The Analog Compact Disc
Back in the days when vinyl was our only source of high-quality recorded music, the first question when playing a new record was often, "How good is the pressing?" LPs varied so much in their mastering and pressing qualities that buying a new one was often a crapshoot.
With the advent of the digital compact disc, however, few of us bother to think about variations in CD quality. We assume that if the CD spins and makes sound, the disc must have been manufactured perfectly.
In reality, compact discs vary greatly in manufacturing quality. The process of creating the tiny pit and land structures that represent the music, then transferring those structures to an inexpensive mass-produced product, is fraught with potential problems. The result is a wide variation in the technical—and sometimes musical—performance of our CDs.
In this article, we'll look at how the CD works, how CDs are made, and what can go wrong in the disc-manufacturing process. In addition, I'll report on the disc quality of a sampling of CDs made at different pressing plants around the world.
This technical evaluation of CD quality was made possible by a unique CD analyzer obtained by Stereophile (see Sidebar) that reveals CD data-error types and rates, and allows an examination of the critical signals coming off a CD. Finally, I'll show you how to identify the factory where a disc was made, and debunk some common myths about data errors on CDs (footnote 1).
How the Compact Disc works
A compact disc is a piece of polycarbonate (a type of plastic) on which a spiral track has been impressed. This spiral track is a series of indentations ("pits") separated by flat areas ("land"). This alternating pit-and-land structure can be seen in fig.1, a scanning electron microscope photograph of a CD surface. The white line at the top of the photograph provides the scale; the line is 10µm (ten micrometers, or microns) long. To put the extraordinarily small size of the pits into perspective, a human hair is about 75µm in diameter. By the scale of this photograph, a human hair would be a foot and a half thick (footnote 2).
Fig.1 Scanning electron microscope photograph of a CD surface. The white line at the top is 10µm in length. A human hair has a diameter of about 75µm. (Photo by Alvin Jennings.)
Digital audio data are encoded in the spiral track of pit and land, waiting to be recovered by your CD transport. The transport's laser beam is focused on the spinning disc, which is coated with a thin, reflective metal layer, almost always aluminum (gold and brass are also occasionally used). The disc's metal coating reflects the beam back to a photodetector, a device that converts light into an electrical signal.
When the laser beam is reflected from the land, the beam is returned to the photodetector at virtually full strength. The laser beam is significantly reduced in intensity when it reflects from a pit bottom because the pit depth is one-quarter the wavelength of the playback laser beam. The portion of the beam reflected from the pit bottom is therefore shifted in phase by 180 degrees (one-quarter wavelength going down, then another one-quarter wavelength going back up) in relation to the beam portion reflected from the land. A 180 degrees phase shift is half a wavelength—equivalent to a polarity reversal. When the two out-of-phase parts of the beam combine to strike the photodetector, they cancel. It's like wiring your loudspeakers out of phase and hearing less bass: when one woofer moves forward, the other moves backward, and the waves cancel each other.
Rather than represent one condition by binary "zero" and the other by binary "one," it was felt better to represent a reflection from both land and pit bottom as a binary "zero," with binary "one" corresponding to the change in beam intensity when the beam is reflected from a pit-to-land or land-to-pit transition. In short, land or a pit bottom are binary 0, transitions are binary 1.
The photodetector therefore outputs a varying voltage in response to the pit pattern—a voltage that contains all the binary 1s and 0s encoded on the disc.
An encoding scheme called "Eight to Fourteen Modulation" (EFM) formats the data to be recorded on the disc according to certain rules. EFM coding creates a bit stream in which binary 1s are separated by a minimum of two 0s and a maximum of ten 0s. The shortest pit or land length therefore represents the binary data "1001," and the longest pit or land length represents the binary data "100000000001." EFM coding creates a specific pattern of ones and zeros that results in nine discrete pit or land lengths on the disc. You can see the discrete nature of the pit and land lengths in the photograph shown in fig.1.
Footnote 1: Before joining Stereophile in the Spring of 1989, I worked in CD-mastering for three and a half years, and helped to build a CD mastering machine. I also co-wrote a paper (with Ray Keating) called "Compact Disc Video (CDV) Signal Optimization," presented at the 1987 New York AES Convention. The paper examined the tradeoffs in signal quality when digital audio was combined with video.
Footnote 2: The pits are 0.56µm wide, with a track spacing of 1.6µm, and vary in length from 0.8µm to 3.5µm.