XO Wave: Guide to Home CD Mastering

CD Mastering is a complex subject. However, not only is it possible to create high quality CD masters at home, but thanks to XO Wave, it's possible to do so on a (tiny) budget. This guide describes what everyone needs to know about the process in order to get the best possible results. Almost everything here applies whether or not you are using XO Wave as your CD Mastering software.

First off, a little terminology: When you and I and most audio professionals talk about CD mastering, we mean setting up the audio, marking tracks, setting up CD Text, and so on, in order to have something to send to a CD manufacturing plant. However, the CD manufacturing plant calls this process "pre-mastering". To them, a CD master is a disc made of glass that has been prepared by them to a client's specifications, and is used to physically stamp its impression onto pieces of plastic, 'pressing' CDs. Audio folks usually refer to this as the "glass master". How the audio world and the CD manufacturing industry ended with conflicting terminology, I don't know, but for this page, I'll use the term 'master' to refer to what you send to the CD manufacturing plant, and 'glass master' to refer to what the manufacturer calls a master.

CD Technology

The CD medium does not lend itself to accurate duplication. The original techniques for preparing the glass master didn't involve burning a CD on your computer because blank CDs and the technology to burn them simply did not exist until some time after CDs became available, and so care was not taken to ensure that burned CDs could be used as masters for the CD manufacturing process.

There are three types of problems that may arise when you use a burned CD as a master for the manufacturing process: chronological offset, disc errors, and manufacturing errors. Chronological offset is essentially a mechanical problem. CD players are not required to play back exactly from the start of a track, and many burners shift the audio in time by up to a few hundred samples. These types of errors are relatively easy to deal with, as long as you know about them and are prepared. disc errors are a result of imperfections in, or poor quality of, blank media or burners. Although this can be a subtle problem, the use of high quality discs and burners will keep you safe. Manufacturing errors happen when your finished disc is not the same as the master you sent. To avoid this type of error, many manufacturers offer to send you a burned CD that will be exactly the same as the CDs they will press. This may add slightly to the cost and time to manufacture your CDs, but there is no substitute for the peace of mind it offers.

In general, if you have any concerns about the process, the best place to seek advice is from the manufacturer. There is no substitute for their knowledge and experience and they know what will and won't happen during the process. If you have concerns after reading this article, you should bring your concerns up with them, and ask how they can ensure that your CDs will be manufactured exactly as you want them.

Chronological Offset

CD technology dates back to the 70's. The hardware used to spin the disc, read data, and accurately buffer the audio and other data on the CD was simply not as sophisticated as what is available today. As a result, the audio CD standard (called "red book" after the color of the book containing the official specification) specifically allows players to do things like start playing a track after the actual location of the track marker. CD burners carry on this tradition, and they also allow a certain amount of inaccuracy when putting the data onto the disc. This offset is usually small (around 100 samples). This offset is usually not important for the way your CD sounds or is played in any noticeable way, and generally pales in comparison to the offset created by CD players when playing back your audio.

It's important to understand that neither of these issues affects the actual sound quality of our CD, just how it is played back. For example, if track 2 of your CD starts right on the first drum hit, everything will sound fine when you play the entire CD from start to finish. However, if the listener skips to track 2 directly, they may not hear the beginning of the drum hit.

In most cases, these problems are easily solved by "padding" your CD tracks a little. That is, move your CD tracks a little earlier in the time-line so that when the CD player plays back that track, it has a little wiggle-room before it has to start playing back. XO Wave automatically pads CD Track Markers that are created using the Auto Create feature of the Memory Locations Manager, but manually created CD Track Markers should be placed a little to the left of the intended location. XO Wave uses 1/8 of a second when padding, and this is what we recommend when placing markers manually.

The best way to correct for these errors is to pad your tracks and listen to the results on a variety of CD players. Ask your manufacturer to check the master you send them to make sure the CD Track Markers are correctly placed and to contact you if there is a problem.

Disc Errors

There are many types of errors that affect the accuracy of the data encoded on CDs. Surface scratches and dust can cause a disc to be unreadable, but because the laser that reads data off the disc is out of focus when it passes through these imperfections, minor dust and scratches my not impair the ability to burn or read a disc at all, and a little extra handling care with your master should help avoid this problem.

More serious, then, is the possibility that the burner, or the disc itself, is inconsistent with the specifications, or is no longer in perfect working order. The solution is to use a high quality CD burner and blank CDs. Despite what some industry groups say, there is a very real difference between the discs sold at your local office supply store, and the discs that should be used for critical mastering. In general, CDs burned on a computer have lower reflectivity, higher error rates, and other problems which make errors more likely. When playing back, CD players are designed to automatically correct for many of these errors and mask others. Correcting errors is good because it means that the original data can be recovered, but masking errors is bad because it means the CD player can't read the data and it is filling in gaps with made-up data. The utmost care must be taken to ensure that the burned CDs are of the highest possible quality to prevent this type of error, because any uncorrected errors on your master will be passed on to your manufactured CDs.

Probably the most significant choice you can make when preparing your CD master is to choose a high quality CD. We discuss this issue and make some specific recommendations in our FAQ. It is also important to choose a high-quality CD burner, such as those found in higher-end desktop computers. Many mastering engineers swear by Plextor drives, though our experience is that many drives can produce high quality results. Generally speaking, it is probably best to avoid laptop drives for real mastering work, because many compromises must be made on those systems for space, power, cost, and other considerations, which generally take priority over reducing error rates. That does not mean, however, that you can't produce great results from a laptop -- it just means you may want to be especially careful when choosing media, and double check your discs to make sure they sound exactly the way you want.

Once you have a good drive and a good blank, make sure you do what you can to isolate the burner from power fluctuations and vibrations. Both problems can cause disc errors, and even a nearby inkjet printer or sub-woofer may be enough to increase your error rates. Be sure to isolate your audio equipment electrically using a dedicated circuit and a high quality UPS or surge suppressor. A high-end power conditioner is best, and is generally a very good investment for anybody working in audio.

If your software offers burn verification, as XO Wave does, always be sure to use it as this will ensure that the data on the CD is the data you meant to put on the CD. Because of chronological offset, some software uses techniques for verifying that may ignore silence on your CD. Though unlikely, it is possible for errors to be missed because of this. (XO Wave's verification does not ignore silence.) As important as verification is, it only proves that your master can be read correctly on your machine, so don't take it for granted that your CD is fine just because your computer says so -- listen to it on another CD player or computer (or several) before sending it off.

An increasingly rare issue is so-called "track at once" ("TAO") burning. Some burners and burning software burn each track separately and "link" them together in a way that usually works on CD players, but may cause problems at the mastering plant. Instead, make sure your software and hardware support "session at once" (sometimes called "disc at once" or "DAO") burning. If you are using XO Wave, you can rest assured that session at once burning is being used because XO Wave does not support track at once burning.

One final thing to consider is so-called "buffer underrun protection", offered by many CD burners. Buffer underrun protection shuts the laser down if the burner is not receiving data fast enough, and resumes burning when the buffer contains enough data. The advantage of buffer underrun protection is clear: it prevents you from having to start a burn over again with a new blank CD if your OS or hard drive is not quite fast enough to keep up with the burner, or is interrupted by another task. The disadvantage, however, is that the burner generally does not start up at exactly the same place it left off, and while it may play back fine, the discrepancy can produce read errors at the manufacturing plant. This type of problem is insidious because it can be very difficult to detect. For this reason, XO Wave turns off buffer underrun protection by default during CD burns. As long as your computer is relatively fast and you are not overtaxing it while burning your CDs, this is not an issue, but you can turn it back on, if you're working on something less critical.

It's a good idea to ask your manufacturer to check over the disc for things like red book conformance and Block Error Rates (BLER). This will ensure that your CD master plays back correctly on their system, and that the data is transfered without loss. While this applies to all disc burning, it is especially true if your master may be compromised for some reason (for example, if you did not use the best blanks or burner, or you didn't spring for overnight shipping). Don't assume that all is well with the disc simply because it plays back.

Manufacturing Errors

When having your CD manufactured, it is possible that the finished product will not be exactly what you sent. This can be for a number of reasons such as poor manufacturing conditions, incorrect transfer of your master (the worst of these types of blunder, such as making an analog transfer, seem to be a thing of the past, thanks to the increasing use of computers) and chronological offsets, described earlier. If you are ordering a small run of CDs, they will be created on a standard CD burner from blank CDs, and may be more likely to suffer from these problems than higher-volume orders which should be "pressed" from a glass master. In both cases, it is the manufacturer's job to ensure that exact duplicates are made of your CD and nothing less should be expected. When your shipment arrives, listen to it and compare it with your master. Listen on several CD players and jump to each track to make sure that you always hear what you expect. Make sure the track lengths are the same on your disc and the manufactured disc. Make sure the "lead-in" or pregap for each track is the same. If your disc had meta-data, such as CD Text or ISRC codes, make sure those are present as well. If there is any difference, contact your manufacturer immediately.

While ensuring that the finished product is exactly like the CD you created at home is the manufacturer's responsibility, communicating your concerns before you send them a master (and throughout the process) will help ensure that you get the expected results. Before you send your master, be sure to find out exactly what they need, and make sure the disc sounds exactly the way you want it to. If the manufactured CDs don't sound exactly like the disc you made at home, be sure to contact the manufacturer to get the problem fixed right away.