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CD PLAYABILITY

A NOTE ABOUT GLASS-MASTERED CDs, CD-Rs AND PLAYBACK RELIABILITY

     Currently there exists an industry-wide problem regarding compatibility issues between CD-Rs and certain CD players. This issue of playability is true for all brands of CD-R and all individuals or businesses that record CD-Rs. This treatise is intended to address its causes and some measures that can be taken to minimize this problem.

     Audio CDs come in two basic varieties: (1) Glass-mastered CDs, which are the silver-colored CDs that you purchase at your favorite CD store, and (2) CD-Rs, which come in various colors and can be “burned” (recorded) on your computer. At VoChor, we offer CDs in both styles, depending upon the quantity of CDs that you order. Because of the manufacturing process involved with glass-mastered CDs, there is a set-up charge regardless of the number of CDs to be manufactured. Therefore, it only becomes economically feasible to make CDs this way when the quantities are large (typically greater than 500). CD-R duplication usually involves much less set-up time and expense, so that they can be made economically in smaller quantities (typically 1-500).

GLASS MASTERING DUPLICATION PROCESS

     Although several methods exist, we will describe what is probably the most common method of producing glass-mastered CDs. First, a laser is used to expose small areas of a photo-resist layer that has been coated on top of a glass master disc in a spiral pattern. The laser is modulated by the ones and zeros making up the digital audio data that were previously recorded on digital tape or a pre-master CD. A chemical developing fluid is then applied to the master and the exposed spots are etched using a chemical etchant, thus forming microscopic pits in the surface of the glass. At this point, a metal alloy, such as nickel-vanadium, is evaporated onto the surface of the glass to provide a conductive surface. A thicker coating of nickel is electrolytically deposited and subsequently removed from the glass master to produce a negative copy of the glass master called a stamper. The stamper is used as a mold for injection molding of polycarbonate CD disks. Next, the polycarbonate CD disk is coated with a thin layer of aluminum serving as a mirror to reflect the laser light emitted by a CD player. A thin lacquer coating is then applied to protect the rear surface of the aluminum and to provide a labeling surface for the finished CD copies. Usually, a silk screen label is applied. The final sandwich of layers on a glass master from the bottom surface upward usually consists of: clear polycarbonate CD disk, metal reflective layer, lacquer coating and silk screen label. The production of glass-master CDs takes place under clean-room conditions, requiring a major expenditure for equipment and facilities, so that relatively few manufacturers perform this service. VoChor, as well as many other recording and CD production companies, sub-contract out the task of producing glass-master CDs to these manufacturers.

CD-R PRODUCTION AND DUPLICATION PROCESS

     A blank CD-R is produced in a similar fashion to a glass-mastered CD. However, different materials are used. In addition, the polycarbonate base is coated with a polymer dye prior to the addition of a reflective metal. This dye has the special property that its light transmissivity is changed when exposed to laser light of sufficient power. The back side of the reflective metal is spin-coated with a thin layer of acrylic plastic to protect the metal layer and to provide a labeling surface. The final sandwich of layers on a CD-R from the bottom surface upward consists of: clear polycarbonate CD disk, light-sensitive dye layer, metal reflective layer, lacquer coating, and label (either paper, ink-jet or thermal printed). This blank CD can then be recorded by shining a modulated high power laser on it from a CD "burner" such as those found in most computers. When the laser light strikes the dye in the CD-R, the light transmission properties of the dye change. A CD-R recorded in this manner can then be played by shining a low power laser on the disk and detecting the changes in reflectivity and converting these changes back into the digital ones and zeros that make up the digital audio signal.

THE PROBLEMS - SKIPS, POPS, AND STOPS

     CD-Rs, regardless of the manufacturer or method of duplication, are more likely than their glass-mastered counterparts to have compatibility problems with certain CD players, resulting in skipping, clicks, pops, or complete stops.


1. Reflectivity. Glass-mastered audio CDs have higher reflectivity than CD-Rs. This may be the main reason for the industry-wide compatibility problems that exist with CD-Rs. In general, glass-mastered CDs can be played successfully in any properly functioning CD player with good results. In addition to the specific properties of the reflective metal coating and the dye itself, the simple fact is that an extra layer (the dye) has been added to the disk, thus reducing the comparative reflectivity of the CD-R. Also, most CD-R manufacturers have patented their own dyes, each of which have different spectral characteristics for both burning and playback. Audio CD players have generally been designed to meet specifications for playing glass-mastered CDs. The best players will include in their specifications the ability to play CD-Rs. Further problems have developed with the advent of players that are designed to play CDs, CD-Rs, CD-RWs, DVDs (-R, +R, -RW, +RW), SACDs, and DVD-Audio disks. The disks for each of these formats have different data recording densities and are made from different materials with different spectral responses, increasing the chances for incompatibilities between players and disks.

2. Data Format. The audio format is problematic in this digital era due to its original design over 25 years ago. Computer CD players and burners have been designed for high-quality reading and recording of data CDs. Data CDs are quite different from audio CDs. When a data CD is written, checksum (CRC) codes are written along with the data so that when the data is read, the checksum from the actual data can be compared to the original checksum "on the fly." If the checksums differ, the CD can be reread many times until the correct data is finally obtained. This generally cannot be done with an audio CD. The audio format was originally intended to be a real-time format. Each bit is read off of the disk at the time it is needed to make up the audio waveform. There is no time to go back and reread a section of the disk - the music would stop! Therefore, error correction is done on the fly. If the error correction routine senses a bit that is just too far out of any normal pattern, the routine substitutes an estimated bit in its place. This cannot be audibly detected at the 44.1 kHz sample rate of the typical CD until you get a long string of substitutions. These long strings can usually be heard as clicks or pops during playback. Occasionally the error strings get so long that the error correction routine gives up and the CD player simply mutes the audio for a period. As if this were not enough, the actual digital data on each disk may vary slightly, making it very difficult to verify that a copy is the same as the original. According to the CD-R specification, there can be a difference in the starting point of the audio data from CD to CD. This variation is not audible because it only affects the amount of silence at the beginning of the disk and time difference is imperceptibly short (on the order of microseconds to milliseconds). This makes a bit-by-bit comparison of CDs very difficult.

3. CD Burning. Manufacturers of glass-mastered CDs use laser "burning" techniques only for production of the glass master and they have methods of verifying the data on the glass master prior to duplication. Once the master is verified, then all subsequent CDs will be identical, bit-for-bit, because each is physically pressed from the same master. In the case of CD-Rs, each one is written individually. As mentioned above, CD-Rs made from the typical burning process may vary from disk to disk because there may be differences in the starting points (although all of the data after the starting point should match bit-for-bit). This means that direct comparisons between CDs cannot be made on a standard player, thus confounding quality control for most duplicators. In addition, since each CD-R manufacturer uses their own proprietary dyes, the laser energy and wavelength required for optimal burning varies from disk to disk. Although the manufacturers try to control the quality of their blank disks, there are still variations among disks from the same manufacturer. Therefore, there can also be compatibility issues between certain brands of blank CD-Rs and certain burners. Many reports have shown that, among other issues, price is a concern because the quality and variability of the least expensive disks are usually inferior to that of the more expensive disks.

4. CD-R Playback Difficulty. CD players may play glass master CDs without a problem, yet may have difficulty with playback of some or all brands of CD-Rs. This problem is more likely to occur with car CD players and portable CD players that have less robust error-correcting abilities, older CD players, and less expensive CD players of any age. On older players especially, the light intensity of the playback laser may be insufficient to read CD-Rs consistently, since CD-R reflectivity is less than glass master CDs. Certain brands of CD-R may cause problems for a particular CD player because of differences in dye formulations among CD-R brands.

5. Storage and Handling of CDs. Because CD-Rs contain a dye that produces the data image through a photochemical change, CD-Rs may be more light and temperature sensitive than glass master CDs that contain their data in a physical impression in the CD. Neither type of CD should be left in a hot car, but this is especially important for a CD-R disk. Direct sunlight and high temperature may cause the dye in the CD-R to deteriorate, making the CD-R unplayable. In addition, high temperature may adversely affect the adhesion of paper labels. For ink-jet printed labels, moisture may cause the printing to smear. Both kinds of CD should be handled only by the edge and/or center hole. Scratches on the bottom clear surface of the CD can cause the laser beam shining through to become unfocused on its way to and from the data layer and cause playback problems. Small scratches on this surface may sometimes be successfully polished out. Much more serious are scratches on the upper surface of the CD (the label side), since the reflective mirror layer (and on CD-Rs, the dye layer) reside on the top surface of the CD. Deep scratches on the upper surface are essentially unrepairable and may make a CD permanently unplayable.

VOCHOR'S APPROACH TO MINIMIZING CD-R INCOMPATIBILITY

1. Most CD-R duplicators can verify the copy accuracy of data CDs but not audio CDs. Data CD-Rs have built-in checksum verification whereas audio CDs do not have this verification provision. We use a CD-R duplicator that has been designed specifically for audio disk duplication. This duplicator, through proprietary firmware, produces "frame-accurate" CD-R copies that CAN be verified byte by byte against the master recording, thus guaranteeing that all duplicated CD-Rs are IDENTICAL to each other and to the master. VoChor's duplicator verifies the copy accuracy of each CD-R produced on it.

2. All our CDs are duplicated using Plextor recorders that are recognized as an industry standard for audio CD-R recording. These drives utilize a feature called "Optimum Power Control" that dynamically adjusts the recording laser intensity for each CD-R being recorded. This technique improves CD-R duplication quality by correcting laser intensity for variations in dye thickness, dust and manufacturing variability of the CD-R blanks.

3. Duplicator manufacturers typically test and recommend only certain brands of blank CD-Rs for use in their machines. VoChor uses only first quality (A grade) manufacturer-recommended CD-Rs in its duplicators.

4. VoChor now uses a thermal transfer CD printer to improve the lifespan of its CDs. Thermal transfer printing has the advantage of being more permanent (waterproof) than inkjet-printed CDs, and avoids the use of paper labels that may have limited adhesive lifetime.

5. In the event that a customer has difficulty playing one of our audio CD-Rs, we suggest that the CD be tried on a different player, preferably on a home stereo CD player or a computer CD drive (as opposed to a boom box, portable or car CD player). If a particular customer playback device has consistent difficulty in playing our CD-R, it is likely that a CD-R brand/CD player incompatibility exists. VoChor, at the customer's request, will do a one-time exchange of the problematic CD-R for one produced by an alternate CD-R manufacturer. VoChor will work with our customers to resolve incompatibility problems to the best of our ability.