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CD-R Frequently Asked Questions

by Jonathan Burt

Based on a talk (on 4th July 2001) given by Jonathan Burt, to the IWPCUG about recording data on CD's.

Questions

  1. What is a Compact Disk?
  2. What Kind of Information Can a CD Hold?
  3. How Much Information Can a CD Hold?
  4. Does the speed of the drive effect the writing time?
  5. How long do the disks last?
  6. What are causes of buffer underruns?
  7. Where can I find Further Information?

Copyright Issues

The first thing we need to discuss is copyright issues regarding CD/R & CD/RW drives. As you all no doubt know, copying Audio CD's without the artists permission is illegal. The same goes for copying commercial software CD's, BUT most terms and conditions of commercial packages allow you to make one (and only one) backup copy!

What is a Compact Disk?

A CD is a medium on which to store information; you can think of it as a very large, write-protected floppy disk. The difference is that, while DOS floppy and hard disks are written using the DOS file format, CD-ROMS are usually written using a file format called ISO 9660. The ISO 9660 standard is a so widely accepted that it can be read back on any computer platform.

The ISO 9660 standard an internationally accepted standard specifying the logical format for files and directories on a CD-ROM. The standard allows different computers with different operating systems to access the same data format.

Joliet is an extension of the ISO 9660 standard, developed by Microsoft to allow CDs to be recorded using long filenames, and using the Unicode international character set. Joliet allows you to use filenames up to 64 characters in length, including spaces. Joliet also records the associated DOS-standard name for each file so that the disk may be read on DOS systems or earlier versions of Windows. The number of characters allowed in a long file name is up to 64, and the number of characters in the long file name plus its file path cannot exceed 128.

Romeo is another extension of the ISO 9660 standard to support Windows® 95 long file names only -- up to 128 characters. Also a file naming option in Easy-CD 95 Software and Easy-CD Pro 95 Software which allows you to write files to disk with names up to 128 characters long, including spaces. This is not part of the Joliet standard, the Unicode character set is not supported, and there is no provision for associated DOS filenames. Romeo filenames can be read on Windows 95 and NT systems. Romeo disks can be read on Macintosh systems if the filenames are not longer than 31 characters. Do not use Romeo if your disk must be read in other conditions.

What Kind of Information Can a CD Hold?

The simple rule of thumb is: whatever you have stored as a file on any other storage media can also be stored as a file on a CD-ROM. Under the ISO 9660 standard, a file is a file, and ISO doesn't care whether it contains pictures, text, or sound.

There are a few special cases where you would want to record a file to CD in a special format. The most obvious is CD-DA audio: if you want to record an audio file to CD so that it can be played back by your home stereo, you need to write a CD-DA (Digital Audio) disk.

CD-DA means Compact Disk-Digital Audio. A CD-DA disk contains tracks with Audio sectors only. In 1982 Philips and Sony introduced the necessary technology for storing digital audio signals on a Compact-Disk, and introduced the CD-Digital Audio.

How Much Information Can a CD Hold?

When you copy data to a CD, you must take care that your data does not exceed the capacity of the CD to which you are recording. Due to the audio origin of CDs, the amount of information a CD can hold is measured in minutes:seconds:sectors. Each second contains 75 sectors, each of which can hold 2048 bytes (2 kilobytes) of Mode 1 user data. Recordable CDs come in 21- (80 mm diameter), 63-, 74-, and 80-minute sizes (both 120 mm diameter), which can contain:

21 min x (60 sec) x (75 sectors) x (2 kbytes) = 193,536 kilobytes = 189 megabytes (184)
63 min x (60 sec) x (75 sectors) x (2 kbytes) = 580,608 kilobytes = 567 megabytes (553)
74 min x (60 sec) x (75 sectors) x (2 kbytes) = 681,984 kilobytes = 666 megabytes (650)
80 min x (60 sec) x (75 sectors) x (2 kbytes) = 737,280 kilobytes = 719 megabytes (700)

(Bracketed figures are calculated using 2000 instead of 2048 for 2 kbytes!)

Factory-recorded CDs can hold up to 74 minutes of data.

And Subtract...
Files on CD do not occupy a space exactly equal to their original size, but usually a bit more. This is because the minimum recordable unit on a compact disk is the logical block. In theory, a logical block could be 512, 1024 or 2048 bytes in size (that is, that you could fit 1, 2, or 4 logical blocks into a sector). In practice, MSCDEX reads only the 2048-byte block size. This means that a file will occupy a space equal to the closest (higher) multiple of 2048 bytes. In ISO, just as in the DOS file system, directories are also files, and also take up space.

The Yellow Book (the standard defining the physical format of CD-ROM) specifies that the CD data starts after a pause of two seconds. This means that the first two seconds on a CD are not available for user data. So, from the theoretical capacity of any CD you must subtract:

[(2 sec) x (75 sectors) x (2 kilobytes)] = 307.2 kilobytes

Furthermore, the ISO 9660 file structure needs space for the following:

More sectors may be needed to store the Path Tables, or the root directory if its size exceeds one sector as files are added. All this adds up to some space which will not be available for your applications and files, a fact which you must keep in mind when determining how much information you can fit onto the CD.

Does the speed of the drive effect the writing time?

Yes!

The length of time it takes to record (write) a disk depends on how much data or how many minutes of audio you have and the speed of your CD recorder or rewritable.

Thus, for example, you have a full 650 Mb of data or 74 minutes of audio;

Some recording software (also called pre-mastering software) have a feature that allows you to verify the data you have written against your source file (disk from which you are recording), and that will take a little longer.

How long do the disks last?

The life span of a recordable disk depends on several things:

Colour of disk: most makers of CD-recordable disks claim that the green/blue (cyanine) disks last up to 75 years, gold (phthalocyanine) last up to 100 years and platinum last up to 200 years. See the manufacturer's information.

Environmental factors: If the disk is exposed to water, sun or heat, the lifetime of the disk may be reduced. Treating your recording media carefully will extend its "life expectancy."

Compatibility

Not all CD-ROM drives can read CD-RW (rewritable) disks. Newer models (those made in 1998 or later) are more likely to read rewritable media, but they must support what is called the "MultiRead standard." There is a MultiRead logo which can be found on the packaging or on the CD-ROM drive itself which indicates that the CD-ROM can read all existing formats (as of the date the drive was manufactured), including CD-R and CD-RW disks. Also some older drives will not read multi-session CD's until they have been finalised (as this finishes the ISO 9660 standard table)!

MultiRead is an OSTA specification that recognizes the importance of having future CD readers capable of reading a wide variety of CD-based media compared to older generations of CD readers. MultiRead CD-ROM drives, for instance, will be able to read newer CD-RW media as well as CD-R and CD-ROM.

Computers being sold now often come with a DVD-ROM drive. All DVD-ROM drives are capable of reading audio CDs and CD-ROMs which are commercially produced. However, only some of the newest DVD-ROM drives will read the CD-R/CD-RW disks you record yourself.

Labelling

Once you have written a disk, you can label it in several ways. You can use an water-based, felt-tipped pen - but don't use a ball point. You can purchase pens that are designed for labeling disks from vendors who sell CD recording accessories if you are unsure.

Since the dye and its protective coating is on TOP of a CD-R disk, it is very important this side be protected. Most people take great care of the bottom of a CD, and this is not where the vulnerability is located. The acid in petroleum-based permanent marker pens can easily eat its way through the top protective coating, reflective surface (silver or gold) and into the dye that stores the data bits.

There are special labels available for use on CD-R/CD-RW media. Don't use regular adhesive labels as doing so may affect the ability of CD-ROM drives and CD players to read your disk since the label may cause the disk to be off balance.

Labels made specifically for CD-R/CD-RW disks are usually available in kits with labels, applicators and, in some cases, jewel case inserts and graphics software. These labelers center labels uniformly on one side of the disk. The label covers the entire disk. Labels can be designed with artwork, photos, etc. depending on the kit you purchase and/or the software you have available. Do not try to remove a label once it is applied as it will damage the recording layer and possibly your CD-ROM reader.

Recording Tips

Disable your screen saver -- it will increase the reliability of duplication. When you are recording a CD, there is a high chance of crash if the screen saver comes on while you are burning.

Also disable background software, like Anti-Virus software, Schedulers, Internet connections, as any unwanted disk access can causes recording session to crash of course as technology is improving some of these precautions are not require but its always best to be safe.

Use only media that is certified to the highest speed of your CD recorder. i.e. Use 4X media with a 4X CD recorder 6X media with a 6X CD recorder, 8X media with an 8X CD recorder and so on. Using media that is not certified to the highest speed of your recorder can waste media, and no one likes to have to start over again -- even at today's low media prices. If the media you are using is certified less than the maximum speed of your recorder then set your recorder to record at the speed of the media.

Defrag your hard disk before you start this will ensure there are no problems, and also allow for quicker loading of the software from the CD. As well as keeping your Hard Disk performing better.

How to avoid CD disk failures!

Recordable CD's are most susceptible to failure during the recording cycle. Once a disk has been "finalized" (with the Lead-Out area and Table of Contents completed) no further recording is possible, and the disk becomes much less susceptible to failure. At this point, a CD-R may be treated with the same care that any commercially made CD is treated, without fear of disk failure.

The "number one" cause of CD-R disk failure is contamination of the recording surface! The key to minimizing disk failures is to keep the disk as clean and dust-free as humanly possible before and throughout the recording process. To do so, use the following guidelines. Make sure, too, that anyone who may use the CD recorder follows these guidelines.

Any mechanical vibrations during recording will put the disk at risk of failure. To avoid such failures, observe the following practices:

Place the CD recorder on a solid, stable surface away from high-traffic areas. The surface must allow the CD recorder to operate in a level, horizontal position.

When recording at 4x speed, always use disks marked with the "4x" symbol. This is your assurance of the best-quality recording medium for 4x recording applications!

Buffer Underruns

CD writing is a real-time process, which must run constantly at the selected recording speed, without interruptions. The CD recorder's buffer is constantly filled with a reserve of data waiting to be written, so that small slowdowns or interruptions in the flow of data from the computer do not interrupt writing. A buffer underrun error means that for some reason the flow of data from hard disk to CD recorder was interrupted long enough for the CD recorder's buffer to be emptied, and writing was halted. If this occurs during an actual write operation rather than a test, your recordable disc may be ruined.

What are causes of buffer underruns?

Buffer underruns can be caused by any process software or hardware related that interrupts the write. The following are possible causes of buffer underruns:

Hard Disk

Hardware

Overall system configuration.

Memory-Resident Programs

Networks

Recording across the network (usually too slow to maintain adequate throughput speed).

Windows 95/98

Files to Be Recorded

Other

Where can I find Further Information?

There is a Glossary of terms relating to writeable CD's here.

Further information can be obtained from the following websites: