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What's Mastering All About?
The basic tools and techniques of the trade
By Dave Moulton

After a recording project has been completed, the next step is to prepare the mixes for their intended release format (usually Compact Disc). The process of doing this is called mastering, and it is considered the last “creative” stage in the music production process. Everything after that is intended to be a “clone” to the extent that is possible.

When the release medium was the vinyl LP, mastering was the process of cutting a master lacquer disc on a lathe in real time. It involved making adjustments to the master tape so the recording would work on vinyl.

People who did this were called mastering engineers. The master tape they worked with was usually fully mixed and edited by the client beforehand. Changes would include eq to correct for monitoring deficiencies, and compression and limiting to protect the cutter head and limit stylus velocities. Further, stereo signals were often run through a shuffler and the A–B low frequencies were rolled off to minimize vertical stylus excursions.

This process has changed a little with the advent of the CD. The mastering stage—the preparation of the glass master, from which CDs are made, at the CD factory—is now separate from the preparatory work of final eq, compression, etc. So what we now call ‘mastering’ is actually part of the process that used to be called ‘pre-mastering.’

Pre-mastering includes all the evaluative study and trial processing that precedes the creation of the “pre-master tape,” the recording that will be used to create the glass master (hopefully an exact clone). Happily, the glass master is a lot less tricky to prepare than the lacquer LP master was. The CD process is considerably more robust, believe it or not.

At the same time, clients have turned over more of the final pre-mastering process to the mastering engineer, including assembly editing (and sometimes, choice of mixes!) and levels management. This involves the modern mastering engineer quite fully in the creative end-game of recording—the mastering role becomes a critically important one.

There’s more: the mastering engineer prepares lots of recordings for release in various media and has a well-developed sense of how “sounds of the studio” translate into recordings for the masses. In this regard mastering engineers function as ears of last resort—they evaluate the master tapes and determine what will be needed to make it sound to the end listeners as the client producer desires. In this regard the mastering engineer acts as a correcting agent, adapting the producer’s vision to the realities of the marketplace with its dizzying array of end-users.

How do mastering engineers ply their craft? What do they do? What are their specific concerns? While I do a moderate amount of pre-mastering, I definitely don’t regard myself as a mastering engineer. So I consulted with Laurie Flannery of Northeastern Digital in Southborough, MA, who does this all day long for a broad range of clients.

First let’s look at the tools that are used in mastering.

Tools mastering engineers choose

Mastering engineers use tools that are similar in function to what we use every day in the studio, but with some significant differences. Mostly these lie in the realm of the precision and accuracy (and therefore repeatability) of their performance.

Further, mastering tools tend to be very high in resolution so the mastering engineer can work effectively at resolutions greater than 16 bits and then reduce the resolution of the master tapes to the release medium of 16 bits—assuming that the source tapes also have greater resolution than that.

Incidentally, some mastering engineers prefer to work in analog during some or all of the pre-mastering process. It’s a matter of personal taste and preference.


Equalizers of choice are parametrics, with extremely precise settings and matching between channels. In addition, widely adjustable shelving eqs and high and low frequency cut filters to work on the ends of the audio spectrum are also used. Notching filters are used for problem-solving. The eq on board the Sonic Solutions workstation is chosen by many, and one hot number as of the time of this writing is Daniel Weiss’s new outboard digital eq. Numerous engineers stick with analog (Neve is a popular brand), and much customization goes on to suit the preferences of the individual engineer.


Compressors are a key tool in mastering. The very precise and sometimes stringent need for levels management coupled with the desire to make such management as “inaudible” as possible makes the use of compressors very tricky and extraordinarily important. One successful mastering engineer has allowed to me that it took him more than two years of working as an apprentice in a mastering facility before he was able to really “hear out” compression.

Often there are profoundly different needs for different sections of a recording or for different components of a mix. Again, precision and reproducibility are key features of compressors. Split spectral bands to compress separately are a very useful feature, along with comparatively complex transfer function curves (an extension of the so-called “over-easy” type of function).

The key element in any compressor is its level detector (a.k.a. envelope follower), and the sonic effect of this circuit in all of its settings is of key importance to mastering engineers. Finally, because mastering engineers work on finished stereo mixes, it is necessary for them to have good control over the signal sent to the two level detectors.

Favorite compressors include units from Neve, Junger, NTP, SSL, Focusrite (Blue Range), and TC Electronic. Note that none of these are normally used in studio production work because of their specialized nature.


Reverberation is very hard to remove from recordings without really affecting the sonic quality. For this reason reverb is usually fixed at the time of mixing, although occasionally additional reverb may be added during mastering for enhanced consistency and smoothness in transition from one track to another. Beyond that, reverb is not a significant element in the mastering process.


Much mastering is done in the digital realm, although many engineers still prefer analog. Because assembly editing—and sometimes music editing that cuts between multiple mixes of a song—has become such a feature of the mastering process, the use of a digital audio workstation (DAW) is extremely helpful. The workstations of choice for mastering seem to be Sonic Solutions (Mac) and SADiE (PC), although other systems are also used quite frequently.

While much of the editing is comparatively easy, the workstation is also the place where we concern ourselves with data management, including truncation or re-dithering, the use of high-quality converters (for analog masters), normalization and general levels management, as well as the placement of track IDs (P and Q codes). This includes cross-fades, hidden tracks, and other cheap tricks. Digital converters are chosen by each mastering engineer for taste and preference, and are usually available at the 24-bit level.

Stereo management

Mastering engineers are often called upon to make the stereo impact of the recordings consistent from track to track. Sometimes they also enhance or reduce the “stereo soundfield.” This is generally done with a shuffler, but there are a variety of other “spatial” devices that are used.

Look what they’ve done to my songs, Ma

In preparation for CD release, mastering engineers assemble the supplied mixes into a desired sequence, fix the level of each song, establish the eq and compression that will be used (including changing it where needed), fiddle with the stereo, fix the bits, re-dither as needed, and then create the master tape into one of several formats used to make CDs. The order in which this is all done varies greatly according to individual preference and working habits.

Levels management

One of the primary goals of mastering, as driven by client preference, is to make the CD as LOUD as possible. We are all constrained by the limiting factor of 0 dBFS in digital recording. Because louder generally sounds better (I’m not going to get into why this is so), the louder of two otherwise identical recordings will be preferred by listeners. The engineer who can push the level right up near 0 dBFS most effectively wins!

There are a couple of ways to do that.

In each track

One can compress each track carefully, adjusting its threshold, ratio, attack, and release to give us maximum musical and dramatic impact. Sometimes it’s good to use a hard limiter with a threshold at 0 dBFS as well. We don’t just want the music loud, we want it dramatically loud, and our compression efforts work on this. Often multiple passes of fairly gentle compression will be used to build up an effective compression scheme.

Sometimes masters venture into the realm of overshoots (“overs”), usually at clients’ requests, increasing the level until they begin to complain about the distortion and then backing off. Not a pretty practice, but a fairly common one. Client is always right, right?

Along this same line is a hot new technique called “bit-shaving.” By stripping away several of the Least Significant Bits, you bring the loudness of the overall recording up. An interesting way to do business, don’t you think? 12-bit audio lives!


Once the tracks are compressed to satisfaction, we normalize them, which is to say we find the point with the highest peak in each track and set that at 0 dBFS. Now each song on the CD is referenced to 0 dBFS, and the overall level of the CD is as loud as we dare make it. Needless to say, if we have crammed a bunch of overs into some songs to make them louder, we don’t then normalize their level downward.

But just because everything has been normalized to 0 dBFS doesn’t mean that the levels of those tracks are matched musically. Normalization levels may need to be shifted to take care of such problems.

Corrective eq

There are two issues in regard to equalization. The first has to do with equalization to correct for perceived flaws due to the effect of the control room monitors, such as when the low frequencies are deficient because the mix monitors were bass heavy. Mastering engineers are really sharp when it comes to this.

The second equalization issue has to do with song-to-song consistency. Often different tunes were mixed at different times, by different people, in different rooms. The mastering engineer uses equalization to reduce these differences, to bring the CD to a more focused and consistent sonic quality. They can really tease exciting mixes to emerge from somewhat marginal tapes by hearing out the frequency problems and fixing them. This is done very carefully, working to avoid the unfortunate possibility that by making the tracks more consistent we also lose the power and magic of individual mixes. Sometimes little tweaks are added in solos, minor events like snare hits, etc., just to bring out the essential quality of the mix.

Management of the digital signal

Mastering engineers get DATs, hard disk files, analog tapes, and occasionally other media from which to master. If the source recordings are analog, conversion is usually handled by outboard converters at 20 or 24-bit resolution at the sampling frequency of the CD: 44.1 kHz.

If the source recordings are digital, the mastering engineer is stuck with the bit resolution and dither provided by the client. If it is at more than 18 bits, and at the right sampling frequency, the mastering engineer is happy, because he or she has some room in which to work.

To truncate or to re-dither?

Once all of the changes have decided upon and the final master tape is ready to be made, the master recording must be reduced to 16 bits for CD release. Usually this will involve throwing away the unused bits and adding dither (from one of a variety of flavors) at the 16th bit.

Another possibility is to simply truncate the recording, throwing away all bits below the 16th bit, and not re-dithering. While this is generally frowned upon, one mastering engineer allowed to me that he does it a fair amount to suit his clients’ occasional preference for that particular sound quality. Truncated recordings, he says, seem to have a little harder, edgier quality that some prefer, while dither tends to make low-level stuff a little rounder, smoother, and sweeter. This has a family resemblance, of course, to bit shaving.

Finding errors

With Sony 1630 format recordings we get a Digital Tape Analysis (DTA) that tells us about overs, mutes, holds, averages, and parity errors. Finding errors on DAT or CD-Rs is much more difficult, and is generally done mostly by ear (which frightens the bejesus out of many mastering engineers!).

Getting an acceptable master

Usually mastering engineers customize their work to suit the chosen CD factory (do they take 1630 format masters, for instance). Also, it is essential to create a safety and a reference, so that while the client is impressing his/her friends the master is not at risk. It is also essential to maintain good communications and relations with the CD factory. Most factories are very open to this communication and co-operative effort.

Interestingly there are only about 15 CD factories in the U.S. All other facilities offering CD production services are actually functioning as brokers. It is essential for the mastering facility (and the client) to know exactly who it is dealing with. Through a broker, obviously, the price of quality is eternal vigilance.

What you can do to prepare your recordings for mastering

It is important to have a track sequence in mind before you arrive for the mastering session—unless you like extra hourly charges.

• Always bring safety copies. If you don’t, you’ll need them. If you do, you won’t! Simple as that.

• If possible, bring everything in at 44.1 kHz. sampling rate. If you can’t, make sure the tape is marked—unless you want the music to be played 6% slow and a half-step lower.

• If you are brave enough to try a rough pre-master in Pro Tools or whatever, remember to bring the original mix tapes along as well as the Pro Tools version.

• To head off horrible dissatisfaction in the mastering suite, it may be advisable to bring in multiple mixes with different vocal and solo levels, called “vocal ups” and “vocal downs,” as well as “solo ups” and “solo downs.” This is because compression at the mastering stage often changes relative balances of the leads to the bed of the track, so some production latitude is extremely handy and calming to the nerves.

•Make an uncompressed mix, along with the compressed mix that you prepare during mixdown.

• Use and bring really good documentation with excellent notes, box legends, clean tapes, test tones. If you are using analog, get a digital safety copy for sure. Analog accidents happen!

• If you want noise removed during mastering via software that reads a noise "signature", remember to include examples of the noise floor you want nuked. A couple of seconds is fine; the reason is that these programs generate filters based on these noise samples.

• It’s a good idea to bring along some recordings that represent a quality of work you aspire to. This will give the mastering engineer a sense of what sort of sound you want.

• Bring phone numbers for everybody you might need to talk to during the mastering session, such as the mix engineer, the bass player, the producer, the label, etc. Let them all know you may be calling them if there are problems and that you’d sure appreciate being able to get through to them quickly.

• Bring mixes without fades in addition to normal mixes, so fades can be changed during mastering if necessary.

• Decide ahead of time exactly who is going to bring the tapes and documentation. Believe me, you haven’t felt foolish until you and the rest of the band is standing in the mastering suite, looking at the clock ticking away your dollars and saying, “Whaddya mean, I was supposed to bring the tapes?”

So you wanna try mastering yourself

There is no technical reason you can’t master your own recording. You assemble your finished mixes, record them on a DAT, make a log of where you want the start IDs to go, and send it off to the CD factory. Hopefully there will be no problem and it will all go swimmingly.

Often, however, you notice that your CD doesn’t sound quite as cool, intense, in-your-face, and dramatic as you’d like (or as your favorite CDs sound). The next thing to try would be some mastering tools. The hardware devices of the 1990s like the TC Finalizer have largely given way to computers running mastering software. There are a lot of programs available for the purpose, and many of them have been reviewed in Recording.

These tools give you a whole bunch of mastering functions with good precision and resolution. Now you’re all set, assuming that you’ve got the editing under control.

The mastering software and/or boxes will solve your mastering problems—once you get good at using them. But that is where it gets tough in a hurry. Most mastering engineers serve a fairly long apprenticeship, where they assist, document, dub, and do other things for their mentoring mastering engineer, as well as get extensive opportunity to practice the same things on the same source tapes.

It is gonna be hard for you to make up for this experience just by your sheer genius and creativity! Think of it this way: you’ve just bought a really good Steinway concert grand piano; maybe you’d better study and practice it some before you go out to give your first concert.

Also keep in mind that when you master for yourself, you are giving up a valuable objective voice that you may really need. As Mark Twain pointed out, “The man who represents himself in court has a fool for a client.” Beware!

On the other hand you may not be able to afford a really strong outside mastering effort, and you still need to release a CD. If you insist on mastering it yourself, take tons of extra time, take trial masters around for extensive listening, seek comments from everybody you can, and be as meticulous as possible. Also, try not to do the mastering the day before the CD factory needs your tape. A certain amount of timeless serenity and calm repose will really help your mastering efforts.

And try not to be disappointed if the released results aren’t exactly what you want, or if there turn out to be some manufacturing problems. That is one of the risks that you take.

Happy masters!

Dave Moulton wants to publicly thank Laurie Flannery of Northeastern Digital (Southborough, MA) for assisting with this article.

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