CD Mastering

Highlights of this page:

What Is A Mastering Engineer?
Why Shouldn't I Call My DAT Tape the "Master"
Seven Reasons Why Mastering is Needed. Why Can't I Just Mix To DAT?
Question Authority, Or the Perils of the Digits
Don't Wreck Your Digital Tape

Introduction

CD mastering is an art and a science. Compare CD Mastering to the editor's job of taking a raw manuscript and turning it into a book. The book editor must understand syntax, grammar, organization and writing style, as well as know the arcane techniques of binding, color separation, printing presses and the like. Likewise, the CD Mastering engineer marries the art of music with the science of sound.

What Is A Mastering Engineer?

The CD mastering engineer must have a musical as well as technical background, good ears, great equipment, and technical knowledge. Ideally, he should know how to read music, and have an excellent sense of pitch. He knows how to operate a range of specialized technical equipment, much of which is not found in the average recording studio. The successful mastering engineer understands many musical styles (and there are a lot out there!), edits music, and puts it all together with sophisticated digital processing tools. He is sensitive to the needs of the producer and the artist(s), and treats each project or CD with individual attention. He must understand what will happen to the CD when it hits the radio, the car, or the home stereo system.

Master vs. pre-master vs. glass master

What's the Difference between the CDR and the Glass Master?

Premastering, not mastering, is the more accurate term, since the true master for a Compact Disc is called the Glass master, which is etched on a laser cutter at the pressing plant. There are two intermediate steps (the father and the mother) before creating the stampers that press your CDs. So, we really should label the material that is going to the plant a PreMaster. The material going to the plant may be an Exabyte DDP tape, a CDR (recordable CD), or a PCM-1630 tape. Even though it's really a PreMaster, it's customary to label the 1630 tape or CDR CD Master--because (hopefully) there will be no further alteration of the digital audio at any subsequent stages. If the pressing plant does its job right, the bits on the final CD will be identical to those on the master that left the Mastering House.

Why shouldn't I call my DAT tape the "MASTER"?

The word Master is overused...I've searched record company libraries, and often found several tapes of a record album, each one labelled master, but in reality, there can be only one Master tape. You should label your tape Mix tape, or Original Session Tape or Edited Work Tape, or Edited Compilation, Unlevelled or perhaps Assembled Submaster. But as you can see, using the label Master will only confuse things later on. Other confusions arise when the producer has second thoughts. He may decide to change the EQ or relevel a song, but forget to relabel the previous master. Certainly, the first thing is to prominently print DNU ("do not use") on the label of a newly "obsolete" tape.

Seven Reasons Why Mastering is Needed. Can't I just mix to DAT?

Every recording deserves good mastering. When you're through mixing, your work is not finished. Mastering adds polish, it sounds more than just a record...it becomes a work of art. The songs work together seamlessly, their sound can take on a dimensionality and life that enhances even the best mixes. Here are seven reasons why Mastering is needed.

1. Ear Fatigue
Most music today is produced by recording a multi track tape. The next step is the mixdown. This mixdown may take anywhere from 4 hours to 4 weeks, depending on the producer's predilections, the artist's whims, and the budget. Usually each tune is mixed in isolation. Rarely do you have the luxury to switch and compare the songs as you mix. Some mixes may be done at 2 o'clock in the morning, when ears are fatigued, and some at 12 noon, when ears are fresh. The result: Every mix sounds different, every tune has a different response curve.

2. The Skew of the Monitors
Monitoring speakers. It's amazing when you think about it, but very few studios have accurate monitor systems. Did you know, placing speakers on top of a console creates serious frequency response peaks and dips? A typical control room is so filled with equipment that there's no room to place a monitor system without causing comb-filtering due to acoustic reflections. And though your heart is filled with good intentions, how often do you have time to take your rough mixes around, playing them on systems ranging from boomboxes to cars to audiophile systems? Usually there is no time to see how your music will sound on various systems in different acoustic environments. The result: your mixes are compromised. Some frequencies stand out too much, and others too little.

3. More Me
The producer was supposed to be in charge. He tried to keep the artists out of the mix room. But something went out of control. The producer was gone for the day, or the bassist had a fit of megalomania. Or the artist decided to be his/her own producer. Whatever....all the mixes sound like vocal, or bass, or (fill in appropriate instrument) solos.

4. May I Have Your Order, Please
When mixing, you (the producer) often have no idea what order to put the tunes until after all the mixes are completed. If you physically compile these songs at unity gain, and listen to them one after another, it probably won't sound like "a record." Some tunes will jump out at you, others will be too weak; you may discover (belatedly) that some tunes are too bright or weak in the bass, or that the vocal is a little weak, or that the stereo separation is too narrow. These things actually happen, even after weeks in the studio, and the problems sometimes don't become apparent until the album is assembled in its intended order, or auditioned in a good monitoring environment.

5. The Perspective of another Trained Ear The Buck Stops Here.
The Mastering engineer is the last ear on your music project. He can be an artistic, musical, and technical sounding board for your ideas. Take advantage of his special ear... many beautiful music projects have passed through his studio. You may ask him how he feels about the order of your songs, how they should be spaced, and whether there's anything special that can make them stand out. He'll listen closely to every aspect of your album and may provide suggestions if you're looking for them.

6. Midi Madness
Lately it sounds like everyone is using the same samples! Acoustic sounds are coming back in vogue, but perhaps you haven't got the budget to hire the London Symphony. So, you had to compromise by using some samples. But you shouldn't compromise on mastering. Good mastering can bring out the acoustic quality in your samples, increasing your chance of success in a crowded music field.

7. Don't Try This At Home
The invention of the Digital Audio Workstation (DAW) and the digital mixer is an apparent blessing but really a curse. Many musicians and studios have purchased low cost DAWs and digital mixers because they have been led to believe that sound quality will improve. Unfortunately, it's real easy to misuse this equipment. We've found many DAWs and digital mixers that deteriorate the sound of music, shrink the stereo image and soundstage, and distort the audio. There are several technical reasons for these problems-usually wordlength and jitter are compromised in these low-cost systems. Therefore, we recommend that you protect your audio from damage; use a mastering studio that employs a high-resolution system that enhances rather than deteriorates audio quality. Prepare your tapes properly, and avoid the digital pitfalls. Use the informative articles at theDigital Domain web site as resources to help you avoid audio degradation. When in doubt, take this advice: mix via analog console to DAT or analog tape, and send the original tapes to the mastering house. You'll be glad you did.


Those are only some of the reasons why, inevitably, further mastering work is needed to turn your songs into a master, including: adjusting the levels, spacing the tunes, fine-tuning the fadeouts and fadeins, removing noises, replacing musical mistakes by combining takes (common in direct-to-two track work), equalizing songs to make them brighter or darker, bringing out instruments that (in retrospect) did not seem to come out properly in the mix. Now, take a deep breath and welcome to the world of CD mastering.

QUESTION AUTHORITY, or THE PERILS OF THE DIGITS

Now that you have some idea of the mastering engineer's role, let's see how you can keep your tape intact on its way to the CD Mastering House.

Let's discuss some digital do's and don'ts.

If you mix to analog tape, the best thing to do is make a safety digital copy, edit the analog (if necessary) with a razor blade, and send the original to the mastering house. A 30 IPS, 1/2" two-track tape contains a wide frequency and dynamic range, and is a superior recording medium. Some will argue that analog tape is more pleasant sounding than 44.1 Khz 16-bit digital tape (is that why so many of us are nostalgic for the sounds of the 50's and 60's?). But the newer digital formats record at 20-bit, at 44.1 Khz sampling or 48 Khz, with 4-tracks (good for surround sound), or at 96 Khz (the first editing system for 96 Khz has just appeared, as have good sample rate converters that support this format). We are living in very interesting (and expensive) times.

A to D conversion is the weakest link in the recording chain. Repeated copying via A/D/A can result in a subtle (or obvious) veil and/or harshness in the sound. That is why, if you prefer mixing to 16-bit digital tape (DAT), you should obtain the best-quality external A to D Converter available, one that is properly-dithered to 16-bits. A good 20-bit A/D is sonically far superior to any converter built into a DAT machine. Now that you have a digital tape, never return to the analog domain. (Well, almost never..there are occasions where analog domain processing is preferable even for a digital source tape. See Below). Your digits shouldn't hit a D/A converter again until they hit the consumer's CD player. That means if you want to use a Pultec, LA-2, or other analog "processor", use it during the mixdown. Interestingly, some mastering houses now have digital equalizers (and processors) that do a very good job of simulating the sound of the venerable Pultec, only in the digital domain.

So, with few exceptions, be sure to keep your sound in the digital domain once it has crossed over the line. What about digital copying? What about digital editing, level changing, equalizion or other processing in the digital domain? There are many reasons why I strongly suggest you leave all that for the mastering house. And here are some of the reasons why...

Question Authority...

Surprisingly, the little bits on your tape can undergo a perilous journey through some of the digital processors and editors on the market. If there's a DSP inside, suspect the worst until you know for sure. There are some tests you can perform on your digital processors and editors (or workstations) without expensive test equipment (the processors are expensive enough!). These tests include linearity, resolution, and quantization distortion, common problems caused too-often by digital audio editors.

In other words, while you may be tempted to save time or money by doing preliminary editing with a digital audio editor, be very careful. A digital editor, after all, is just one big computer program; computer programs have bugs (there's not one bug-free program in existence!) and one of those bugs could be guilty of distorting your digits, in a big, or very subtle way. The sophisticated digital mastering systems at CD mastering houses also have bugs, but undergo regular testing to verify proper sound quality. We have received recordings with truncated fades (where the audio sounds like it dropped off a cliff!), distorted audio on the fadeouts; music with poor low-level resolution that is a shadow of its former self; music whose soundstage (stereo width and depth) appears to have collapsed, or recordings that have an indescribable "veil" over the sound compared with their sources. Here are some pointers that will help you avoid these problems:

Don't wreck your digital tape...

• STOP right here if you want to re-order your tunes before sending them to the mastering house. You won't save time copying your DAT or reordering it in an editing system before sending it for mastering. If you're not careful, your digital copy (reordered) can have glitches on it. If you must pause the recorder when dubbing, roll it for at least 30 seconds before the tune begins. This guarantees the playback machine can lock up without glitches or noises. (Most DATs can lock up in 1 to 2 seconds, but who wants to play with fire?) Don't stop the recorder until you are sure the music has faded completely--DAT tape is cheap!
• In short, it's a good thing to make safety copies and put together some tests to find a good song order, but it's actually better to send that "raw" original DAT to the mastering house (with a good written log of where to find the cuts). There is less chance of degradation or missing a piece (digital copies are theoretically perfect, but the people who make them are subject to human error). A bonus in sending the original mix tape is there may be outtakes or other sections the mastering engineer can use to repair noises you may not have noticed. The mastering engineer will reorder the tunes, carefully smooth fadeins or fadeouts, place black or roomtone between the tunes, in extremely efficient time. Plus, at the mastering studio, each fadeout or level will be controlled with dither.
• If you want to produce a compiled DAT made from several source DATs (to save some time in the mastering session), you can copy digitally with no loss (but make two DATs, DAT A and DAT B, in case something happens to one). Some mastering engineers may claim that a digital copy multiplies jitter, but we have found that every digital bit is clocked out of the hard disk mastering system with a very stable clock, and therefore, the copy sounds better than the source! (It's all in the playback, surprisingly!)
• When you mix, DON'T fade in or fade out. Leave all the decay you can, because the mastering engineer has precise digital tools for performing artistic fades. He may even suggest a segue (where two musical pieces overlap) as an artistic alternative. Remember, the mastering process is like whittling soap. You can remove a piece, but it's hard to fix what was chopped off! However, there are some tools for adding tails. If, for example, the musicians talked before the ringout was over, or the bass player dropped his bow (shit happens) , or the assistant stopped the recording before he was told, there are ways to add convincing tails to a song that are indistinguishable from real life, and sometimes even better!
• As mentioned above, if you decide you have to use a digital audio editor, TEST IT first. Do this for each software revision. You really can't trust the manufacturer when your precious music is at stake. Listen for degradation of soundstage width and depth, graininess, increased brightness or hardness. Listen on the finest reproduction system possible, or these changes may be perceived as too subtle and you won't know you've ruined your material until it's too late!
• "Don't try this at home! " You have excellent ears! That's why you're in the music business. So let's keep those little aural tickles you mixed so carefully into your music. If you're going to edit digitally, and if your digital editor passed the tests, please do NOT change the gain on your music. Do not raise it or lower it. Don't perform any fadeins or fadeouts! Don't use any of the fancy "plugins" that "maximize" the sound. Don't equalize or compress using the DSP in the editor. Don't normalize. Don't pass through external digital processors (including digital reverbs). And finally, turn the DITHER (if available) off. Every one of these processes can deteriorate sound, especially if the tape is to undergo further digital processing. Cumulative digital processes (if improperly performed) can be very degrading to sound.
• Here's why: The reason (and many engineers are not aware) is that almost every DSP computation adds additional bits to the wordlength. The wordlength can increase to 24, 56, or even 72 bits if the DSP designer knows his stuff. The right thing to do is keep your newly "lengthened" words as long as possible, until the final stage, where they will be dithered down to 16 bits for the CD. So, if you work with a 16-bit editor, and change the gain, for example, you are actually truncating and distorting your sound. Even if the editor has built-in "dither", you are adding a subtle veil to your sound.
• What makes the CD mastering house different? All the processors at the CD mastering house produce 24-bit output words whenever possible. If the mastering engineer employs digital processing on your tape, he/she will endeavor to keep your tape in the 24-bit domain until the final stage. When properly applied, 24-bit (and longer word) processes maintain a degree of warmth and space that is hard to believe. And that's why it can sound so good!

Analog versus Digital Processing

Earlier in this article, I cautioned against returning to the analog domain once you've converted to digital. Ideally, you only want one of these conversions, once in the original recording, and once in the CD player playback.

But what about Pultecs, tube and solid state equalizers, tube and solid state compressors, limiters, exciters.... Most mixing engineers can cite a plethora of famous processors that perform their work with analog circuitry. While useful for effects patching during a mixdown, a good number of these processors are unsuitable for mastering purposes. For example, an old, unmaintained Pultec may be a little noisy, but still be suitable to process a vocal or instrument during a mixdown. But would you pass your whole mix through that noisy box (maybe yes, if you like the sound!)? However, every processor used by a mastering studio (a good mastering studio) will be used in matched pairs, have calibrated positions, be quiet, clean, well-maintained. Calibrated positions are important for re-mastering, or for revisions. Clean means low-distortion and noise. Matched-pairs keeps the stereo image from deteriorating.

If a mastering engineer has a favorite analog EQ, or processor he wishes to use to create a particular sound from a DAT tape, he should carefully balance out the cure versus the disease. There is always a loss in transparency when passing through analog stages, particularly A/D/A. Anyone who has patched processors in their Consoles is aware of these tradeoffs. In other words, you have to carefully weigh the veil and fogginess that results from patching the DAT via analog versus the changes the processor can give versus bringing the DAT into a high resolution Digital editing and mastering system and performing the processing in the digital domain.

There will be an inevitable slight (or serious) veiling or loss of transparency due to each conversion. However, perhaps the mastering engineer feels the music will benefit from the sonic characteristics of a vintage compressor or equalizer...maybe he's looking for a "pumpy" quality that can't be obtained with any of today's digital processors (many people complain that digital processing is too "clean"...certainly a subject for another essay). There are many vintage "sounds" and other effects that still can only be obtained with analog processors. And finally, some mastering engineers claim that analog processors sound better than digital processors. I'm not one of them; I won't make that blanket statement. But I agree that analog processing is the "bees knees" for many musical productions. For example, I transferred a client's DAT to 1/2" analog tape and then back to 20-bit digital. Why? In short, because it sounded better. The analog tape stage did just the right thing to the source. I also had to make the fine choices of tape type, flux level, speed and equalization. Each of these decisions helped attain the spacious, warm, yet transparent sound quality my client and I were looking for. Ultimately, we used (and preferred) the analog dub to the original digital source for 8 out of the 10 tunes!

Even without going through the analog tape, I have always maintained that A/D and D/A conversion processes are the most degrading processes that can be done. When we think about using an analog process on a digital tape, the first thing I ask the producer is "why didn't you mix to analog tape in the first place?" Then there would be less questioning about which route to take. When we do go back to the analog domain, I use the highest-quality 20-bit D/A converter (works well even on 16-bit tapes), carefully calibrated levels, short analog signal paths and quality cables, and when converting back to digital, an extremely high-quality 20-bit A/D converter. Then, the losses in transparency due to conversion will be minimized, and in many cases we consider the improvement due to the unique analog processing outweighs the losses of an extra D/A/D conversion.

Most of the time, I personally have found the digital process to be the most transparent of the two options. Perhaps this is because I am very comfortable in both the analog and digital domains. Other mastering engineers agree or disagree with me on this point, and our choice of processing depends a lot on personal taste, habits developed over the years, ignorance (or knowledge) of the power of good digital processors, the quality and transparency of their monitoring system (if it doesn't show the degradation, then maybe it isn't there?), and so on. I have many clients with excellent ears who cannot believe that these results were obtained with (god forbid!) digital EQ and processing.

Unique Digital Processes

There are also some unique (and proprietary) techniques that I perform only with 24-bit DSP, one of which I call microdynamic enhancement, and the other I call stereoization. If the material needs it or warrants it, these processes can only be done in the digital domain.

For example, my invention, called microdynamic enhancement, can restore or simulate the liveliness and life of a great live recording. I've used it to get more of a big-band feel on a midi-sample-dominated jazz recording. I've used it to put life into an overly-compressed (or poorly-compressed) rock recording. It's really useful and extraordinary in helping to remove the veils introduced in multi-generation mixdowns, tape saturation and sound "shrinkage" that comes from using too many opamps in the signal path. My microdynamic enhancement process is achieved totally digitally.

I've invented another totally digital process called Stereoization, which I use on unidimensional (flat-sounding) material. Stereoization is very different from the various width-altering processes that are now-available. Stereoization actually captures and brings out the original ambience in a source. The degree of stereoization is completely controllable. Instruments in the soundfield have natural space around them, as if they were recorded with stereo microphones. The process is totally natural, utilizing psychoacoustic principles which have been known for years, and it's fully mono-compatible.

The above remarkable processes can only be achieved digitally. And DSP engineers are constantly inventing new ways to simulate all the traditional analog processes. So there's a lot to be said for digital processing, and I have no doubt that will become the dominant audio mastering method in the next five years. Whether analog or digital processing is the better choice today is very dependent on your music, and the talents and predilections of the individual mastering engineer.

-Bob Katz, revised 5/97

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Thanks for reading!
Bob Katz, President, Digital Domain, New York City

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