Once recorded, CD-R discs can be played back or read in any standard CD drive. CD-R discs are useful for archival storage and creating master CDs, which can be duplicated for distribution within a company.
CD-Rs function using the same principle as standard CD-ROMs. The main difference is that instead of being stamped or embossed into plastic as on regular CDs, CD-Rs have images of pits burned onto a raised groove instead. Therefore, the pits are not really raised bumps like on a standard CD, but instead are rendered as dark (burned) areas on the groove that reflect less light. Because the overall reflectivity of pit and land areas remains the same as on a stamped disc, normal CD drives can read CD-Rs exactly as if they were stamped discs.
Part of the recording process with CD-Rs starts before you even insert the disc into the drive. CD-R media is manufactured much like a standard CD-a stamper is used to mold a base of polycarbonate plastic. However, instead of stamping pits and lands, the stamper imprints a spiral groove (called a pre-groove) into the disc. From the perspective of the reading (and writing) laser underneath the disc, this groove is seen as a raised spiral ridge and not a depression.
The pre-groove (or ridge) is not perfectly straight; instead it has a slight wobble. The amplitude of the wobble is generally very small compared to the track pitch (spacing). The groove separation is 1.6 microns, but it wobbles only 0.030 microns from side to side. The wobble of a CD-R groove is modulated to carry supplemental information read by the drive. The signal contained in the wobble is called absolute time in pre-groove (ATIP) because it is modulated with time code and other data. The
time code is the same minutes:seconds:frame format that will eventually be found in the Q-subcode of the frames after they are written to the disc. The ATIP enables the drive to locate positions on the disc before the frames are actually written. Technically, the wobble signal is frequency shift-keyed with a carrier frequency of 22.05KHz and a deviation of 1KHz. The wobble uses changes in frequency to carry information.
To complete the CD-R disc, an organic dye is evenly applied across the disc by a spin-coating process. Next, a gold or silver reflective layer is applied (some early low-cost media used aluminum), followed by a protective coat of UV-cured lacquer to protect the reflective and dye layers. Gold or silver is used in recent and current CD-R discs to get the reflectivity as high as possible (gold is used in archival CDRs designed for very long-term storage), and it was found that the organic dye tends to oxidize aluminum. Then, silk-screen printing is applied on top of the lacquer for identification and further protection. When seen from the underside, the laser used to read (or write) the disc first passes through the clear polycarbonate and the dye layer, hits the gold layer where it is reflected back through the dye layer and the plastic, and finally is picked up by the optical pickup sensor in the drive.
The dye and reflective layer together have the same reflective properties as a virgin CD. In other
words, a CD reader would read the groove of an unrecorded CD-R disc as one long land. To record on a CD-R disc, a laser beam of the same wavelength (780nm) as is normally used to read the disc, but with 10 times the power, is used to heat up the dye. The laser is fired in a pulsed fashion at the top of the ridge (groove), heating the layer of organic dye to between 482°F and 572°F (250°-300°C). This temperature literally burns the organic dye, causing it to become opaque. When read, this prevents the light from passing through the dye layer to the gold and reflecting back, having the same effect of cancelling the laser reflection that an actual raised pit would on a normal stamped CD.
The drive reading the disc is fooled into thinking a pit exists, but no actual pit exists—there’s simply a spot of less-reflective material on the ridge. This use of heat to create the pits in the disc is why the recording process is often referred to as burning a CD. When burned, portions of the track change
from a reflective to a non-reflective state. This change of state is permanent and can’t be undone, which is why CD-R is considered a write-once medium.
All CD-R drives can work with the original 650MiB (682MB) CD-R media (equal to 74 minutes of recorded music), as well as the now-standard higher-capacity 700MiB (737MB) CD-R blanks (equal to 80 minutes of recorded music).
Some drives and burning software are capable of over burning, whereby they write data partially into the lead-out area and essentially extend the data track. This is definitely risky as far as compatibility is concerned. Many drives, especially older ones, fail when reading near the end of an over burned disc. It’s best to consider this form of over-clocking CDs somewhat experimental. It might be useful for your own purposes if it works with your drives and software, but interchangeability will be problematic.
Some vendors sell 90-minute (790MiB) and 99-minute (870MiB) media to make over burning easier. Most standard CD-RW drives can reliably burn up to 89:59 of music onto the 90-minute media, and the resulting CD-R can be played on a variety of late-model auto and home electronics players.
CD-R Media Colour
There has been some controversy over the years about which colours of CD-R media provide the best performance. Table 11.6 shows the most common colour combinations, along with which brands use them and some technical information.
Some brands are listed with more than one colour combination, due to production changes or different product lines. You should check colour combinations whenever you purchase a new batch of CD-R media if you’you have found that particular colour combinations work better for you in your applications.
The perfect media for you will be the ones that offer you the following:
¦ High reliability in writing (check your drive model’s list of recommended media)
¦ No dye or reflective surface dropouts (areas where the media won’t record properly)
¦ Durability through normal handling (scratch-resistant coating on media surface)
¦ Compatibility across the widest range of CD drives
¦ Lowest unit cost
If you have problems recording reliably with certain types of media, or if you find that some brands with the same speed rating record much more slowly than others, contact your drive vendor for a firmware upgrade. Firmware upgrades can also help your drive recognize new types of faster media from different vendors.
CD-R Media Recording Speed Ratings
With CD-R mastering speeds ranging from 1x (now-discontinued first-generation units) up through the current 48x-52x rates, it’s important to check the speed rating (x-rating) of your CD-R media.
Most branded media on the market today is rated to work successfully at up to 52x recording speeds (some are limited to 48x). Some brands indicate this specifically on their packaging, whereas you must check the websites for others to get this information. If necessary, install the latest firmware updates to reach maximum recording speed.