POP GOES THE PROBLEMS

The first time I walked into a record shop that sold Compact Discs, the salesperson put on quite a performance of his own. Eager to show me how durable the new little discs were, he dramatically threw one of them onto the floor with all his might.

The floor, I might add, was nicely carpeted. To add to the effect, he then ground his heel onto the surface of the disc. (The label side was facing up, of course.) He picked up the disc, popped it into a CD player, and punched a few buttons. Miracle of miracles, the disc still played without so much as a glitch or a skip.

During the brief ensuing history of Compact Discs, we've all learned to be a little more careful with our collections. That the disc survived the salesperson's demonstration was more a matter of luck than anything else. We now know that even minor scratches on the label side of a CD can potentially wreak havoc when we try to play it on all but the most forgiving (and sophisticated) CD players. But what about the ability of Compact Discs to withstand other forms of environmental stress? Not too long ago, several new terms crept into the language of optical disc technology. I became fascinated with one of them: "laser rot." It seems that there was a growing number of reports from owners of laser videodiscs that the discs literally began to "fall apart" after being played a number of times. I should point out that laser videodiscs, unlike CDs, are basically made of two slabs of plastic, both of which are coated with the metallic reflecting surface that constitutes the information layer read by a laser pickup. Since laser videodiscs can be played on both sides, much like a vinyl recording, the two slabs are sandwiched together.

What has been described in various terms, including "laser rot," turned out to be nothing more than a physical separation of the two "halves." The separation was ultimately attributed to adhesives of poor quality, elements in polluted atmospheres that reacted chemically with the adhesives, unusually severe temperature/humidity conditions, or a combination of these three factors.

Whenever I give a presentation concerning Compact Discs and the state of their technology, one question that always arises is, "How long will a CD last?" Clearly, there is no simple answer to this question. Treat a CD like the salesperson did in 1983, and it may or may not last five minutes. Keep the CD in its jewel box case, at room temperature, at a relative humidity of between 35% and 50%--and play it once a year--and it's likely to outlast your great-grandchildren. As we all know, however, few of us are likely to take this kind of care of CDs. And now, with CD players becoming more popular in automobiles, new questions arise regarding the proper care of these little plastic wonders. Is it safe to leave CDs in the glove compartment of a car in all kinds of weather? Will a CD play immediately after being subjected to winter temperatures in Minnesota--or after sitting inside a car in Arizona in midsummer? How about a Compact Disc's odds for survival in the 100% humidity of someplace like the Florida Everglades in mid-July? I decided to conduct my own experiment, using a few expendable CDs.

Let me state immediately that the environmental variables involved in an experiment such as mine are so numerous that I make no claim for total scientific control or methodology. After all, CDs are manufactured in dozens of plants, and there's no easy way to check whether the plastic compounds and base materials used in each factory are identical. Even within one batch of CDs of the same musical material, there may well be variations in quality and in the number of original digital burst errors (as well as their length or severity). Still, keeping all these variables in mind, I hoped to determine in a very general way just what extremes of temperature and humidity a half-dozen Compact Discs of mixed origin could take.

The CDs I chose ranged in date of manufacture from 1983 to the present-the most recent one having been acquired just weeks before I began my experiment. They were also chosen for diversity of label. As for the experiments themselves, they were quite simple. I started my tests by storing all six discs overnight in my food freezer. The freezer is reputed to bring temperatures down to around 0° F (around -18° C). With the discs at this low temperature, I planned to insert each disc in a state-of-the-art CD player and in an inexpensive, portable CD player known to have a minimum of error correction/servo-tracking capabilities. Each track was to be played, in turn, using the track skip feature of the player. Following this test, all surviving discs were to be placed in an oven which was regulated to a temperature of 212° F (100° C), the boiling point of water.

When the disc surfaces were stabilized at the higher temperature, I would play each track of each disc on both CD players-again, requiring the laser pickup to go from track to track by using the next-track feature. Following this test, I would place the discs in an enclosure containing a steam vaporizer, thereby elevating the relative humidity in the vicinity of the discs to as close to 100% as possible. The only concession to the CD players I would make during the test was to wipe surface moisture from the disc before inserting it in the two machines.

Following these three tests, all discs-those that survived, at least--were to be returned to the freezer for another round. My plan was to repeat the entire sequence at least 10 times.

After subjecting all six discs to the deep freeze treatment the first time, I was able to play all tracks on all six discs on both CD players. After the first heat treatment, however, I was able to play only the first four of six tracks of an early, CBS-label CD without any malfunction. Although the laser pickup was able to find tracks 5 and 6, it was unable to stay on course, skipping merrily through them in a matter of a few seconds! Examining the disc with a magnifying glass, I detected a 3/4-inch scratch on the outer edge of the disc. I also found a tiny "bubble" in the plastic near the scratch. It was apparent to me that both defects had been present even before the test began. I also detected warpage on one section of the disc. It was almost as if one section of the plastic had expanded at a nonuniform rate and an entire edge had "lifted upward." Once I found this, it was clear that no servo correcting system could adequately compensate for the severe deformity in the disc. I saw no point in subjecting it to the high-humidity test, which the remaining five discs passed with no difficulty. I did feel, however, that subjecting the now-defective disc to the freezer test for the second go-round might prove instructive. Perhaps the cold temperature could reverse the warpage and make tracks 5 and 6 of the disc playable once more. Unfortunately, such was not the case. Instead, the warpage became more pronounced. When I attempted to access either track 5 or 6, the player was hung up in an endless and unsuccessful search. I finally pressed the "Stop" button.

After one more complete cycle o heating and freezing, the defective disc still could not successfully play track 5 or 6. The laser pickup on the more sophisticated player did locate these tracks, but it could not successfully play through them without skipping randomly.

I saw no point in including this defective disc in my experiments, so the number of discs for the remaining tests was reduced to five. A third cycle through the freezer, oven, and vaporizer yielded no further defective discs. At the end of a day's testing, all discs were kept in the freezer overnight, rather than in the oven. This choice was more practical than scientific: I did not relish the idea of leaving an unattended oven on overnight. The second day's tests were essentially the same as those of the first day. I was able to complete four test cycles for the five remaining discs. By the end of the second day, with seven complete cycles of heat, cold, and high humidity, all five discs were tracked accurately through all their selections by both CD players.

I ran all five discs through three more cycles on the third day. One of the five, a very old Philips sampler given to me in 1983 by the co-inventors of the CD, did develop a slight warp during the next to the last heat cycling.

The warpage was enough to cause problems for the portable unit when I played the disc's outer tracks; the other player went through all outer tracks without missing a beat.

On the basis of this admittedly small sample of Compact Discs--and allowing for the somewhat arbitrary number of hot/cold/high-humidity cycles--I can offer these general conclusions:

1. Extreme heat seems to pose more of a threat to CDs than does extreme cold or high levels of humidity.

2. For whatever reasons, older CDs might be more easily damaged by extremes of heat than the more recently pressed discs. Whether an aging process and some change in the chemistry of the plastic is involved here (or whether modern pressing plants are using better grades of plastic) is something these tests were unable to establish with any degree of certainty.

3. Extremes of humidity have no effect upon the longevity or durability of CDs, but moisture can render the surface of a disc temporarily unreadable.

It can also cover the tiny laser pickup lens, rendering it somewhat opaque.

Neither humidity nor moisture is a permanent condition, however, and normal operation is always restored once the moisture has evaporated or been carefully wiped away.

4. Most important, my tests proved that the occasional "separation" of the two halves of a videodisc is not likely to occur in CDs, which are manufactured in a completely different molding process. Even the one disc that underwent severe warpage retained its integral structure. There was no evidence of any separation of the thick, transparent plastic coating from the aluminized reflecting surface that bears the digital data read by the laser pickup. So much for CD rot!

(adapted from Audio magazine, Aug. 1988)

= = = =