DISC SIZE AND CAPACITY

What are the physical sizes of CD-R and CD-RW discs?
CD-R and CD-RW discs come in standard 12 cm (120 mm) and 8 cm (80 mm) sizes. The most popular is the larger 12 cm type which has the same physical dimension as most commercial audio CDs and computer software CD-ROMs. 8 cm discs are less common but, thanks to their smaller size, are gaining popularity for use in consumer electronic devices such as portable compressed digital audio players, digital still image cameras and data storage products like miniature CD recorders.

What about business card CD-R and CD-RW discs?
Beyond the conventional 8 cm and 12 cm sizes some manufacturers offer discs shaped like business and credit cards or in other novelty forms. These do not conform with Orange Book specifications and, as a result, may not write and play back in all recorders or reading devices. Following manufacturer instructions is always the best course.

What capacities of blank CD-R and CD-RW discs are available?
Manufacturers commonly express disc capacity in terms of how much Red Book digital audio (in minutes) and computer data (in megabytes) a disc can contain. Historically, 63 minute/550 MB (12 cm) and 18 minute/158 MB (8 cm) discs were once available but are now rendered obsolete by advances in recording technology. Currently, 74 minute/650 MB, 80 minute/700 MB (12 cm) and 21 minute/185 MB (8 cm) discs are the market standards.

What about 34, 90 and 99 minute CD-R discs?
A few media manufacturers have recently introduced 34 minute/300 MB (8 cm), 90 minute/790 MB and 99 minute/870 MB (12 cm) CD-R discs. To achieve these higher capacities such discs do not conform to Orange Book specifications and, as a result, may not write in all recorders, be accessible to all software or readable in all players and drives. Using 34, 90 and 99 minute CD-R discs is therefore not recommended.

How much information can actually be stored on CD-R and CD-RW discs?
The amount of information that can be written is determined by the disc’s recording capacity as well as the physical and logical formats used.
Each of the five main CD physical formats devotes a different amount of space to user data (audio = 2,352 bytes/block, CD-ROM Mode 1 = 2,048 bytes/block, CD-ROM Mode 2 = 2,336/bytes/block, XA Form 1 = 2,048 bytes/block, XA Form 2 = 2,324 bytes/block). For any given data format disc capacity can be calculated by multiplying the appropriate user data area size by the CD data transfer rate of 75 blocks per second by 60 seconds by the minute size of disc. For example, a 80 minute disc written in CD-ROM Mode 1 format: user data area of 2048 bytes/block x 75 blocks/second = 153,600 bytes/second x 60 seconds = 9,216,000 bytes/minute x 80 minutes = 737,280,000 bytes. This rounds to roughly 700 MB (dividing by 1,024 to convert into KB and again by 1,024 to convert into MB). It should be noted, however, that in the real world capacity can vary slightly among discs from different media manufacturers.

For discs written with computer data the logical format used also consumes space available for user information. For example, the overhead for the first session of a multisession disc consumes 22 MB of space and each subsequent session thereafter uses 13 MB. And in the case of CD-RW discs which are formatted for random packet-writing, usable capacity is reduced by roughly 23%.

CD-R and CD-RW Disc Capacities
(capacities indicated in bytes)

What is the difference between 74 and 80 minute discs?
The only meaningful difference between most 74 and 80 minute discs is their storage capacity. Typically, this increase in usable space is achieved by tightening the coils of the pregroove (track pitch). This allows the disc to accommodate a longer pregroove and therefore a larger recordable area.

Are there any compatibility issues when using 80 minute instead of 74 minute discs?
Originally, 80 minute discs were specialized products for use in audio premastering studios but now have become commonplace and compatible with most software, recorders, readers and players. In some instances, however, older recorders and premastering software must be upgraded to accommodate 80 minute discs. It is, therefore, advisable to check with the manufacturers of your products and ensure that the latest versions of software and firmware are being used.

What is overburning?
Overburning (sometimes called oversizing) is the ability to write beyond the manufacturer’s declared capacity on a CD-R or CD-RW disc. This is accomplished by using the disc’s Lead-Out Area (reserved to indicate to a reading device that the end of the data has been reached) to store the additional user information. Although some recorders and premastering software packages have the ability to overburn a disc the practice is not permitted by Orange Book standards. Overburning might affect product warranties and result in lost data so it is not recommended.

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