Understanding CD-R & CD-RW Technology


NWA-PCUG Newsletter Article, March 2003
Understanding CD-R & CD-RW Technology
by John C. Lee
TOPCC - Thousand Oaks Personal Computer Club
johnclee@adelphia.net (click to email author)

As the CD-R and CD-RW technology advances and prices for the CD- RW drives and their media drop rapidly, it is no surprise to see that almost every personal computer sold today comes equipped with a CD-RW (ReWritable) drive. There is, however, a lot to learn about this technology, if you want to get the most out of your CD-RW drive. This article, which is prepared with this objective in mind, is composed of three parts:

    Part I covers the discussion of the functional performances of CD-RW drives and the points to be considered in purchasing a new CD-RW drive. Discussion on the structural and functional differences between the CD-R and CDRW media is also included.

    Part II discusses various aspects of the software used with the CD-RW drives, including data and audio file formats, CD writing methods, digital audio extraction, image file recording, system tests and CD copier.

    Part III contains illustrations showing the steps to be followed in creating different types of CDs.

Part I: Functional Performances of CD-RW Drive

Most CD-RW (recordable and rewritable) drives sold today are capable of performing the following 3 main functions:

    ???Record CD audio tracks or computer data onto CD-R or CD-RW media

    ???Write or rewrite computer data onto CR-RW media as onto a Zip drive disk or a floppy disk

    ???Read data from CD to the PC or play music CD as in a CD-ROM or DVD drive. It also performs ripping which is a process of converting CD music tracks to WAVE audio files on the PC.

Drive Speeds
The RW drives are generally classified by the maximum speeds they can run to perform the above three functions. For example, a 16X- 10X-40X CD-RW drive is capable of recording at a maximum speed of 16X, rewriting at a maximum speed of 10X and reading or ripping at a maximum speed of 40X. When the first CD-ROM drive was introduced, it was run at a speed of 1X, which could play CD music and could read data at about 150 KBps. Since then great advances in drive technology have been made, and it has been reported that the fastest CD-ROM drive can now reach a speed of 52X (7.8MBps), and the fastest CD-RW drive can reach speeds of 40X-12X-48X equivalent to 6MBps-1.8MBps-7.2MBps. Many RW drive manufacturers have made claims that a 50-minute audio CD can be recorded in less than 3 minutes. So when you buy a CD-RW drive, speed is the first criterion you should consider. You should also make sure that the speeds of the media you are going to use are compatible with your CD-RW drive.

Notice that the CD-RW drive speeds are all quoted as maximum speeds, which are the maximum data transfer rates attainable when the head is recording over the outer circles of the disk. If the drive is run at a constant angular velocity (CAV), the transfer rate decreases as the laser head moves toward the inner periphery of the disk. With the CAV method the minimum transfer rate may be about 0.45 of the maximum value and the average transfer rate is around 0.73 of the maximum value. Several CD-RW drives employ constant linear velocity method (CLV). In this method, the data transfer rate is held constant by adjusting the disk angular speed according to which portion of the disk data is being recorded on. CLV drives are somewhat noisier, because they have to be run at higher angular speeds that may result in higher loads on the drive mechanism. One drive maker, Yamaha employs a partial CLV-CAV arrangement, in which the disk is run at CLV over the outer peripheral circles, and CAV over the inner peripheral circles. It is always a good practice to set the recording speed of the CD-RW drive at half of the maximum speed of the slower drive.

Buffer Under-run
When you record audio or file data from a source CD-ROM (or DVD) drive to a target CD-RW drive, the target drive must receive a continuous flow of data (without interruption) from the source drive via the PC. In order to safeguard against perturbations in data transfer rates between the drives, buffer memory (varies from 2 to 8MB with different drive makers) is usually provided in a CD-RW drive to store data from the PC. But if the source drive is consistently slow and lapses in sending data to the target drive, the buffer memory can eventually be depleted. This condition is termed as "Buffer Under-run", and the result is an aborted recording and a ruined CD-R disk. Buffer under-run usually occurs when the CD-RW drive is set to run faster than the source drive. So it is important to take precaution to set the recording speed on the CD-RW drive lower than that of the source drive. Normally this is not a problem if you use a high-speed CD- ROM drive or a hard drive as the source drive (unless the hard drive is highly fragmented). But if you use a DVD as a source drive, which has a limited maximum speed of 16X, you should not set the recording speed to more than half (8X) of the maximum speed. In some cases, you may be able to achieve a faster recording by using Image File Recording (discussed in Software section) instead of using the DVD as a source drive.

Recording can also be interrupted and aborted, if a residence program on the PC starts to run during recording.

Therefore precaution should be taken to close all programs that may be scheduled to run during the recording session.

Burn-Proof Technology
Some CD-RW drives are equipped with burn-proof sensors to detect and prevent the buffer under-run. When the sensor detects that a buffer under-run is about to happen, the drive will pause and record the location where it stops.

As soon as the buffer is fully replenished by the PC, the sensor starts to synchronize the last recorded point with the new data collected in the buffer and resumes the recording. For more details on burn-proof technology, log on http://www.sannet.ne.jp/BURN-Proof/faq/. If you are thinking about purchasing a new CD-RW drive, make sure the drive is burn- proof.

Though the burn-proof technology provides some protection against occasional buffer under-run damage, it is still advisable to take the previously mentioned precautions to avoid repeated buffer under-runs.

CD-R Media versus CD-RW Media
Two types of CD media (CD-R and CD-RW) are used with the CD-RW drives. CD-Recordable (CD-R) disks are coated with green, gold, or blue dye. The CD-RW drive writes data by using the laser beam to burn tiny pits into the dye layer. The disk is read by a head that simply looks for the absence or presence of the reflected laser light. The burning process is irreversible and the burned pits remain on the disk permanently. For this reason, CD-RW drives are often referred to as CD-Burners. CD-Rewritable (CD-RW) disks are lined with crystalline materials. The CD-RW drive writes data by altering the alignment of the crystalline structures in the media. These changes in the crystalline structure alter the phase of the effected light, when the disk is read by a laser. The disk can be "wiped clean" in an erase process that realigns the crystalline elements. Tests on these media have shown that the disks can be re-recorded up to 999 times. The estimated storage life for the CD-RW disk is about 30 years, and that of the CD-R disk is 75-200 years. CD-R media recorded with audio tracks are compatible with nearly all CD-ROM drives, DVD drives and Audio CD players (except the earliest models of CD and DVD players). Data recorded CD-R can be used on CD-ROM or DVD drives. CD-RW media can only work on CD-RW drives, although some Multiread (MR) CD-ROM and CD players that are compatible with CD-RW media have been recently developed, but they are still rare and expensive.

Both CD-R and CD-RW media disks can be used for performing the three functions as previously discussed. Nowadays, the cost of CD-R disks has come down so low that most users do not even bother to use CD-RW disks for recording. I personally like to use the reusable CD-RW disk for tests or trial runs in recording before I burn a permanent CD-R disk. Of course, CD-RW disks have to be played on a CD-RW drive.

Part II: CD-R and CD-RW Software
The first comprehensive software for recording and rewriting CD was developed by Adaptec. Recent versions of the software are made by Roxio, a subsidiary of Adaptec. A year or two ago, a German software company, named Ahead Software GmBH, made its debut in this country by offering a CD-R and CD-RW software called Nero Burning ROM.

The software is similar to Roxio software in most respects except some minor differences in features. Quite a number of drive makers have switched to the use of Nero software with their drives mainly for cost reasons I guess. The following discussion will be mainly directed to Roxio software, but the mechanics of the processes are equally applicable to Nero software. The Roxio software consists essentially of two separate programs: Easy CD Creator for recording and DirectCD for rewriting.

Easy CD Creator
You can use this program to perform recording of audio CD and computer data as well as reading computer data from data CD or ripping (audio extraction) audio files from an audio CD. Clicking either the Audio or the Data button in the initial menu opens the program. The main display of the Easy CD Creator program is an Explorer type window, with the directories and subdirectories shown in the top left pane and folders, files or music tracks shown in the top right pane. You can select from this top pane data files or music tracks that you want to record by clicking the ADD icon on the toolbar or dragging and dropping them to the lower right pane. The lower left pane lists the file folders and icons for the type of CD you select to create. You can also rearrange or change the recording order of these files or music tracks by dragging the selected file to the position you want it to appear in the lower right pane.

You can click the arrow next to the NEW icon to display a drop- down menu. The menu will list the different types of CDs you can select to create, including Audio CD, Data CD, Multi-session Data CD, CD Extra, Mixed Mode CD and Bootable CD. Before we delve into details of these types of CDs (in Part III), we will first discuss some of the other aspects of the Easy CD Creator program that are pertinent to create these types of CDs.

Data and Audio File Formats
When computer data or audio tracks are recorded to a CD-R or CD- RW, they assume file formats that may not be supported by all types of drives or environments. In the following we will discuss some of the more important CD file formats used in the Easy CD Creator software program.

    ISO 9660: This format is used for creating Data CD. The CD in this format can be read in CD-ROM and DVD drives on platforms including DOS, Macintosh, OS/2, Windows and UNIX. However, the ISO 9660 standard limits the directory and file names to less than 8 alphanumeric characters and extension to less than 3 characters as in DOS systems.

    Joliet: This is the format used to record most Data CDs and is the default option in Easy CD Creator software. You can use file names up to 64 characters in length. Joliet can also records the associated DOS-standard name (8+3 characters) for each file, so that the CD may be read on DOS systems or earlier versions of Windows.

    CD-DA: CD-DA stands for CD-Digital Audio format with a file extension of .cda. It is the first standard for the audio CD that can be played in all CD-ROM, DVD drives and Audio CD Players. All audio tracks or audio files are recorded to CD-R or CD-RW media in this format. Music CDs generally belong to this category. It has the CD digital sound quality with a bitrate in the same range as the Wave file. Wave is the equivalent format to CD-DA when written into the hard drive. Bitrate is the amount of digital audio information recorded on a CD or hard drive in Kbits per second. The higher the bitrate, the better is the sound quality.

    WAVE: Wave format with a file extension of .wav is commonly used for computer audio files. It is part of the general RIFF Standard (Resource Interchange File Format). It is an uncompressed audio format with a bitrate of 1411 Kbps, and hence has a large file size. The file can take up 10.5 MB space for each minute of playing time. Wave files can be recorded from the hard drive to audio tracks (with CD-DA format) using CD-R or CD-RW media in the CDRW drive. A reverse process called ripping can extract audio tracks from an audio CD to audio .wav files in the hard drive.

    MP3: MP3 stands for MPEG-1 Audio Layer 3. It is the most popular compressed audio format used for portable MP3 music players and for transferring audio data via the Internet. At an Industry standard bitrate of 128 Kbps it retains almost the same CD quality as a Wave or CD-DA file, which means a reduction of file size to 0.96 MB for each minute of playing time. As with the Wave files, the MP3 files can be recorded from the hard drive to CD-R or CDRW audio tracks. Similarly MP3 files can be extracted to the hard drive from audio CD tracks. Easy CD Creator software has plug-in encoders to extract audio tracks to Wave, MP3 or WMA files.

In some OEM versions of Easy CD Creator, you can only extract to MP3 with bitrates less than 56 Kbps, which gives Stereo quality instead of CD quality. So you may want to elect an alternate approach by ripping the audio tracks to Wave files, and then converting the Wave files to MP3 files with higher bitrates using some other software. Some of the more sophisticated software such as RealJukebox Plus or Music Match is capable of encoding MP3 files with different fixed bitrates as well as variable bitrates (VBR). With VBR, more complex sections of a track, such as those with many instruments playing simultaneously, may require more "bits" of computer information to create a highquality sound. These sections of a song are recorded at a higher bitrate based on the quality level you choose (that is, if you choose 128 Kbps, the most complex parts of your track are recorded at this bitrate). Less complex sections can be accurately represented using less information, and may be recorded at a lower bitrate. Hence the VBR will give further reduction in file size with hardly any detectable degradation in sound quality. WMA: This format with an extension of .wma is mainly used by the Windows Media Audio Player for "audio streaming" on the Internet. A bitrate of 96 Kbps is the usual default setting, and its sound quality cannot match that of the MP3 format even at a bitrate of 128 Kbps. Like Wave and MP3 files, WMA files can be recorded to or extracted from the CD audio tracks.

There are several other audio file formats such as .mes, .lqt, .au, .aif, .rm, .ra, .rmx, and .rmj. The last four of them are used by the RealNetwork Player, mainly for Internet audio streaming like WMA. I personally like to use the RealNetwwork Player, because it usually delivers faster buffering on the Internet and better sound quality than the Windows Media Player.

CD Writing Methods
Three ways of writing data or audio tracks to the CD-R or CD-RW disks are used in the Easy CD Creator program: Disk-at-Once, Track-at-Once and Session-at-Once. The method you should use would depend on the type of CD you are going to create. For Audio CD (or audio part of a CD Extra or Mixed Mode CD), a track is referred to as a music track or a song in the music CD; while for Data CD (or data part of CD Extra or Mixed Mode CD), a track is referred to as a stack of data that may contain one or a number of data files or folders. A session is referred to as a single continuous pass of writing without pausing. A session consists of a lead-in, data or audio tracks and a lead-out. The lead-in and lead-out areas signify the beginning and end points of a session. They are not part of the data, but contain information about the session itself.

    Disk-at-Once: This method is used when writing a complete disk in a single operation without pausing (also in one session) followed by the closing of CD. This means that file or audio data cannot be added later, even if the full capacity of the blank disk has not been used. This writing method is used in creating Audio CD, Data CD or Mixed Mode CD.

    Track-at-Once: This method is used when writing data to a disk one track at a time. More tracks can be added later if there is enough space left on the disk. The Track-at-Once method is sometimes referred to as Multisession. If you want to record data later after a session, be sure to leave CD open by unchecking the Close CD button. Multisession Data CD uses this method. Make sure to set Mode 2, CD-ROM XA as the writing mode for Multisession CD. Otherwise some CD-ROM drives may not be able to read the Multisession CD.

    Session-at-Once: This is a 2-session writing method used to create CD Extra format CD. The audio tracks will be recorded in the first session and the data will be recorded in the second. In the lower left pane of the main window CD Layout, you will see two folders: one audio and the other data. Click the first folder and add audio tracks to be recorded. Then click the second folder and add data to be recorded. Both sessions will then be recorded one after the other.

Digital Audio Extraction (Ripping)
Audio extraction is a reverse process to Audio CD recording. You can select one or a number of music tracks (CDDA format) from an audio CD and convert them to Wave, MP3 or WMA files on the hard drive, by clicking the Extract button on the display window of the Easy CD Creator program. On the Extract dialog box, you can also choose the bitrate for the extracted files.

These audio files can only be played using audio programs such as RealJukebox Player, Window Media Audio Player or Music-Match on the PC. Sometimes, you may want to store these files in a CD in their own formats. To record these audio files to a CD-R or CD-RW in their respective formats, you have to treat them as data files and use Data CD program to record them. If you use the Audio CD program, these files will all be converted back to music tracks in CD-DA format. Anytime you use Audio CD (or the audio portion of CD-Extra or Mixed Mode CD) to record audio files or audio tracks to CD-R or CD-RW media; they will always be recorded to audio tracks in CD-DA format.

Image File Recording
Normally, you record a CD from a source drive (CD-ROM or DVD) to a target CD-RW drive. But if you only have a single CD-RW drive, you can still create a CD using the CD Image (an option in the File menu). First you insert the source CD (data or audio) in the CD-RW drive to create a CD image file (in Windows Temp directory of the hard drive). A CD image file is an exact representation of the whole set of data as it appears on a CD, in terms of both content and logical format. This may be an ISO 9660 image (adhering strictly to the ISO 9660 standard), or some proprietary format such as the .cif format used by Easy CD Creator. After the image file is created, you will be prompted to remove the source CD and insert a blank CD-R or CD-RW media. You can then record the content of the image file to the CD-R or CD-RW disk.

If you intend to make a number of copies of CD from a single source CD, it is more expeditious to employ the image file recording instead of recording from the source CD individually. After you record the first copy, you should save the image file. Subsequent copies of CD-R or CD-RW disk can then be recorded directly from the saved image file in the hard drive, without having to re-create the image file. DVD drive has a limited maximum speed of 16X. In some cases you may be able to obtain a faster recording using the image file than using DVD as a source drive. Creating an image file is usually carried out at the CD-RW drive reading speed, and the recording from the hard drive is usually carried out at the CD-RW drive recording speed, which may be higher than the 16X DVD speed. In fact, some RW drive makers recommend the use of the image file recording instead of recording from a source drive, because the image file recording is more reliable and free from the buffer under-run problem.

System Tests
The main purpose of the system testing is to find out how well your system's components: CD-ROM, DVD, CD-RW and hard drive can work together. The Easy CD Creator contains three tests:

    The Data Transfer Rate test calculates the average speed at which data can be read from the source drive.

    The Audio Extraction rate test determines the average rate at which digital audio information can be read from the source drive (CD-ROM, DVD or CD- RW drive) to the hard drive. The Recording test determines the recording speed at which your system can support.

    You can perform the first two tests, when you first acquire your CD-RW drive to get some idea about the transfer rates and extraction rates of each of your drives, but these two tests are not essential for later CD recordings.

    The Recording test is an option you can choose when you create a CD. The test actually performs a simulated recording to determine and set the speed at which the CD should be recorded, before the final recording is carried out. Hence, the whole process including the test and the actual recording will take twice as long as if you perform the recording without the test. You can skip this test to save the time by setting the recording speed yourself, once you know the speed your system can support.

CD Copier
CD Copier is a subprogram of Easy CD Creator. It allows you to make exact copies of Audio CD, Data CD, Photo CD, Video CD, CD Extra or Mixed Mode CD (results are not certain with Mixed Mode CD). CD copying should all be carried out using the Disk-at-Once writing method. You can also create an image file to copy CDs with the CDRW drive only or to make multiply copies, as described in the section of Image File Recording. To create an image file you have to select the option of Advanced>Disk Copy>Copy Source CD to the Hard Drive First in the CD Copier window.

DirectCD is a program that allows you to write files directly to a CD-R or CD-RW disc in much the same way that you copy files to a floppy diskette, a removable drive or another hard drive. DirectCD lets you read and write your files directly to your CD with any software application that can read from and write to a drive letter. Some examples include:

    ???Software applications, such as Microsoft Word, when you use the Save or Save As commands

    ???Windows Explorer when you erase files or drag and drop files to and from the CD-RW disk

    ???The Windows Send To command

The DirectCD program uses a file system based on UDF v1.5 and this file system gives you a drive letter access to your CD-RW drive. The program also uses packet-writing technology to write data to the CD-R or CD-RW disk. Using the packet writing method, data can be written in much smaller chunks than are possible with the current Disc-at-Once and Track-at-Once methods used in Easy CD Creator program. Some CD space (roughly 100MB) will be assigned and used by the file system. So in a 650MB disk, only 550MB is usable space for writing data. DirectCD includes a Wizard that guides you step-by-step through the process of preparing and ejecting CD-R and CDRW disks. It also includes a data compression feature that allows you to copy more data files to the disk. Before you can write data to your CD, you must first format the disk, as you would have to do with a floppy diskette or a hard drive. When you insert an unformatted blank CD-R or CD- RW disk in the RW drive, the Wizard will automatically prompt you to format the disk. After the formatting, the Wizard will cue you that the disk is ready for use. If you are formatting a CD-RW disk that has been formatted before, you can select either Quick format or Full format in the Format Disk window. You can also select the Compression option if you are using a CD-RW disk and you want to enable data compression on the disk.

DirectCD will work with both CD-R and CD-RW disks. However, there is no real advantage of using CD-R disks with the DirectCD program, because once the data is written on a CD-R disk, it is burned on it permanently, and you cannot erase any part of the content. "Deleting" files from a CD-R disk only makes the files invisible to the file system but does not free up any space on the disk for re-writing as in a CD-RW disk A word of caution: Once a disk is formatted in DirectCD, it can only be used with the DirectCD program in the CDRW drive. It can neither be unformatted nor used with Easy CD Creator program.

Part III: Create Different Types of CDs
Part III of the article will cover the discussion of the procedures you can follow to create different types of CDs, including Audio CD, Date CD, Multisession Data CD, CD Extra CD, Mixed Mode CD, Digital Audio Extraction, Rewritable CD and Bootable CD. You can access these CD Layouts by clicking the arrow next to the NEW menu on the Easy CD Creator window, except the Rewritable CD, which is from the DirectCD program.

Audio CD
Audio CD uses the Disk-at-Once writing method. The audio tracks or/and audio files are written onto a complete disk in a single session without pausing. Audio data cannot be added later, even if there is remaining space left. The source input can be one or a number of audio tracks from one or several music CDs. It can also be one or a number of audio files from the hard drive such as WAV, MP3 and WMA files, or a combination of audio tracks and audio files. If the input audio tracks are from several source CDs, then you will be prompted each time to insert the CD, in which the audio track to be recorded resides.

The output to the destination CD-RW drive with a CD-R or CD-RW disk will be always audio tracks in CD-DA format, which can be played in all CD-ROM, DVD or audio CD players. If the source inputs are audio files from the hard drive, they will be encoded and also recorded to destination CDs in CD-DA format. In some instances, you may want to store audio files on a CD with their own formats (such as WAV, MP3 and WMA) instead of CD-DA format, in which case you should treat these files as data files and use Data CD program to record them to a destination CD (also see Digital Audio Extraction in Part II). You can record audio files or tracks from a source CD-ROM, DVD or hard drive to the destination CD-RW drive, or you can use the Image File to read and record on a single CD-RW drive (see Image File Recording in Part II).

Data CD
As in Audio CD, Data CD also uses the Diskat-Once writing method. The file data is written onto the disk in a single rack and a single session. One difference is that the Audio CD can comprise of a number of audio tracks per session, while the Data CD can comprise only one track per session. But a Data CD track can contain one or a number of data files or/and folders. The source input can be one or a number of data files or folders from one or several data CDs or/and from the hard drive.

The output to the CD-R or CD-RW disk will be in either ISO 9660 or Joliet format. Both of these two formats can be read on CD-ROM or DVD drives. Joliet format is the default format in Easy CD Creator program and should be used if you want to retain long file names (see Data and Audio