In a previous Daily Drill Down, ”All About IDE,” I explained the latest formats, performance ratings, and compatibility issues with IDE devices, focusing mainly on hard disks. In this Daily Drill Down, I’ll examine CD-ROM formats, and I’ll help you make sense of the acronyms and specifications you may encounter when selecting CD-ROM hardware.
The colors of CD-ROM
First, let’s take a look at CD-ROM standards, which are specifications that detail how a CD-ROM stores data and how a drive reads that data. The specifications exist so that anyone can read any disc without worrying about the drive brand or model. You won’t need this information when selecting a new CD-ROM drive because all new drives conform to all of these standards, but you might find the specifications helpful in determining whether an older drive can handle a specific type of modern content.
The main set of CD-ROM standards is classified by color. Why? Because the original CD-ROM standard was bound in a book with a red cover, and it came to be known as the Red Book standard. For consistency, later CD-ROM standards were also assigned colors.
For backward compatibility, today’s CD-ROM drives are capable of reading discs that conform to any of the standards that were released prior to the drive’s manufacture date. That means the same CD-ROM drive can play both data and audio CDs and can read a home-recorded CD-R disc. However, if you have a very old CD-ROM drive that was manufactured before some of the newer standards were invented, you may encounter problems reading CD-R or CD-RW media or discs that contain more than one type of data (such as a combined audio and data CD).
The Red Book specification, designed for playback at a constant speed of 150 kilobytes per second (KBps), was developed for audio CDs. Since audio CDs typically play from beginning to end without a lot of skipping around, this speed is more than adequate.
A drive that plays at a maximum speed of 150 KBps is considered a 1X drive. Since 150 KBps was the first speed specification, it is the baseline speed from which other drives are measured. Don’t be confused and think that the speed, or “X,” of a drive is directly related to the book colors—it’s not. I’ll discuss more about “X” specifications later in this Daily Drill Down.
The Yellow Book (also called International Standards Organization [ISO] 10149) specification further details the way audio data is stored on a CD-ROM and also introduces standards for storing computer data. It was originally developed in 1984 and is still the primary standard to which all CD-ROM content conforms. There are several pieces to it, the most important of which are:
- High Sierra (HS) and ISO 9660: Originally separate, these two standards were merged into a single standard under the name ISO 9660. Together, they specify how data is stored on a CD-ROM. Data is placed in sectors and tracks similarly to the way it’s placed on hard and floppy disks. Each disc contains a Volume Table Of Contents, which is stored in a specific area.
- CD-Digital Audio (DA): This standard defines specifications for a CD-ROM drive that can read either audio CDs or data CDs. (Note that it is not the standard for drives that can read a single disc containing both audio and music. That’s the CD+ format, which I’ll discuss later.)
- CD-ROM Extended Architecture (XA): This standard, often called CD-ROM/XA, provides for multisession recording, in which each session has its own Volume Table Of Contents. It also specifies how data of different types can be interleaved to make multimedia playback possible. For example, it allows the video and the sound of a person speaking to be stored together so there are no synchronization problems between the sound and a speaker’s lip movements during playback.
More about CD-ROM/XA
CD-ROM/XA is a very lengthy, very technical topic that can’t be completely covered in a few paragraphs. However, here are the main terms you should know regarding this format:
- Mode 1 and Mode 2: Mode 1 refers to a normal data CD (not CD-ROM/XA). Mode 2 refers to data stored in CD-ROM/XA format. Data stored as Mode 2 is split into two form types: Form 1 and Form 2.
- Form 1 and Form 2: Each block of data on a Mode 2 disc is broken into two forms. Form 1 is a compilation of several different types of data stored together: 2,048 bytes of user data, an error-detection code (EDC), and an error-correction code (ECC). Form 2 contains 2,324 bytes of raw data and is used for time-dependent information, such as voice or music, which must synchronize with the regular Form 1 data.
The White Book standard is used for storing videos on CD (VideoCD format). Prior to DVD technology’s introduction, this format was widely used in Southeast Asia for movie distribution. This standard isn’t popular in the United States.
The Orange Book standard defines how writable CD-ROMs operate, including both magneto-optical (Part I of the standard) and CD-R (Part II). It covers single-session and multisession recording specifications, as well as packet writing for CD-RW drives.
The Green Book standard combines standards from the Red Book and Yellow Book. It also throws in a few extra specs for a format called CD-Interactive (CD-I) that allows users to view stored data on the CD and also to write to it interactively. It’s a rather specialized system, used primarily for kiosks and presentations, and requires a special CD-I-capable drive.
The Blue Book standard encompasses CD+ or Enhanced CD and was developed by Philips and Sony, with some help from other companies. This standard defines a mixed-mode CD specification that combines audio and data tracks on a single CD. The audio portion of the disc is normal Red Book standard, while the data portion is CD-ROM/XA Mode 2. Audio-only CD players ignore the data track. A CD-ROM/XA compatible CD-ROM drive, however, can play both the audio and the data.
Mixed-mode CD formats were not original to the Blue Book standard; they existed in various forms before the Blue Book standard came out and worked with a varying degree of success with most audio players. Prior to the Blue Book standard, the main problem was that if an audio-only player tried to read the data, it would make a horrible noise that would potentially damage the speakers. Various record companies and manufacturers attempted to create a standard, including CD Extra, CD Plus, and CD-ROM Ready.
What does the X mean in a CD-ROM specification?
The X refers to the drive’s speed—the theoretical maximum data transfer rate for that drive. X also refers to the rotation speed of the disc, because the transfer rate is determined by how fast the disc whizzes by the read head. (Depending on the drive type, there’s a little more to it than that—I’ll cover this topic below.)
The original CD-ROM drives (1X) read 75 2,048-byte sectors of data per second, which comes out to 150 KBps. Since then, drive speeds have advanced in multiples of 150 KBps. For example, a 2X drive reads 150 2,048-byte sectors per second, for a rate of 300 KBps, while a 4X reads 300 sectors for a rate of 600 KBps.
Data transfer rate is only a theoretical measurement for a drive, because no drive actually achieves its rated transfer rate in sustained performance. When a program calls for something to be read from the disc, the disc must start spinning and get up to speed, and the read head must move in and out to find the spot on the disc where the requested data resides. When reading a specific spot on the disc, the actual speed also depends on whether the drive is CLV or CAV.
An emerging technology, called TrueX/Multibeam, uses several laser read heads to read more data at a time than on a normal CD-ROM. Since these drives read more data per rotation of the disc, their data transfer rate and their rotation speed are not the same. Unfortunately, these drives are not yet widely available.
CLV and CAV
The two technologies used for CD-ROM drives are Constant Linear Velocity (CLV) and Constant Angular Velocity (CAV). Generally speaking, CLV is found in older, regular CD-ROM drives and in multipurpose drives such as CD-RW. CAV, however, is found in newer, regular CD-ROM drives.
All CD-ROM data is recorded using CLV. No matter what spot is being recorded on the disc, it is recorded at the same area per second. That means that when recording near the center of the disc, the drive must spin faster because there’s less area to be covered. As the recording reaches the outside of the disc, the drive must slow down. If you have a CD-R or CD-RW drive that also plays CDs, it plays them using CLV. A CLV drive has a single speed measurement, such as 8X, 16X, and so forth, because the amount of data it reads per second does not vary. (The actual rotation of the disc—in rotations per minute [RPM]—does vary.)
Most of the higher-speed regular CD-ROM drives sold today play CDs at CAV. That means the CD rotates at a constant speed (RPM), and the amount of data being read per second varies depending on the spot on the disc being read. Since these drives do not have a single data transfer rate, they can’t have a single, accurate “X” rating. You’ll often see them advertised with two X ratings: a minimum and a maximum (such as 28X/32X). The smaller number is the data transfer rate at the center of the disc, and the larger number is the rate for the outside edge. Such drives almost never achieve their top speed, however, because data on a CD is written from the inside to the outside, and most CDs are blank at the outer edge.
CAV is typically found in drives 12X and above, and almost all 16X-and-above drives sold today are CAV. If you see a single speed rating for a drive above 16X, you can guess that it probably refers to the highest speed for that drive and that the lower number has been omitted for marketing purposes. CLV drives, on the other hand, are available with speeds up to 48X. These drives aren’t common, so read the entire drive specification to be sure what you’re getting. High-speed CLV drives tend to be noisier and more expensive than comparable CAV drives.
What is access time?
Access time is the amount of time that elapses between your PC’s request for data from a CD-ROM and the drive’s delivery of the first of that information. It’s a measure of the drive’s mechanical ability to move the read head to the correct spot. Access time is not directly related to the drive’s speed (X), although drives with faster speeds tend to also have superior inner mechanics that allow for better access time.
A 1X drive has a typical access time of around 400 milliseconds (ms). Today’s best-performing drives have an access time of around 75 ms.
What else determines a CD drive’s performance?
Two big determinants of a drive’s performance are its rotation speed (X) and its access time. In real-world use, however, several other factors also play a part:
- Drive interface type: Traditionally, the SCSI interface has been considered superior to IDE for all types of drives, but recent advancements in IDE technology have helped level the playing field in terms of drive performance. (For more information, see the TechProGuild articles ”All about IDE” and ”A SCSI primer: Seven generations of high-performance hardware.”) SCSI still has a slim advantage, however, because of its decreased CPU utilization. There are various types of SCSI interfaces, from the low-cost SCSI-3 to the top-of-the-line Ultra2Wide. Low-end SCSI is appropriate for ordinary CD-ROMs, while higher-end SCSI would benefit CD-R or CD-RW drives. Portable CD-ROM drives are also available with parallel or USB interfaces. Parallel is very slow; USB is very fast.
- Competition for interface bandwidth: If your CD-ROM drive shares an IDE subsystem (that is, if it’s on the same cable) with another IDE device that’s used heavily (such as your main hard disk), you might experience performance problems with your CD-ROM drive, especially in time-critical applications like CD-R or playback of DVD movies. When feasible, place the CD-ROM drive on an IDE cable by itself for best performance.
- Drive cache: The larger the size of the built-in cache (buffer) in the CD-ROM drive, the less it must reread data from the disc when subsequent requests are made for the same data. A good drive should have at least a 512-KB cache.
- CPU usage: Drives vary as to how much of your computer’s processing time they require to do their jobs. (Less is better.) In very broad terms:
SCSI is better than IDE because SCSI operates more independently of the processor than IDE.
CLV is better than CAV because CLV uses a modest fixed amount of processor time, while CAV varies widely from very little (at the center of the disc) to a great deal (at the outer part of the disc).
A large drive cache helps minimize CPU usage.
- DMA usage: DMA stands for direct memory access. It’s a method of drive access in which the drive communicates directly with the operating system, bypassing the processor whenever possible to improve performance. Taking advantage of DMA requires a bus-mastering IDE controller. To use DMA, enable your operating system’s CD-ROM drive properties (look under Device Manager or Control Panel), but first make sure that your hardware is capable of utilizing this method.
File system standards for CD-ROM
On a PC running MS-DOS or Windows, the standard file system is known as ISO 9660. Yes, it’s our old friend, the Yellow Book standard, providing the specifications for file storage so that any CD-ROM drive can read any disc designed for DOS or Windows.
When Windows 95 introduced long filenames, the ISO 9660 file format became problematic, because it allows for only 8.3 filenames (eight characters plus three extension characters). The Joliet file system was created to circumvent this limitation. (Technically Joliet is an extension to the ISO-9660 file system, not an entirely new spec.) In order to maintain compatibility with DOS, the primary volume descriptor on Joliet discs meets the ISO 9660 standard. Then a supplementary volume descriptor (SVD) is used to support long filenames, long folder names, and long paths.
On a Macintosh, the file system is different. Mac data CDs use the Hierarchical File System (HFS), the same as the Macintosh operating system uses for other disks such as hard disks and floppy disks.
Understanding CD-ROM specifications can help you choose the best device for your system. In addition, it can help you troubleshoot problems with your CD-ROM drives, including older drives or drives moved around as you added and configured hard disks. With a better picture of the hardware and specifications behind CD-ROM technology, you’ll be able to evaluate and select CD-ROM hardware with confidence.
For more information about CD-ROM formats and related technology, read these articles:
”Using DVD-RAM for network storage”
”Installing CD-Writers under Linux”
”Why would I want to use an SCSI drive?”The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.