Frozen CDs?

Just when you thought it was safe to put green paint around the edges of your CDs without ridicule, there's yet another CD tweak that's sure to bring howls of laughter from the skeptics: cryogenically freezing CDs. They won't be laughing for long, however, when they hear for themselves the sonic results of this process.

Ed Meitner, designer of the Museatex line of electronics, has discovered that cryogenically freezing a CD changes the physical structure of polycarbonate, the plastic material from which CDs are made. The result is reportedly an audible improvement in sound quality. In this process, CDs are placed in a cryogenic freezing chamber and the temperature is slowly reduced over eight hours to 75 Kelvins, or about -300 degrees Fahrenheit. This is approximately the temperature of liquid nitrogen, the chamber's cooling agent. The temperature is then slowly brought back to room temperature over another eight hours.

This technique reportedly relaxes the lattice structure of a material (polycarbonate in the case of CD) that has been previously distorted by heat or pressure, both of which are present during CD injection molding. By reducing the molecular bonds holding the material together, the internal stress in the material is reduced, thus changing its resonant characteristics. Indeed, a treated disc feels slightly more flexible than an untreated disc.

But how could freezing a CD possibly affect its sound quality? So what if the polycarbonate has a different structure? The data are all ones and zeros. Furthermore, uncorrected data errors are almost nonexistent in most discsci without treatment, ruling out improved data integrity as an answer. I posed these questions to Ed Meitner and got the following explanation (footnote 1).

Mechanical vibration of the disc causes the HF signal to become noisy and have excessive jitter. The HF signal is the raw signal output from the CD player's photodetector (footnote 2). By freezing a CD, the disc's mechanical resonance is lowered, improving the quality of the HF signal retrieved from the disc. Although theory states that noise and jitter in the HF signal will have no effect on sound quality—the HF signal is squared, buffered, decoded, filtered, and clocked out of another buffer with quartz-crystal accuracy—many digital designers maintain that HF signal quality does affect the sound.

Ed Meitner claims that the HF signal improvement from a cryogenically treated disc is easily measurable. I looked at the HF signal on an oscilloscope from the Esoteric P2 transport with treated and untreated discs. I could see no difference in the signal quality. However, it is very difficult to make comparisons without seeing the two HF signals side by side.

Meitner is talking to some audiophile labels about mass-treating their releases. Apparently, the process is efficient and economical, with the ability to treat thousands of discs at once. Liquid nitrogen, which doesn't come in direct contact with the CDs, is inexpensive and readily available. Interestingly, this process is said to yield similar sonic improvements with a vinyl phonograph record. In addition to CDs and LPs, the process has been used on LaserVision-format video discs, speaker cable, interconnects, integrated circuits, and musical instrument strings.

Cryogenic freezing is also used to treat machine tools like drill bits, copper welding tools, and saw blades. The process reportedly improves their wear characteristics, thus extending the tool's useful life. The treatment doesn't always work, however, and there is no consensus among metallurgists that the process is always beneficial. In fact, the effects of cryogenically freezing materials is not well understood; little scientific research has been done to explain the phenomenon (footnote 3).

Another tweak developed by Ed Meitner is painting a CD's top surface black. This reportedly improves sound quality by improving the signal at the CD player's photodetector. Before describing how this works, let's look at the playback laser beam's path through the disc.

The playback beam enters the disc through the surface without the label. It travels through the 1.2mm disc thickness where it encounters pits impressed in the polycarbonate. To reflect the beam back through the disc and to the photodetector, a thin layer of aluminum is deposited on the disc surface, which conforms to the pit structure. A protective coating of varnish seals in the aluminum and prevents it from oxidizing. The label is then silk-screened on top of the protective coating.

Footnote 1: Please note that in this article I merely relate the explanations of physical phenomena as described by others. I do not necessarily subscribe to the views presented.—Robert Harley

Footnote 2: See my article "CD: Jitter, Errors, and Magic" in Vol.13 No.5 for an explanation of how data are retrieved from a CD.—Robert Harley

Footnote 3: I refer the interested reader to an excellent article in the June 1988 issue of Popular Science for more information on cryogenic freezing. The piece includes tests on treated and untreated machine tools, with mixed results.—Robert Harley

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