Tedium - Mind The Pregap 💿

The tale of the pregap, a weird quirk of the compact disc era.

Hunting for the end of the long tail • July 06, 2024

Today in Tedium: Having gotten through a massive Animal Well journey, I’d like to make a proclamation: More things need hidden secrets, like Easter eggs. Usually, games and applications are where we find hidden secrets, but during the 1990s, the music industry had a monopoly on all the good ones, thanks to the compact disc, which kicked off a hidden track renaissance. As cool as hidden tracks were, the best CD trick was something called a pregap, referring to the space immediately before the first track of an album. It led to numerous innovative ideas—but it also pointed out the risks of going off-spec, as anyone who has ever tried to pull a pregap from a CD-ROM can tell you. Today’s Tedium ponders where pregaps came from and how it became the music industry’s weirdest hiding spot. – Ernie @ Tedium

pause encoding

The term the compact disc specification uses for what is better known as the pregap section of audio CDs, according to the standard IEC 60908. (This is not a particularly easy document to access, by the way, because it costs money. You won’t find any copies on the Internet Archive, though related documents are online that refer to pause encoding are online, if you’re curious.)

(Cross Duck/Flickr)

If you have a favorite CD from the ’80s or ’90s, it might have a really tiny pregap

“Why do older CDs usually have a 0.33 second pregap before the first track?”

As nerdy questions go, you don’t get much nerdier than this particular question, asked about a year ago on the Steve Hoffman Music Forums, a popular forum for music nerds and audio engineers. Apparently, the user noticed it when trying to rip CDs using the tool Exact Audio Copy, an application well-known for its ability to rip CDs that other programs can’t.

It may be the most specific question I have found in my near-decade-long history of looking for specific questions.

Exact Audio Copy, the program that exposed the extremely specific pregap hidden on many pre-2000 CDs.

This is not something the average person would notice (after all, when was the last time you used a CD that allowed you to go backwards exactly a third of a second?), but it proved a problem in this specific case, because it apparently caused an issue with the user’s external drive:

The 32/33 frame pregap has no impact on my built-in drive’s ripping speed in EAC, but does impact the speed of my rips on my external ASUS drive—it seems to spin up to full speed a few seconds into a rip on a CD without this pregap, but doesn’t on ones with, meaning rips take about twice the length.

A commenter on the thread who identified as a professional audio engineer suggested the poster was “agonizing over trivia.” (This engineer will probably throw a chair when he sees this piece.) But honestly, it reflects a truism about disc mastering: Over time, people got better at it. There is speculation that the discs with pregaps were made at one factory over another, which would require speaking to specific engineers who may have left this mortal coil decades ago.

(More recently, the same user asked another very specific question: “Which CDs from the 1980s were mastered using first generation tapes?”)

The weird part is, he’s not the first person to notice this! About a decade ago, a user on the Hydrogen Audio forums found pregaps on albums as diverse and well-known as Beck’s Mellow Gold, Guns N’ Roses’ Use Your Illusion I & II, Nirvana’s Nevermind, and, um, the score to Schindler’s List. These albums all had a pregap of either 32 or 33 “frames,” with a frame representing a length of about 1/75th of a second, per Hydrogen Audio’s Wiki. This apparently is not a specification made up by EAC, but dictated by the Red Book, the compact disc guidebook that sets specifications for the format. According to The CD ROM Handbook, a 1994 guide to the mastering and setup of compact discs of both audio and digital forms, each of these sectors includes exactly 2,352 bytes of digitized audio data.

To offer a small correction to the original question now that we know we’re talking about 74 frames per second rather than 60 or 100: we are talking about a period of just shy of half a second—a period that most CD players could not access. But the interesting part is that this is actually a shorter period than the format actually suggests, a length of between two and three seconds—an essential separation for data. But over time, it appears that audio CD makers realized that such a gap was not particularly necessary.

These data points obviously do not affect what you’re listening to. You may not even notice them! But small differences in CD length add up when you’re doing things like adding music into a database like MusicBrainz. Hence why nerds ripping CDs notice things like this!


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The production of this Willie Nelson album inspired a pregap-related patent filing.

The patent filing that made Willie Nelson a pregap hidden track innovator

When I say the name “Willie Nelson,” the phrase that comes to mind is probably “outlaw country.” But I’d like to make the case that Nelson deserves another term tied to his name: “Hidden track innovator.”

It’s true. In 1994, the country legend, between major labels, took a chance on a small indie jazz label called Justice Records for his record Moonlight Becomes You. It was small potatoes compared to the majors he usually worked with, but Nelson rolled with the dice and got a Grammy nomination from it—one of 57 to his name. It’s also an album that you will not find on any streaming service, likely because he chose to work with a small indie label, though the songs are on YouTube if you’re curious.

The vibe is much more Sinatra-ish than Nelson usually goes, as the above performance of the title track shows. (It’s in the vein of his 1978 album Stardust, a hugely successful record of pop standards that presented Nelson as a master interpreter.) But Nelson happened to be working with a label whose founder had just filed a patent for the pregap hidden track. And per a Houston Chronicle review, Nelson apparently used it to explain why he went with Justice Records rather than another big label.

Willie’s album was actually produced by the guy who filed the patent, Randall Jamail, with the label’s COO David Thompson, also appearing on some patent filings.

That patent filing, titled “Method of selectively concealing magneto-optical compact disk data for playback upon demand,” describes a process for “impressing data on the media between the first play location and the begin play point.” (I like sharing patent drawings, but there’s really no point with this one—it’s just a flowchart that describes the process of adding the track, no fun visuals here.) The invention, which the label called Justice Soundboard, was promoted as a big deal by the Houston label.

The ambient “Running Man” by the heavy metal band Course of Empire, either the first or second pregap hidden track, based on how you measure this.

There was just one problem: A producer in Dallas had actually come up with the technique first, a few months before Jamail. Course of Empire, a heavy metal band signed to Zoo Entertainment (a label best known for bringing the world Tool), had used the exact same technique on their 1993 album Initiation, which had some weirdly hidden tracks, including one buried inside the mono channel on another track, which is kind of insane.

The debate, which played out in Billboard in the spring of 1995, seems to have been the result of weird timing.

Castell notes that he was unaware of the Justice Soundboard. However, he insists that he came up with the invention himself months before the fall ’93 Justice planning meetings for the Nelson record, at which the idea for the Soundboard was hatched, according to Jamail.

Castell, like other audio professionals surveyed for this story, is skeptical of the patentability of the feature. In fact, he says it never even occurred to him to file for a patent.

Ironically, Jamail says he also was initially skeptical about the patent-potential of his idea, but proceeded with the application at the urging of his colleague, Thompson, who discovered after a week of research that neither Philips nor Sony—the inventors of the CD—had patented the procedure.

(Another, unrelated claimant is Electric Light Orchestra, which reissued an album with a pregap in 1993. For completeness’ sake: Wikipedia claims another act with an unprintable name, initials A.C., also was early to the trend. However, per my Discogs research, the EP in question was released on vinyl first, then on CD years later. Whew. I didn’t want to have to make this whole section about that band.)

Ultimately, the patent ended up sticking around, which was great for Justice Records, which had to sue Sony and Philips to allow his label to license its technique. Jamail quickly won the case, per the Dallas Observer.

“This is going to be given to the consumer for free,” Jamail told the alt-weekly. ”I’m going to license Soundboard for pennies a disc, and with CD pricing, there’s room for the record companies to absorb that.”

So, if your favorite record had a hidden pregap track on it, just know that it probably made Justice Records a bunch of money.

(Rodrigo Sena/Flickr)

Five interesting details about pregaps

  1. Technically, every track can have a pregap. If you want to put some weird audio in the spot between a given track, it’s technically possible to do. As The CD ROM Handbook explains, the Red Book spec lets CD-makers add two-second audio pregaps between each track, though they’re optional, as proven by the fact that your favorite albums aren’t loaded with giant pauses. The only pregap that’s actually required is the one directly before the CD.

  2. The first format to hide stuff in the pregap wasn’t an audio CD. In the early ’90s, Philips bet big on the CD-i format. Thanks to the fact that Philips had directly developed the CD format with Sony, it built some CD-i specific standardization of its own. One of those standards was the CD-i Ready format, which hid the CD-i data in the pregap area, about two years before Randall Jamail filed for his patent.

  3. Many printing plants won’t press CDs with pregaps. As we noted in our piece on hidden tracks—and first reported by the late publication Wondering Sound in 2014—They Might Be Giants reportedly faced a lot of drama with their label for using a pregap, because it made printing the CDs harder. These days, hidden pregap tracks are unpopular with CD plants—to the point where plants won’t even consider doing them, according to mastering engineer Russ Hepworth-Sawyer.

  4. There’s also a thing called a postgap. Similar to a pregap, these are used as a pause on the CD, but they are specifically used on data CDs to signify the end of the data section. In other words, they represent an actual pause. If you’re running a CD-ROM for a game console (see this doc for the PlayStation development platform PSYQ), for instance, you may see a postgap there.

  5. Your CD-R can print pregaps and postgaps. There’s a terminology used in CD-R software called track-at-once a burning style that effectively adds gaps between different tracks, essentially turning off the laser between burns. This is useful if the disc is a multi-session CD-R, because the postgap allows you to add more tracks later on.

“In fact, most industry experts estimate as many as half of currently installed CD-ROMs won’t be able to play the new discs.”

— A Billboard piece from the fall of 1995, mere months after the magazine was home to the showdown between Justice Records and Course of Empire, discussing the rise of the Enhanced CD, a standard that aimed to bring music albums to the PC and Macintosh. Nothing like a standard that doesn’t work for most people!

An example of the kind of content you might find on an Enhanced CD, from the Barenaked Ladies’ Shoe Box EP. So, essentially, home videos.

The Enhanced CD highlighted why hidden pregaps were so problematic

If you bought a CD from a major label in the mid-’90s, odds are it might have included some additional digital stuff for your computer. One band that embraced this strategy was the Barenaked Ladies, whose Shoe Box EP first used the technique in 1994. (Here’s a video collection from the CD, with like half an hour of random video content.) These albums, called Enhanced CDs, came with multimedia stuff like music videos, band photos, or even a multimedia experience that leveraged the benefits the format already offered.

BNL found success with the format, replicating the idea again about a year later for its live album Rock Spectacle, which proved the band’s U.S. breakout. The band’s Tyler Stewart, speaking to Billboard in November 1996, said the ECD format allowed the live album to better present the band’s on-stage humor to CD buyers.

“We’re lucky that we have the extra dimension of humor,” Stewart said at the time. “Good ECDs are probably more difficult to make if the band is serious.”

The ECD format was a hybrid CD, and the format had Apple’s fingerprints all over it at a time when Apple wasn’t exactly at full strength, thanks to many ECDs utilizing QuickTime. As Josh Warner wrote in the Apple-sponsored Enhanced CD Fact Book:

It’s important to know where you came from to know where you are. Until recently, most multimedia discs targeting the music market were full-blown CD-ROMs, such as Xplora1: Peter Gabriel’s Secret World or JUMP: The David Bowie Interactive CD-ROM. These discs are meant to be played on the CD-ROM drive of your computer. That’s the only way you’ll get at what is important–the multimedia.

Enhanced CDs are different. The primary focus of enhanced CD is the music. The multimedia part is the bonus. For this reason, the name is revealing. It recognizes the format as an enhancement of the popular audio CD, which follows an industry standard referred to as the Red Book.

“Enhancement of the popular audio CD” sounds good. There was just one problem: It wasn’t easy to do any of this. It was sort of the Wild West, and it required a lot of experimenting to get right. Warner’s guide described the use of three approaches for making ECDs: The first, a mixed-mode CD, puts data on the first track, threatening to destroy the speakers of any given music fan if they leave the CD on repeat. Warner made it clear this idea was a nonstarter:

Mixed-mode enhanced CDs are commonly stickered with an alert like “SKIP TRACK ONE OR ELSE.” Warning labels are OK if you’re a pack of cigarettes, but imagine how artists and record labels feel. That’s why you don’t see very many mixed-mode discs.

Despite this, the format deserves credit for kicking off the trend, with Sarah McLachlan’s The Freedom Sessions often credited as the first Enhanced CD to be released by a mainstream artist.

Don’t listen to the first track of Sarah McLachlan’s The Freedom Sessions, believed to be the first Enhanced CD by a mainstream artist. It might bust your speakers. (via Discogs)

One alternative, as you might guess by the topic of this piece, was to shove the data in the pregap, hiding the data layer from the CD player. One problem: Most CD-ROM drives didn’t actually support pregaps, because of course they didn’t.

The final one, which eventually won out, was called a Multisession CD, also known as CD Plus, which essentially treated the two content elements (music and data) as separate parts based on the machine it was in. Unlike the pregap, the data was technically at the end of the CD, rather than the beginning.

“The audio CD player never gets to the interactive data (which now follows the audio session), virtually eliminating the chance for CD Plus discs to play improperly in a regular audio CD player,” Warner wrote.

Problem is, even fewer CD-ROMs supported the multisession format at the time, which required reprogramming the CD-ROMs to look for a secondary data track. This was easy on Apple’s end—they just created a driver to fix this—but the rest of the PC industry clearly was poorly positioned to handle this.

As Enhanced CDs showed, CD-ROMs did not handle gimmicks well. It was either create a substandard user experience from an audio CD standpoint, or know that the CD-ROM portion won’t work in like half of all CD-ROM drives. What a weird thing to invest in!

And even when the ECD format was out in the wild, breaking changes were still occurring at random. BNL, which saw more success with them than most acts, actually had to delay the release of Rock Spectacle by a couple months, per Billboard, because the disc originally used the pregap Enhanced CD approach. The reason? Microsoft inexplicably decided to change how Windows 95 read CDs, making the pregap approach to Enhanced CD entirely useless.

Despite this, the record industry had been quick to paint the Enhanced CDs as the future, with RIAA then-president Hillary Rosen telling Billboard:

The E-CD targets the nearly 28 million IBM and Mac users, the most rapidly growing consumer lifestyle. E-CD puts us squarely in that market and will ensure our continued leadership in the packaged media for that market.

“Continued,” you say? This technology faded pretty quickly in the late ’90s, especially as the internet proved a much better vessel for this kind of tertiary information.

And because they relied on proprietary technology—like Apple’s QuickTime, or executables that only worked on Classic MacOS, Windows 3.1, or Windows 95—it meant that as soon as the record industry got bored with Enhanced CDs (which it eventually did), it meant the information was essentially useless to the average consumer.

It was as if the recording industry was in a huge rush to timestamp music CDs, which at their best can be timeless, as things that go immediately obsolete. They should have just used the pregaps for hidden tracks.

When the compact disc format was first released in the early 1980s, it was shared with the industry with a script for how the format should be published and produced. While lengthy, the Red Book was a clear list of standards that any label or mastering plant willing to spend money on that book could take advantage of.

The problem is, the compact disc was capable of more than was allowed under these basic parameters. And as a result, people started to stretch it. Sometimes they filed patents for said stretching.

There was one problem, though: While the compact disc media could be easily modified, the compact disc hardware, which generally didn’t have user-upgradeable firmware unless it was attached to a computer, could not. And that meant there were always significant edge cases that emerged when the discs you used didn’t match the norms.

The pregap was one of the most notable examples of this, but it was far from the most divergent—the DualDisc, which David Buck wrote about in 2019, was so far afield from the Red Book standard that it couldn’t even call itself a compact disc.

In this light, the pregap, the tiny little area that initially only meant something to audio engineers before becoming the home of Enhanced CDs and artists who felt confined by a 74-minute limit, was an oddity. It was something specified for a specific use on the CD, only for its purpose to morph as people actually got their hands on the idea.

But we love finding secrets—and the pregap gave us another place to look for them.


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