I came across the following claim in an aging post about very expensive “$2000 glass CDs”. The author, a well-known audiophile writer, insists that the optical properties of a compact disc can affect the sound of the resulting playback. “I’ve long been fascinated by the idea that a CD’s optical properties can affect the sound in an analog-like manner even though the datastream remains unchanged,” he wrote. Regular readers will know that I’m of the camp that says that the same digital bits output from a glass disc or a conventional CD will result in exactly the same audio fidelity…provided the clocks are completely reconstructed as they are on high-end equipment and the rest of the signal chain is the same. This viewpoint is the basis of digital audio and everything else stored as digital information. Unlike vinyl LPs or analog tapes, the medium has no impact on the delivery of the message.
Is it possible that audible changes can result by using glass instead of polycarbonate? Can comparisons such as the following really be true? “I hate to rely on that old cliché in describing improved digital sound, but the glass CD sounded more ‘analog-like.’ The glass CD was smoother, more spacious, more open, deeper, and had greater ease. By comparison, the polycarbonate CD was flatter and had less air between images; instrumental textures were less natural, sounding slightly synthetic by comparison. The polycarbonate CD by contrast overlaid timbres with a patina of glare.” Who writes like this except audiophile reviewers?
This whole debate requires a firm understanding of how a CD or any optical disc works for the matter. I think most of us have a basic understanding of how a compact disc is made but I think it’s worth explaining the process once again. The digital information from a digital master tape (Sony 1630) or digital audio file (DDP) is used by an LBR (laser beam recorder) at the mastering room of the replication facility to create a glass master. The LBR burns away very small pits in the surface of a pristine glass disc, which is then used as the master in a sequence of steps that results in a stamper. A metal stamper is good for about 15,000 disc pressings. However, contrary to the author’s claim that, “Digital data are stored on the CD in ‘pits’ (indentations in the disc) and ‘lands’ (the flat disc surface),” a series of manufacturing steps actually results in exactly the opposite physical arrangement on the surface of the discs that we play. They are bumps NOT “pits”.
A stamper is used in the pressing lines to replicate discs. Polycarbonate pellets are heated and fed into the pressing machine where the stamper is slammed up against the material. The hot plastic forms a disc with millions of tiny “pits” in it. At this point, the new disc is completely clear. The disc is “sputtered” with aluminum to make it reflective and then a thin layer of additional clear polycarbonate is layered on top by dropping a small bead of liquid plastic and spinning the disc rapidly. The disc is then turned over and the disc artwork is silkscreened onto the surface. The disc when read by the optical pickup in your player shoots the laser through the clear layer and hits the “bumps” or the “lands” between the bumps and is interpreted as the two states required for digital information. There is a complex process of conversion to the actual digital stream but it not affected by the surface of the discs.
To be continued…