Mixing in Preparation for Mastering - Part 1: Audio File Formats, Sample Rates & Bit Depths

In order for your mastered tracks to have that sparkle and clarity that you envision, it is important to follow some mixing guidelines. For these discussions we'll assume the project is for CD-Audio, but all the basics apply no matter what the final delivery medium.

First we'll address digital audio file formats, sample rates and bit depths.

1.) DO NOT SAMPLE RATE CONVERT. If you are mixing 'in the box' or through an external digital mixer, the mix files should be at the same sampling rate as the session. That means NO SAMPLE RATE CONVERSION. Hopefully you tracked at 48k or ideally 88.2k or 96k, but many engineers make what seems a logical assumption that since CD-Audio has to be 44.1k, then they should convert their mixes to 44.1k even though the session was tracked at a higher sample rate. There are three reasons not to do this. First, for ultimate sound quality the project should be mastered at the higher sample rate at 24 bits and then downconverted to 44.1/16 as the final step before the Production Master CD or DDPi is cut. Second, it is likely that a real mastering room has a much better sample rate converter than those found in even large studios. And third, there is a growing trend of artists and sites offering hi-resolution downloads, so it is advisable to have the mastering room create a separate data master containing the mastered 24 bit files at the higher sample rate.

2). MIX AT 24 BIT RESOLUTION. Even though your session may have been tracked at 16 bit, it is important to mix at 24 bit resolution. The reason is that if you do anything more than play one sound file at unity gain out of one buss, even something as simple as changing the gain by 1/10th of a dB, your system has to expand to a 24 bit word length internally. So if you output your mix as a 16 bit file, you will create truncation distortion and lose fidelity. Since hard drives are so inexpensive these days, you really should be tracking as well as mixing at 24 bit resolution.

In the mastering world we are seeing a disturbing trend of people wanting to deliver mp3's as sources. mp3 is barely acceptable as a consumer audio format, and is certainly not suitable for mastering.

CAVEAT: Know Your System! If your DAW or digital mixer operates at 32 bit or 32 bit floating, you need to apply 24 bit dither if you are outputting 24 bit mix files. Or you should output them as 32 bit files and inform the mastering room of this.

Relating to points 1 & 2, There is no 'getting back' quality. If you have created 16 bit mixes, loading them into a program that will output them as 24 bit files will do nothing for you. All it will do is add 0's to the lower eight bits. The sound quality will be the same but the file will just take up more disk space. Likewise upsampling a 44.1k file to a higher sample rate will often decrease quality except in the best of sample rate converters, and then the quality will likely be the same, it will again just take up more disk space. If all you have are mp3's of your mixes, converting them to .aif or .wav will not improve their quality. In fact, you will lose quality with such 'transcoding'. If all you have are mp3's any qualified mastering engineer will know how to handle them to result in the best quality possible. The same is true if all you have is an audio CD-R for a master - it's not doom, many great records have been made from 44.1/16 masters.

3) AUDIO FILE FORMATS. All mastering rooms should accept any of the standard file formats - .aif, .wav, .bwf or .sd2. Normally interleaved stereo files are preferred at the mastering room, but split L/R is fine and there is no difference in fidelity. Also, keep your file names under 15 characters. It's not necessary to name the file with the full song title. Just the first two or three words of the title is fine. See the documentation topic below.

4.) DELIVERY MEDIUM. Most mastering rooms should offer you the option of uploading your mixes. You should put all of your mix files into one folder and then zip that folder prior to uploading. Zipping a folder will perform and internal checksum which ensures that nothing was corrupted during the upload/download. If you want to deliver your mixes on disc, make sure you are creating either a data CD-R or DVD, not an audio CD-R. If you have tracked and mixed at 48k or above at 24 bits, and then you put your files onto an audio CD-R, you have lost fidelity. The reason is that an audio CD-R, like CD-Audio, can only be 44.1k/16 bit, so even though you may not realize it your CD burning software has converted your files to 44.1k and then dropped the lower 8 bits, and you have lost fidelity through both an inferior sample rate conversion and the file has been truncated. On a data CD-R or DVD, the audio files can be of any sample rate and any bit depth. Plus the method in which a data disc is written is much more accurate than an audio CD-R, so there are no errors on the data disc. Your 24 bit higher-sample rate files are much larger than CD-Audio files, so an album's worth will fit on one DVD-R, but will require 2 or more CD-R's, which is fine.

Any mastering room should also accept your sources on a Firewire or USB drive.

5.) MULTIPLE SAMPLE RATES. Do not mix sample rates unless clearly noted. Mixed sample rates often occur when multiple studios and/or engineers have been involved in a project. One engineer may have tracked and mixed at 48k, another at 88.2k etc. If you have a situation where your mixes are at different sample rates, put them into separate folders indicating the sample rate of the files they contain. Bit depth does not matter, if some of your mixes are 16 bit and some 24 bit but all are at the same sample rate this will not cause a problem at the mastering stage.

6.) EDITING AND OTHER PROCESSING. It is important that you deliver the first generation mix files for mastering when possible. They should not be loaded into any program to convert .aif to .wav, or split L/R to interleaved or any other seemingly transparent process. They ESPECIALLY should not be loaded into any workstation for normalizing, fades, edits etc. Your mastering engineer can easily perform fades and edits, and normalizing should not be done in any case. In most cases eq, compression or limiting should not be applied across the stereo buss while mixing. We'll address that in the next post.

CAVEAT: If your project will require a lot of editing, sometimes budget will not allow editing in the mastering room. If you do your editing in the studio, just be sure that everything stays 24 bit, the sample rate stays the same and that no level changes are done if at all possible. If your editing requires a section of a song to be raised or lowered in volume, then 24 bit dither should be applied when the finished edit is output.

7.) DOCUMENT, DOCUMENT, DOCUMENT. Too much information is a good thing. Your sources should be delivered with documentation indicating the sample rate and bit depth of the files as well as the title of the file and the corresponding full title of the song. It is also helpful if your mix engineer can provide details of whether the mix was done in the digital domain or through an analog mixer, if analog tape was used at any stage and whether stereo buss processing was applied and if so what kind and how much was applied and if any dither has been applied at any stage.

8.) ONLINE DELIVERY. If you are going to make your finished project available through download only, do not assume that you can lower the quality of any steps in your production. Please read our first blog entry Audio Production in the Age of mp3 and AAC for details.

Don't Forget the Future - Archive Your Music

3/24/11 - Some very important news you need to read concerning archiving. After you read this post, please go to Alarming News.

When an artist/producer finishes a project, they are often so beaten down by it that they don't think of ordering at least a production master for their own archive. (Archiving means to store important information for a long period of time.) And by the time they realize this it may be too late to obtain one. Many CD plants will not return the Production Master CD (PMCD) or DDPi that you send them. That way you are locked into using them for additional runs. Glass masters are not archived at CD plants as many tend to think. A CD plant will only have a certain number of glass discs that are then re-used over and over again. If you order a new run of CDs they will cut glass again from your master that they have on file. Or in some cases the plant creates an image file of your PMCD, or copies your DDPi to their drives, and then destroys the actual disc. And what many have learned the hard way, most of the ads you see for CD manufacturing are not CD plants, but brokers acting as a go-between that normally use several different plants. So as those people go out of business, there is often no record of what CD plant was actually used for a given project, so the master is lost.

But producers, artists and labels should consider not only having an additional PMCD or DDPi of their own, but a data backup of the mastered project in high resolution as well. Here is how it works in the mastering process (if you are dealing with a real mastering room):

Ideally your original source mixes were at 24 bit resolution, and mixed at the same sample rate at which the session was tracked. And hopefully it was tracked at 88.2 or 96khz. Regardless, the mix engineer should not have sample-rate converted the mixes. We know that CD-Audio can only be 44.1khz, 16 bit resolution. But the mastering should be done at the higher sample rate of the sources and at 24 bit resolution, then downconverting to 44.1/16 should be the final step before the PMCD or DDPi is cut. Even in the worst case where the source mixes were delivered at 44.1/16, the mastering will still be done at the 24 bit level. So you can still get mastered files at 44.1k/24 bit, which will have a sound superior to the CD.

So why would you want these high resolution files? We can look at two possible scenarios:

First, if we follow the current trend where the majority of audio will be delivered online, and if things stay at the relatively lo-fi of our current mp3 and AAC formats, then you will have the best sounding masters possible to create new bit-reduced files as you need or as technology changes. As I explained in my first post, your master has to be of the utmost quality if the bit-reduced versions are going to sound decent. So if you have mastered files at a higher resolution than even CD-Audio, then any down-converted files you create from them are all the better.

Second, and more importantly, we are now seeing what industry insiders have predicted (hoped for) for a couple of years - that as internet speeds increase, and as storage gets smaller and cheaper, we will see high-resolution download options for consumers interested in better sound quality. You now see many bands offering at least CD resolution WAV files and even better on their personal sites as a more expensive option to the mp3 download. Radiohead's "King of Limbs" was offered as 44.1/16 WAVs for instance. And we are beginning to see sights like hdtracks.com that offer current and classic albums in as high as 176.2k/24 bit! So as this trend hopefully continues you will eventually (or even now) be able to offer high-resolution downloads of your music.

1/18/12 - Click Here for important update on tracking and mixing in HD.

Less likely, if a new high-resolution consumer audio format is released that is actually accepted by the general public, then you have high-resolution mastered files that you can release on the new format. Unfortunately, DVD-Audio and SACD were miserable failures. And the majors aren't likely to get behind any new hard delivery mediums anytime soon, if ever. 

A qualified mastering room will be able to offer a DVD containing your high-resolution mastered tracks as well as a Production Master DDPi file set for CD manufacturing, and your unmastered source mixes. At DES Mastering it is our standard practice to also include text files of the pq and delivery logs, metadata verification as well as mastering details. Ideally you would get both a Production Master CD and a data DVD as described for your archive. As this will probably run a few hundred dollars, you should be able to arrange with your mastering room to have all of this created at a later date. Mastering rooms can usually create archival masters within a two or three month period after the project is finished.

IF YOU HAVE DAT MASTERS, YOU NEED TO GET THEM TRANSFERRED NOW!!!

If you have important music on DATs you need to get them transferred to standard audio files as soon as you can. There are two reasons for this; number one is that the DAT is a mechanical format that is subject to failure. Even in mastering rooms with well maintained machines a snag can happen and a tape can be ruined. Also, depending on how the tape was stored, the data can become unreadable.

One of the biggest reasons however is that DAT machines are no longer made, and are no longer commonly seen in most studios. And fewer and fewer technicians can repair them or have access to parts. So before they all go away, get your DATs transferred!

Audio Production in the Age of mp3 and AAC

A lot of people are suddenly questioning whether their recording and production quality standards can be lowered if they are strictly using online delivery, and the music is going to be reduced to the quality of mp3 and AAC anyway.

Here is the surprising truth: The quality of your production chain is MORE important if any of the data-compressed formats are going to be the primary delivery medium. AAC for iTunes or mp3, even at the highest kbps rate, are not near the quality of CD audio. When your music is converted to these formats, it will lose quality. So the better the quality of your final mastered project, the better the data-compressed files will sound. That may seem like a very obvious statement to some, but surprisingly a lot of people see it backwards; the delivery medium is of relatively low quality, so the quality of the original recording is less important. They couldn't be more wrong. In this age of internet delivery, from tracking to mixing to mastering you need to pay more attention to quality than ever before.