Introduction

As a teenager I wanted to make records. When it became clear to me that no-one else wanted me to (what do record companies know! eh?) I decided to do it for myself. Bit by bit I taught myself everything I needed to know and slowly built up a modest collection of equipment. One process, however, remained a mystery, audio CD mastering and duplication. It seemed to me an expensive and esoteric practice, and so it was until the arrival of fast affordable home computer systems and cheap CD recorders.

During this article I will discuss ...

Sound recording mastering processes

CD formats
CD recording hardware
CD recording software
Computer based CD recording systems
the steps necessary to create a Red Book audio CD

Sound recording mastering

The production of an audio CD LP (long player) requires four distinct processes.

1 Multi tracking

During multi tracking the individual components of the recording are originated and recorded to the separate tracks of MIDI and audio multitrack systems.

2 Mixdown mastering

This involves assembling these components into balanced, effected and finished 2-track stereo mix masters conventionally recorded onto a DAT recorder, 2-track analogue tape recorder, CD-Recorder or hard disk recorder.

3 Production mastering stage 1

Production mastering (my term) involves two stages. This first stage involves a degree of art and science.

First all the individual 2-track mix masters are "tweaked". This will be done at the highest sample rate and bit depth possible. Typically 48KHz or 96KHz at 24-bit or whatever the original (multitrack) recordings were made at. They may be EQ' d to ensure overall consistency throughout the LP and compressed and enhanced. Some of these processes may be carried out in the analogue domain. This stage of mastering is more art than science and I am not qualified to write with authority on it. Seek out advice from mastering engineers.

When the tweaks are complete, all the individual stereo mixes will be (down) sample and bit rate converted to conform to the audio CD standard (44.1kHz 16-bit for audio CDs).

This technical aspects of these processes is brilliantly discussed by Bob Katz in his book Mastering Audio is a must read. Find it on the net, buy it and read it!

4 Production mastering stage 2

In the second stage, all the tracks are assembled together into a coherent LP using CD mastering software. Running order is decided upon, tracks are normalised to ensures that dynamic range is maximised, the relative level of each track determined, and fade-in, fade-outs and crossfades are set. Then all the necessary settings are made which relate to the way CDs store and replay tracks including the PQ Code data (more on this later).

The final product of the production mastering process, the Production Master, will usually take the form of either a Sony PCM-1610/1630 2-track stereo master (stored on a U-matic video cassette) or a red book audio CD, or a a hard drive of audio files ready to be sent to the CD factory for duplication. You will be interested to know that more and more factories are accepting Red Book CDs as Production Masters. This has significant advantages for home music makers as you will see.

The rest of this article concerns itself with the second stage of Production Mastering and how these processes can be completed at home using an affordable computer based software and CD-Recorder system.

Mastering Studios

In the past Production Mastering has had two disadvantages. Firstly it was the point at which the home recordist (and indeed studio producer, engineer, and artist) relinquished total artistic control. Going to a mastering studio involved putting your precious work into the hands of a mastering engineer, listening to his advice and trusting that the unfamiliar sound coming from his monitors was all for the best.

Admittedly there was (and is) an advantage in that a good mastering engineer will know better than you how a well balanced recording should sound, offer an invaluable objective view, and have at their disposal an arsenal of esoteric and wonderful "audio sweetening" machinery. For these reasons most major record label funded commercial LPs are mastered by industry renowned mastering engineers such as Bob Ludwig and Ted Jensen.

The second disadvantage is more compelling and has to do with an arm and a leg. Professional mastering can be prohibitively expensive for the home recordist, especially one only seeking to duplicate short runs to order using a CD-Recorder at home.

Before we go on to discuss how we can accomplish stage 2 of the production mastering processes at home we need to know something about CD formats and CD recording hardware.

CD data formats

There are several different formats of CD. Originally CD was developed by Phillips and Sony as a replacement for vinyl. They defined a standard audio format to be used by all domestic CD players and published it in a book with a red cover, thus we have the Red Book Audio CD standard format!

CD areas

All CDs have three distinct areas. The Lead-In Area contains a TOC (Table Of Contents) and is the first area on a CD occupying the innermost 0.4mm of the disc. The TOC contains a list of everything on the CD including the Track ID positions (or start times) of all the audio and data tracks on the disc. The Program Area contains all the actual audio or data tracks. The Lead-Out Area simply marks the end of the Program Area.

CD tracks

All CDs can have a maximum of 99 tracks. Tracks IDs mark the beginning of tracks. A track can be a minimum of 4 seconds long (600 sectors).

CD sectors

The data on all CDs is stored in chunks called sectors. There are 330,000 sectors on a CD each of which can hold 2352 bytes. This adds up to a total data capacity of approximately 744MB (Mega Bytes). Different CD formats use this capacity in different ways..

There are now a number of 80 minute CDs available but beware, your software and hardware may not be able to write CDs longer than 74 minutes.
Red book Red Book audio CDs use nearly all of their storage capacity to store audio samples. A single minute of 16-bit 44.1kHz stereo audio uses 10MB of storage. The maximum running time of an audio CD is 74 minutes using 740MB of its total 744MB capacity. The remaining 4MB holds error correction and sub-channel data.

PQ sub-channel data

Sub-channel (or sub-code) data is designated by the letters P through to W. In the Lead-In Area the Q sub-channel contains the TOC complete with Track ID positions. In the Program Area the P sub-channel data indicates where music tracks start and end while the Q sub-channel contains time information. During CD Production Mastering it is often necessary to edit the PQ sub-channel data in order to set the Track ID positions, particularly where music tracks crossfade into one another. Incidentally, sub-channels R to W are mostly unused at present.

The Q sub-channel contains four other data fields which we will need to set during Production Mastering.

Pre-emphasis Pre-Emphasis On/Off is a sub-code which turns a CD players De-Emphasis circuitry on to allow it to decode CDs encoded with Pre-Emphasis, a now largely unused early form of digital noise reduction.
ISRC (International Standard Recording Code) code This sub-code holds the "serial number" of each audio track in a format prescribed by the Red Book standard. It can be used by music industry organisations to keep track of copyrights and royalty payments for record companies, composers and artists etc.
Media catalogue number This sub-code defines a 13 digit bar code for CDs and should be set if the CD is to be duplicated for commercial distribution in retail shops. Codes are allocated by the EAN and UPC authorities.
Copy prohibit on/off This is a sub-code that domestic digital recorders (such as DAT, DCC, Mini-Disc and CD-R recorders) can read to determine whether or not digital copying (or cloning) is to be permitted by their SCMS (Serial Copy Management System) circuitry.

Sessions

A session is a collection of tracks. Every time data is recorded to a CD a session is created. Red Book audio CDs must be written in one complete session without pauses, a process called "disk-at-once" by CD-Recorder manufacturers. Because domestic CD players only read the TOC in the Lead-In Area of a disc, a subsequent session which places a second TOC followed by audio tracks on the program area of a disc will never be "seen" by the player. CD-ROM drives are able to recognise all the sessions on a disc and treat them as separate drive volumes.

Yellow book

After the Red Book standard had been established it was realised that CDs which store digital audio could just as well store any other type of digital data and the CD-ROM format was defined (Yellow Book). CD-ROMs use a significant number of their bytes for error correction and thus the room left for storage of actual data is approximately 650MB.

CD-ROM drives are included as standard with all desktop computers and with the appropriate software these drives can extract Red Book audio CD data as computer files and copy it to the computers hard drive where it can be treated like any other audio file.

Blue book

Subsequently many other formats have been developed (Orange, White, Green and Photo CD) but Blue Book (CD Extra) is probably the other format of interest to music makers. Essentially it is a mixed format that allows both audio and CD-ROM data to be included on the same disc. Such a disc will have two so called "sessions". The first will include all the audio data complete with its TOC. The second will be a CD-ROM session, unplayable by a domestic CD audio player but recognisable when placed in the CD-ROM drive of a computer system.

The idea is that an artist can include some new (multi) media material such as animation, interviews, videos etc to complement the audio section. Because a CD-ROM session will eat into the total storage capacity of the disc this material may only be a taster for a more comprehensive and separately available CD-ROM production. Recently several LPs (including Coldcut's brilliant "Let US Play") have included a complete second CD-ROM disc. Many sound recording magazines include Blue Book cover CDs containing PC, Mac and audio sessions.

CD physical formats & recorder types

Now we know something about CD data formats we can move on to discussing different types of CD recorder hardware and the CD discs they are capable of creating. There are currently two recorder/disc types ...

CD-R (Recordable) These recorder/disc combinations can only write discs once. This means that disc data cannot be erased and new data rewritten at a later date.
CD-RW (ReWriteable) These recorders/disc combinations are capable of writing, erasing and writing once again. WARNING! CD-RW discs often cannot be replayed in conventional CD players unless they are MultiRead capable.

Both of these formats are implemented in two hardware types ...

1 Domestic hi-fi recorders

These recorders can accept either a stereo analogue signal or an SPDIF (Sony Phillips Digital Interface) digital input from a device such as a CD player, or DAT recorder. They are capable of automatically converting DAT ID markers into CD track markers. Although offering a quick and easy way to duplicate CDs they suffer from three primary disadvantages ...

Firstly blank discs cost are high. £5+ for CD-R, £9+ for CD-RW. This because the music industry, in fear of widespread infringement of its copyrights, insisted on a royalty levy for every blank disc sold.
Secondly these recorders cannot produce Red Book CDs.
Thirdly they cannot be employed to record any other type of digital data, such as computer files stored on a computers hard drive.

2 Computer front ended recorders

The second recorder type are computer front-ended. This means they are connected to a computer system like any other storage device, usually internally or externally using a FireWire or USB 2 interface/cable which supports the kind of sustained data transfer rates necessary for creating an audio CD. The data to be recorded is prepared and stored on a hard drive and an appropriate application is employed to transfer it to the recorder and write, or burn, the disc.

Such a system has several advantages over hi-fi recorders ...

Firstly branded blank CD-R discs are cheaper (but you should always use good quality branded media for your Red Book masters and duplication).
Secondly, these recorders are capable of producing any format of CD including Red Book and Blue Book.
Thirdly they can backup ANY data residing on the hard drive so you could, for example, archive your multitrack MIDI and audio data as computer files.
Fourthly, data to be written can be comprehensively arranged and formatted prior to recording. You can, for example, set tracks to crossfade and determine the precise locations of Track IDs.
Fifthly these recorders cost less than their hi-fi counterparts and come packaged with CD writing software.

Disc-At-Once vs Track-At-Once

Another issue that affects both recorder types is whether they are capable of writing a disc from beginning to end without pauses, a necessity if a single session is to be created as demanded by the Red Book Audio standard. Disc-At-Once (DAO) recorders can do this, Track-At-Once only recorders cannot. Make sure the recorder you buy is capable of both Track-At-Once and Disc-At-Once recording modes.

Writing speed determines how fast your disc will be written. Computer front ended recorders can write discs faster than real time. At 4 x real time a 60 minute disc will be written in approximately 15 minutes (although the optional verification procedures will take a further 15 minutes if you choose to run them). 6 x and faster drives are now available but BEWARE! Faster write speeds may be convenient but many engineers believe they produce more errors and degrade sound quality. It may be better to stick to 4 x speed.

Sustained data transfer rates

To transfer data from your hard drive to your recorder without interruption requires a robust and fast interconnect technology and SCSI and FireWire (USB 2 also) have been proved to be just that.

Drivers

You need to ensure that the CD Recorder you are planning to buy is supported by the CD mastering applications you want to use. That means visiting relevant manufacturer web sites and checking that drivers and extensions have been developed. I wanted to be able to use Toast, Jam and MasterList CD (of which more later) and found that all of these applications supported the Yamaha 400TX drive. A word of warning here. It is the drive mechanism that is important. My drive is actually a One Technologies unit (essentially a box with power supply and SCSI connector) but the CD-Recorder mechanism fitted inside is the Yamaha.

Software

If you choose a computer front-ended recorder (and you should!) you will need some software to drive it. Popular programs include ...

Roxio Jam

Jam is fully Red Book compliant. It allows you to select your audio files and then determine how they will play on the finished CD. Fades and crossfades may be set, relative levels between tracks tweaked and PQ sub-channel codes fully edited and set. During writing Jam processes the audio files on-the-fly according to your settings (providing you have a fast enough computer, otherwise you will have to create a disc image first).

Waveburner

Apples Waveburner application (which comes with Logic Pro) is an excellent professional program.

Wavelab

SoundForge

Bias Peak

Others

No doubt there are others, just ensure they can produce Red Book CDs.

Host computer

Older computer systems may struggle to write discs effectively whatever the spec of your software and recorder. You will need to satisfy yourself that your CPU, RAM. hard drive and interconnect technology are all up to the job in the first place.

RAM cache

It's worth having a reasonable amount of RAM allocated to your software during writing to allow a large RAM cache to be set. This will help prevent the damaging effects of data transfer bottlenecks should they arise whilst writing a CD. Essentially a reservoir of data is kept topped up in RAM during writing.

Summary so far

Let us summarise what we have learned so far.

The second stage of Production Mastering involves preparing and arranging digital audio data ready for CD duplication.
The traditional process of creating a CD Production Master on a Sony U-matic video tape is expensive.
There are several formats of CD. Red Book is the standard for audio CDs, Blue Book for mixed format audio and CD-ROM CDs.
During Red Book audio CD mastering various sub-channel codes must be edited and set.
To be suitable for use as a Production Master a CD must be Red Book.
Discs created on CD-RW recorders suffer from playback compatibility problems.
Computer front-ended CD-Recorders are more flexible than their domestic counterparts and the discs they use are considerably cheaper.
To create a Red Book CD a recorder must support Disc-At-Once mode and write all the data in one complete session.
It is important to match your recorder to the software you wish to use.
Red Book CD writing applications allow considerably flexibility in determining how tracks will replay.
Your host computer system must be up to the job.

So, armed with all this knowledge, how do we go about creating a Red Book CD suitable for selling to customers or sending to a CD factory for duplication? Read on.

Creating a Red Book audio CD

Finally we get to the crux of the biscuit. I'm going to assume that you have completed all your mixes and the first stage of production mastering. Now here's what you do.

Transfer all your finished stereo audio files into a separate folder, preferably on a separate hard drive or hard drive partition.
Launch your Red Book CD writing application.
Specify a generous RAM Cache (if necessary).
Choose Disk-at-once mode (if necessary).
Choose the slowest burning speed (this will minimises disk errors).
Import your files.
Place them in your desired running order.
Make any changes in levels between tracks.
Apply any mastering plug-ins desired.
Set the desired pause lengths between tracks.
Set any fade in/out and cross fades between tracks.
Edit Track ID start positions.
Set pre-emphasis to OFF.
Set Copy Prohibit as desired.
Leave the ISRC field empty (unless you have obtained the correct codes).
Leave the EAN/UPC fields empty (unless you have obtained the correct codes).
Check that the disc will write successfully by activating Simulation Mode and checking the speed.
Write the disc.
Bore all your friends with your music by pretending you need to "check your mixes" on their CD players.

CD formats & red book CD audio mastering by Matt Ottewill