CD-R & CD-RW Media Technology Primer

Different technologies for different needs
Look at recordable CD technologies, and you may wonder why there's a price difference between CD-R equipment and supplies and seemingly similar CD-RW gear.

It's not simply that CD-R technology is older and has had time to come down in price from its original introduction to the marketplace — although that does impact prices and availability. The price difference between CD-R and CD-RW technologies focus on the complexity of the devices and media. While the discs look alike to the naked eye, CD-R media and drives use a less complex technology, providing lower-cost replication for distribution of music, family photos, or computer data. CD-RW media use a more complicated recording process, and use a different technology for the recording layers of the discs.

Here's a closer look at each technology:

CD-R Technology
CD-R is a "write once" technology that can be used in both CD-R and CD-RW recorders, and read by nearly any computer CD drive or audio CD player.

In a CD-R, the recording layer is made with organic dyes — greenish cyanine dye, golden phthalocyanine dye, or silver-blue azo dye, depending on the disc manufacturer. While there are differences among the dyes, the differences are probably most important to chemists, not to CD-R users. Each dye's quality is such that which dye is used makes no difference for disc use and life.

The laser of your CD-R/CD-RW drive heats the dye to a temperature of about 200° C, irreversibly melting a pitted pattern into the recording layer. A plastic layer alongside the dye expands into the newly available space, creating a pit pattern similar to that of a conventional CD. Your CD player reads this highly reflective pattern for playback. Because the plastic layer melts into the dye layer to set the pattern, CD-R discs cannot be re-recorded.

CD-RW Technology
Unlike CD-R discs, a CD-RW disc can be reused — in fact, up to 1,000 times with current discs, and perhaps even more with upcoming media improvements. However, older computer CD drives and audio CD players cannot read CD-RW media.

In a CD-RW disc, the recording layer is made of an alloy of silver and other metals — indium, antimony, and tellurium. Out of the case, this layer has a polycrystalline structure. When you record to the CD-RW, your CD-RW drive's laser selectively heats tiny areas of the recording track to a temperature above the layer's melting point (500 - 700° C) — a much higher heat than the laser in a CD-R recorder can reach.

The energy delivered by the laser beam melts the crystals in the heated areas into a non-crystalline phase — also known as "pits." These pits reflect less light than the remaining crystalline areas, creating the playback pattern for your Multi-Read CD-ROM drive or specialized audio CD player.

Because the recording layer is self-fixing, a CD-RW disc can be erased. A CD-RW recording drive erases a recorded CD-RW by returning the pitted material in the recording layer to a crystalline state. The drive's laser heats the recording layer to a temperature of about 200° C and maintains that temperature for an extended period (more than half an hour for a complete disc, in most cases). The result is a blank disc, ready for reuse.

A recorded CD-RW may also be overwritten, saving time compared to erasing then re-recording. Overwriting combines the recording and erasing procedures. The laser writes a new data pattern to the disk, and also creates new crystallized areas between the newly recorded pits. The beam switches between the higher-energy recording level and the lower-energy erase level, resulting in simultaneous erasure of the old information on the disk and recording of a new track of data.

The complexity of the recording process makes clear why CD-RW technology still costs more than CD-R technology. However, more and more people are finding CD-RW gear to carry more value for the increasingly slim price difference in drive cost, thanks to its versatility and ability to record to both CD-R and CD-RW media.