDM)
ELEMENTAL AUDIO SYSTEMS
As you might have guessed from the name, the bargain-priced Firium ($99) is an FIR-based plug-in from newcomer Elemental Audio Systems. Firium is a linear-phase EQ with some novel features (see Fig. 3). Like GRM Tools Equalize ST, Firium displays a spectral analysis of the audio output. Superimposed over the depiction of the output is a line of 50 connected balls that represents Firium's EQ curve. You change the EQ by drawing a line on the screen. The spectral analysis of the audio updates to reflect the effect of the EQ on the signal.
Firium's FIR algorithms have a clean character that is suited to the subtleties of mastering. It is also able to significantly change the character of a snare drum, although it wouldn't necessarily be my first choice for that application.
Firium's user interface is clearly oriented toward stereo processing, and it offers a variety of linking modes connecting left and right channel adjustments. The left and right spectral analysis and EQ curves are displayed in different colors, and you can switch rapidly between sides for precise adjustments. Handy sliders for scaling and smoothing the EQ curve are also a great touch.
Firium's purpose is mainly to produce colorless, phase-accurate mastering, and it exhibited some of the same sonic characteristics found in the other FIR-based mastering plug-in we tested. However, one panelist characterized it as having “muddy lows,” while another described it as having a “cloudy low end and thin high end.” The impression of a “thin” high end apparently reflects the fact that Firium did not enhance the highs, and that's fine, because it isn't supposed to do that. We were more concerned about the lows, because if this truly is a transparent EQ for mastering, the lows should not be muddy or cloudy.
The panelists found Firium to be more useful for subtle applications than for making extreme changes. Most of the listeners considered it to be a viable and useful product, and everyone loved the user interface. It's also the only plug-in in the roundup that currently supports Apple's Audio Units (AU) standard.
Focusrite d2 ($595, bundled with the d3 compressor-limiter) is an EQ plug-in that has been around for several years and has seen wide distribution. The result of a collaboration between Focusrite and Digidesign, d2 is modeled visually and sonically on Focusrite's Red 2 hardware equalizer (see Fig. 4). The d2 plug-in is available for all flavors of Pro Tools: TDM, RTAS, and AudioSuite for Mac and PC.
This 6-band equalizer consists of highpass and lowpass, high-shelf and low-shelf, and high-mid and low-mid peaking filters. The EQ does not allow the filter types to be changed (for example, allowing the low band to be selectable as shelving or peaking), which means it's not quite as flexible as it could be. However, the Q setting is adjustable for the peaking filters, allowing curves from gentle slopes to sharp mountains. D2 also features individual or linked stereo input- and output-level adjustments as well as a nice diagram of the EQ curve.
To conserve precious CPU bandwidth, d2 offers three modes: 2-band, 4-band, and 6-band. Each mode takes progressively more DSP power. Stereo instantiations of d2 offer either separate left and right adjustments or a linked option that lets you adjust both channels from one set of controls. If you're a speed demon who prefers keyboard control to mousing around with virtual knobs, you can type all frequency values from the keyboard. For example, clicking on a filter and typing “4k” would set the filter to 4 kHz.
Focusrite d2 has been around long enough that no one really expected it to measure up to the new, hip, high-profile EQs with which we were comparing it. You can imagine our surprise when we consistently rated d2 near the top as a program EQ. Everyone agreed that it had a very clean, solid-state kind of sound. It had a good presence and was quite musical and full. It's a good sculpting tool for sounds and appropriate for a wide variety of applications. We also liked the user interface, which the panelists described as “uncluttered” and “fairly direct” (that is, intuitive). Perhaps this shows that newer is not always better, even in the digital realm.
GRM TOOLS EQUALIZE ST
The FFT-based Equalize ST plug-in is part of the GRM Tools ST (Spectral Transform) bundle ($549). It's a 31-band graphic equalizer with an interesting twist: it analyzes the frequency content of the source audio and shows the results in real time on the display behind the graphic faders (see Fig. 5). That lets you adjust the EQ visually, correcting problem areas in the frequency spectrum that you might not have noticed otherwise.
The user interfaces for GRM Tools plug-ins have always been oriented toward changing the timbre over time; a slider at the bottom of the screen lets you morph smoothly between different settings during audio playback. This approach is terrific for special effects and sound design but somewhat less useful for musical applications unless you are pasting takes together. I found Equalize ST to be great for generating wild, unexpected changes in the audio material. However, I noticed some strange warbling and a cold, metallic character in the upper frequencies.
The plug-in's user interface rated high among the panelists; everyone loved the flexibility and ability to easily automate cool EQ changes over time. The spectral-analysis display of the audio superimposed over the graphic EQ area was similarly impressive. Unfortunately, Equalize ST was not a favorite in terms of sound quality. The consensus was that the sound was a bit cold. In particular, Equalize ST's high end had a ringy metallic edge; in general, the sound did not flatter any of the source material we used. That said, the plug-in is popular with many people involved in techno and experimental electronic-music production, where this sort of sound could be just what you need.
The flexible interface lets you morph and animate EQ settings smoothly, and the plug-in provides power that I haven't seen in any other EQ. If your focus is on techno or experimental music, Equalize ST definitely offers a unique choice.
The algorithms for Sony's OXF-R3 EQ plug-in ($850) are derived from the company's highly acclaimed Oxford digital mixing console. It has seven bands: highpass and lowpass filters and five parametric bands with variable Q, gain, and frequency. The low and high parametric bands can be peaking or shelving EQs. The plug-in can handle sample rates of up to 192 kHz.
Oxford OXF-R3 is clearly designed for serious professional use (see Fig. 6). The carefully chosen frequency bands and relatively fine-resolution Q and gain structure allow for subtle EQ adjustments. The plug-in offers four different EQ types, modeled after various industry-standard analog equalizers. Selecting among the different types alters the relationship of Q to gain, essentially changing the steepness of the Q factor depending on how much gain has been applied. A fifth EQ type, the GML option, is based on George Massenburg's GML 8200 EQ; it's available for an additional $350.
As soon as the panelists started working with Oxford OXF-R3, we knew that the engineers at Sony had indeed built a better digital mousetrap, proving that sonically superior results are achievable with a well designed plug-in on a Mac or PC system. One panelist called it “a good Swiss-army-knife mastering tool” and “probably the best tool of the bunch.” Another called it “very pleasing” and “near our analog benchmark in character.” Another liked the fact that “you could push the EQ to extreme gain changes and have it still be useful.” Everyone agreed that setting up an EQ curve and then switching between EQ types to explore different sounds was immensely cool.
Oxford OXF-R3's user interface received mixed reviews. As in most mouse-based systems, the virtual knobs are difficult to manipulate. If a control surface is thrown in, however, OXF-R3 becomes quite easy to work with. Like Focusrite d2, the Oxford EQ has a number of configurations with different numbers of EQ bands to allow you to maximize DSP efficiency.
I recently mixed an album of classical Indian music by sarod maestro Ali Akbar Khan, using Oxford OXF-R3 as my sole equalizer. The results were excellent; I was able to enhance the high-quality original acoustic tracks with nuance and subtlety. The Oxford OXF-R3 plug-in is the most expensive of the bunch by a decent stretch, but if you can afford it, you should check it out.
LINEAR PHASE EQ
Like Elemental Audio's Firium, Waves' Linear Phase EQ is an FIR-based equalizer that is designed to eliminate phase smear and thus create more accurate, transparent signal processing. The plug-in is part of Waves' Master's Bundle (native, $900; TDM, $1,800). Although Linear Phase EQ is primarily oriented toward mastering, it can certainly be used to process individual tracks as well; its colorless nature would make this EQ a good choice for serious track-related corrective surgery, such as removing DC offset or low-frequency rumble.
The Linear Phase EQ plug-in uses the same “Paragraphic EQ” interface approach that Waves uses in its Renaissance EQ and Q10 plug-ins (see Fig. 7). The front-panel colors are different, and there are now nine EQ curve types instead of three and six bands instead of five, but Waves recognized a good thing and stuck with it. If you've ever used Renaissance EQ, you'll feel right at home with Linear Phase EQ.
Linear Phase EQ's first band is dedicated specifically to low-frequency manipulation and offers very fine control resolution in the low end. The other five bands are more general and work across most of the frequency spectrum. The nine EQ-curve types are selectable per band and feature precision digital curves as well as curves modeled on analog designs that feature a bit of a resonance bump at the filter cutoff frequency.
The plug-in offers three EQ modes: normal, accurate, and low-ripple. Each mode is aimed at different tasks; a different version of the plug-in is included specifically for low-frequency tasks. That version has three low-frequency and no broadband filters, allowing precise manipulation of the low end alone. Needless to say, it is an extraordinarily flexible tool for mastering engineers and other high-end audio professionals.
This is a truly clinical mastering EQ; in spite of its power and flexibility, none of the listeners chose this plug-in as their favorite for program enhancement. One panelist described it as having a “lack of character,” which is exactly right; Linear Phase EQ is supposed to be transparent. Several members of the panel had a problem with this because we are accustomed to using EQs that flatter the material; this was a mistake that you shouldn't make. Remember that each EQ design has its purpose, and this one is supposed to deliver clean, precise adjustments without phase problems, neither more nor less.
Apparently Waves has heard engineers remark about the Linear Phase EQ's “clinical” sound before, because the plug-in also includes filters that were modeled on resonant analog designs. After the initial tests, I examined these filters, and I liked what they did to the midrange and low end. They still would not be my first-choice filters for big, analog sounds, but I liked the sound a good deal more than I did the digital filter curves that we had listened to earlier. When I applied gain to the high end during this second exam, the EQ still seemed a bit too sharp for my taste, even when using the analog-filter model.
I conclude from this that fundamentally, Linear Phase EQ is, as advertised, a superclean, colorless, precision tool that causes no phase distortion. It can be used for some types of enhancement but it doesn't shine in that application. It's also noteworthy for its very low levels of quantization noise (which is especially important for DVD mastering).
ROUND 'EM UP
It's a challenge to compare products with different purposes in a way that makes sense overall. In the end, we found that in some cases, we couldn't compare these EQ plug-ins directly so much as contrast them.
Waves Linear Phase EQ and Elemental Audio Firium are both aimed mainly at mastering and precision editing, and as such, attempt a more neutral approach to altering sound. Both products are in widespread use among pro and amateur mastering engineers. When comparing Linear Phase EQ and Firium directly with other types of general-purpose or program EQs that are specifically designed to hype or flatter the source material in a musical way, they can seem more clinical. And that is exactly the point: they are not intended to flatter program material; they serve primarily as precision scalpels for balancing and correcting.
Of the program EQs, the panelists especially liked Sony Oxford OXF-R3 and Focusrite d2. That Oxford OXF-R3 was a favorite was no surprise, and it's good to see that it lives up to the hype and justifies its higher price. Focusrite d2, on the other hand, was a pleasant surprise. It has been around for years and is often included by Digidesign in free plug-in packs that accompany Pro Tools. If you have to purchase it, d2 (bundled with the d3 compressor-limiter) is not exactly cheap, but it's a good value and certainly easier to afford than the Oxford.
When it comes to bang for the buck, it's hard to beat Elemental Audio's Firium. A professional plug-in with a terrific interface for $99 is a great deal that gets even better when it's bundled with IIR-based companion plug-in Eqium for $129. Firium's drawing-based approach combined with its real-time spectral analysis, smoothing and scaling tools, and flexible left-right connectivity makes it an undeniable bargain.
No one picked Bomb Factory's Pultec EQP-1A plug-in as an everyday, slather-it-on-like-ketchup type of program EQ, but we agreed that it's a good flavor to have in your arsenal. Opinions were mixed on the high frequencies, but everyone agreed the low end was wicked cool.
So there you have it. I set out to discover whether the differences between assorted digital equalizer plug-ins with widely varying quality levels somehow justified their widely varying price tags. What I learned is that digital EQs do indeed have widely varying sonic qualities, but you have to look more closely at what each EQ is designed to do and what the trade-offs are.
True, the highest-priced program EQ (the Oxford) also sounded the best for enhancing program material. After that, however, all bets were off: the panelists disliked the sound of some relatively expensive products and liked the sound of some inexpensive ones. The lesson is simple: downloading a demo before you buy is a very good idea. Let your own taste be your guide.
Nick Peckplays the piano, makes sounds, and is a lifelong resident of Marin County, California. He does not own a hot tub.
Mike Bemesderfer is a freelance recording and mastering engineer in Berkeley, California. He is also a singer and flutist.
Tom Corwin is a Mill Valley, California-based producer, engineer, and bassist.
Ben Emmerich works in Novato, California, as a freelance recording engineer, studio designer, and bassist.
Jeff Kliment is director of the sound department at LucasArts Entertainment. He was a staff engineer at Russian Hill Recording in San Francisco for many years.
J. White is sound supervisor at LucasArts Entertainment. He is also a composer and recording artist.
bandpass filter: A circuit or algorithm that allows signals at frequencies within a specified range (band) to pass through unaffected, while the level of signals at other frequencies is reduced.
equalization: A circuit or algorithm that allows frequency-selective manipulation of a signal's amplitude.
filter: A device that attenuates or removes certain elements or data from an audio waveform or data stream.
graphic EQ: A frequency-shaping device having multiple filter bands, each operating at a fixed frequency and bandwidth.
highpass filter: A circuit or algorithm designed to attenuate, or cut, frequencies that fall below some designated point, while allowing higher frequencies to pass through unaffected.
lowpass filter: A circuit or algorithm designed to attenuate frequencies that occur above some designated point, while allowing lower frequencies to pass through unaffected.
parametric EQ: A circuit or algorithm designed for frequency-selective attenuation or boosting of a signal's amplitude, with independent controls for gain, center frequency, and bandwidth (including continuously adjustable Q).
Q: In filters, the ratio of a bandpass or band-reject filter's center frequency to its bandwidth. (Higher Q values indicate a narrower bandwidth.)
shelving EQ: A circuit (or algorithm) that offers the ability to cut or boost gain above or below a given frequency. Examples include the treble and bass controls found in home stereo systems or guitar amps.
This glossary is based on definitions from Tech Terms: A Practical Dictionary for Audio and Music Production (Hal Leonard, 1993) by Steve Oppenheimer and George Petersen. The material is used with permission of the copyright holders.