Education and Resources

Premastering for Enhanced CD
BY Dr. Toby Mountain
(revised from the original article that appeared in MIX magazine, April 1997)

Just when you think a technology is dead, it sometimes gains a second life. Such is the case with Enhanced CD.
The format was officially pronounced 'DOA' nearly three years ago by the press, but has recently enjoyed a renaissance. Since then over 3,000 Enhanced CD titles have been released with the numbers growing steadily. NDR has seen a 500% increase of ECD masters over last year.

Enhanced CD may play a greater role in the music industry than we think. It actually represents a milestone in atortured ten year odyssey of building a successful interactive music disc.Now Enhanced CD is poised to become a key bridge technology to both DVD-ROM and DVD-Audio.
The purpose of this article is to familiarize audio professionals with the format and demonstrate the best strategies to premaster Enhanced CD. Before launching into a "hands-on" discussion, I'd like to review with you some compact disc history to illustrate the bizarre journey that Enhanced CD has taken.

Music Plus"Something Else"

When Sony and Philips created the Red Book compact disc standard almost fifteen years ago they hardly envisioned the plethora of CD "offspring" that would emerge. In 1985 an elite consortium of computer companies called the High Sierra group laid the groundwork for Yellow Book CD-ROM standard for storing computer data on a CD. Within a few years publishers were not only putting text on CD-ROM, but also color graphics, animation and even video. Having witnessed the huge success of MTV, record executives couldn't help but notice the impending explosion of multimedia. They were eager to come up with a way to add extra information to their CD releases. Early experiments of stuffing lyrics and low resolution graphics into the unused R-W subchannels of the Audio CD resulted in CD+ and CD+G. More elaborate formats like Commodore's CDTV and Philips' CD-Interactive interleaved music with video and graphics. All were consumer flops. Early failures of such proprietary formats forced record companies to refocus their efforts on the personal computer to deliver multimedia with music. The obvious solution was to fuse two standards, Red Book and Yellow Book.

The first incarnation was Mixed Mode CD-ROM, which consists of a ROM track followed by any number of Red Book audio tracks. But Mixed Mode has two problems. First, the CD-ROM track can be accidently accessed by some audio players, causing an ugly static-like sound and potential speaker damage. Second, retailers don't know whether to sell the discs as audio or CD-ROM discs. Despite several impressive music titles like Sarah McLachlan's "The Freedom Sessions," Mixed Mode has had a limited appeal to music afficianados. The inadequacies of Mixed Mode led people in the industry to come up with clever solutions. One of these, known as Pregap, actually "hides" the computer data at the front of the audio CD between index 0 and index 1 of track one. A CD player still thinks it's an audio CD, while a computer is fooled into thinking that it's a CD-ROM. Pregap improves upon Mixed Mode by not allowing the audio user to access the Enhanced CD track directly, but other problems exist. Intense controversy has erupted over who owns the patent and receives royalties for Pregap, so developers are staying away from it. Worse still, a 1996 revision to Windows 95 mysteriously made the Pregap track inaccessible to PC users (Billboard 8/17/96).

The best solution to date, CD EXTRA (formerly CD Plus) inverts the track structure of Mixed Mode by creating two separate sessions: first audio, then data. Major players in the computer and record industry have already endorsed CD Extra under a new compact disc standard called Blue Book. Blue Book is fully compatible with Red Book , ie. it works safely on all CD Audio players. It also establishes a minimal file structure within the data track which may be utilized by smart CD Audio Players and future DVD Players. CD Extra's one drawback is that it's dual session format makes it inaccessible to first generation CD-ROM drives and out of date software drivers. Within the context of this article I have chosen to focus on the CD Extra/Blue Book standard. Because it has become the defacto standard, I'll simply refer to it as Enhanced CD.

Mastering Enhanced CD

Enhanced CD utilizes a multi-session format, with the following disc geography: Session 1: "Red Book" Audio - Session 2: "Yellow Book" CD-ROM-XA, mode 2. The Extra/Blue Book spec imposes a minimum file structure requirement within Session 2 in addition to any stand-alone multimedia applications. We'll discuss the specific requirements of "Blue Book" later.

It is possible to send the music and data separately off to the CD plant and assigning them the task of putting the Enhanced CD together. Several plants are capable of doing this, but I would discourage this route. It is very important for the artist, producer, and label to receive and approve both programs on one disc before replication. The Enhanced Master should be done in Premastering to a recordable CD (CD-R) to avoid expensive extra steps at the plant.

Session 1: Creating Audio

The audio section of a Enhanced CD must be a Disc at Once session that is unfinished to allow the second data session to be written. Any of the premier audio mastering softwares like SonicStudio�Ѣ or Sadie are now capable of writing unfinshed audio sessions. Even though our Enhanced CD audio initially gets created within SonicStudio�Ѣ, we prefer to have a professional CD-R software like Adaptec's Jam write the unfinished audio to CD-R. Sonic can generate an audio image file (actually a SoundDesigner II file compatible with JAM) that includes all tracking, subcode, and ISRC data.

Some audiophiles and engineers may question the integrity and convenience of converting the audio into a computer file format before writing to CD. Bit for bit testing (see below) has proven that writing an unfinished Red Book audio session using JAM produces an identical copy of the Sonic master. The audio image file can be conveniently archived or tranferred to any computer workstation without error or degradation. Since multimedia sessions of Enhanced CD projects tend go through several stages of revisions, having the audio "cached" as a computer file frees up our audio mastering rooms from costly rewrites.

Why We Don't Use Digital Audio Extraction

I would highly discourage the practice of Digital Audio Extraction, which so many inexpensive CD-R packages offer. This process captures the samples from an audio CD-R played played on a CD-ROM drive and writes them via the SCSI bus to a computer audio file on a hard drive. Since you can't monitor the copying process, it's more or less an act of faith.

While copying computer files from a CD-ROM to a hard drive is generally a straight-forward, error free process, digital audio extraction is somewhat more daunting and less reliable. An audio CD has a larger sector size (2,352 Bytes) than CD-ROM (2,048 Bytes) which means the CPU has to work about 15% faster while also decoding the audio data. Unlike CD-ROM sectors, CD audio sectors have no headers, just Q Subcode information. When the laser searches the Q subcode to find it's previous location, there can be an error of +/-5 CD frames! This must be compensated for by smart copying software which performs 'overlap reads' by collecting redundant audio samples and deleting the ones that have already been written to the hard drive. For this reason, Digital Audio Extraction is not a reliable linear process.

Aural Verification

Regardless of the method you use to write the audio of an Enhanced CD, you need to be assured that the transfer process did not compromise the audio. When comparing the source and copy programs aurally, be sure to audition them through the same CD player or D/A. This will minimize the varying jitter characteristics of different digital audio media (CD, DAT, HD) and transfer protocols (AES,SPDIF,Optical). Remember, a digital audio copy pretends to be nothing more than a list of numbers with a generic sampling rate. Regardless of all the hyped up "anti-jitter" boxes and cables that exist in mastering studios these days, the potential jitter characteristics of a replicated CD come down to the final playback during glass mastering. Rest assured that most CD plants cut their glass masters with extensive buffering prior to a highly accurate, independent low jitter clock.

Checking the Audio Samples

With little control over jitter, our priority is making sure the audio data is identical to the source. There is a bit for bit test we perform that I mentioned earlier. We load both "before" and "after" programs into our Sonic System with all input dither and DSP disabled. Then we synchronize the two programs to the audio sample. We play all four channels through a stereo output mixer but with one of the stereo programs out of phase. If the files are identical, they cancel each other out. A Sony PCM-1630 video screen confirms zero output with a black screen. We perform this test free of charge for Enhanced CD clients who are concerned about the integrity of their audio on a dual session disc.

Session 2: Enhanced CD

Once we've survived the first round of audio premastering, then we're ready for round two: adding the multimedia track. Usually we're spared the agonies of authoring the multimedia, but at the very least the artist or developer will ask us to let them know how much room is left for the Enhanced CD track. Here's a simple formula that we use:

650MB -Total audio minutes x 10MB = MB available for Session 2.

The multimedia developer usually provides us with the Enhanced CD data on one of several currently popular storage formats (Zip, Jaz, CD-R). Our job is to successfuly format and write this data after the audio to a CD-R that the plant can successfully master. Most of the time, the release is for both Mac and PC, so we have to create a Shared Hybrid session, which will consist of an HFS directory and an ISO 9660 directory with common files. We also have to be vigilant about ISO 9660 naming restraints, since some companies want their release to be compatible with Windows 3.1. We create the Enhanced session in CD-ROM-XA format or more specifically, CD-ROM Mode 2, Form 1. XA stands for "extended architecture" and is a variant of CD-ROM used for multi-session.

We take extra care to make the Enhanced section functionally efficient and appealing. This means defragmenting and optimizing the files on the CD-R for optimum performance. It also means enhancing the appearance of the project on the desktop: creating custom icons for both Mac and PC, creating AutoStart capability, and discretely hiding or making invisible files and folders that the end user need not directly access.

Sometimes our role extends beyond premastering. Because we have a lot of multimedia authoring experience, many record companies rely on us for Beta testing and troubleshooting their project on both Mac and PC. This sometimes means correcting a Macromedia Director bug or even creating a Director Projector. Some clients need a music video converted to digital video (QuickTime or MPEG). Currently, the most popular feature that we add to an Enhanced CD is a Web Link, which enables the end user to directly connect to the artist's Web page.

View from the Top

So why would a record company want to go to the extra hassle and expense of enhancing their audio CDs, especially when charging extra at the retail level has not been a viable option?

Labels and musicians are finding that the benefits are more subtle and hidden. Sony has used the Enhanced CD as a 'watering trough' to their artists' Web sites. This means better fan awareness of group activities, concert schedules, and future releases, which all translate into increased product merchandising. Smaller labels have found they can sustain their loyal audiences with captivating "insider" artist info and videos.

This has always been the strategy of Rykodisc, an Indie well known for venturing into uncharted waters. They released the first ever "multi-session" Enhanced CD (Sugar "Besides") in August 1995. Says Lars Murray of Ryko, "Our policy from the start has been to try and provide worthwhile multimedia for the fans, but not to let the multimedia drive up the price of the disc or confuse the public with a separate SKU. It took some labels longer to figure this out. We started simply, with full-length videos, and kept the budgets down so we could provide the goodies for free. As developers work out the kinks, I expect enhanced discs to become more commonplace, and less risky to produce and market."

Aside from financial incentives, many labels see Enhanced CD as a springboard to DVD or other future multimedia formats. And it's affordable and "do-able" right here and now. They figure they had better "get their feet wet" before multimedia convergence in the home actually becomes a reality in the 21st Century. They sense that the cautious gait toward interactive audio/video in the home may someday become a stampede.