Sampling

In analog sound recording, such as that on phonograph discs, audiocassettes, and standard audiotapes, an analog of the source audio waves is physically produced. Playback then requires an abrasive physical device to literally trace the recorded sound wave. Digital sound recording, such as that on compact discs, DVDs, and CD-ROMs, instead involves taking multiple discrete measurements of the voltage levels of the continuous source audio waves, a process known as sampling. The most common sampling rate is 44.1 kilohertz (kHz), or 44,100 times per second, which guarantees at least two measurements of any humanly audible sound wave. (The typical sound range audible to a person is 20 Hz to 20 kHz.) The accuracy of the recorded voltage measurements depends critically on the number of binary digits (bits) used to record the measurements. More bits enable finer distinctions to be made in audio voltage levels and, in turn, enable a closer approximation of the original sound wave. The industry standard of 16 bits is sufficient to produce an audibly smooth curve with very little distortion. For even greater fidelity (that is, greater accuracy in approximating the effect generated by the true sound), music studios sometimes use 24-bit encodings for their master tapes. Because the recorded bits are read from the internal reflective layer of the CD by a laser, the disc remains untouched by any physical object and thus does not degrade under normal use.

Dynamic range

The number of bits determines the maximum attainable dynamic range. Dynamic range is the ratio of the loudest undistorted sound to the quietest discernible sound, expressed in decibels, that a system is capable of producing. The compact disc’s dynamic range is about 90 decibels, compared with about 70 decibels on the best phonograph discs, thus accounting for the distinct, clear sound obtained from even the cheapest CD players. Nevertheless, some audiophiles maintain that the best phonograph recordings stamped on polyvinyl chloride (or “vinyl”) discs deliver subtle musical overtones that are almost invariably lost in the digitization process.

Laserdiscs and CD-ROMs

Because any type of information—not just audio—can be digitized, other forms of digital discs have been produced. Among them are the larger (120 to 300 mm [4.7 to 11.8 inches]) laserdisc, or videodisc, which combines digital audio and analog video data and has become an obsolete format, and the CD-ROM. (ROM is an abbreviation for “read-only memory.”) CD-ROM discs are identical in appearance to audio CDs but are typically read by a computer or similar device and generally contain up to 680 megabytes of computer programs and data, rather than just audio or video data.

DVD

In 1995 Philips and Sony introduced a new type of disc, known as a digital videodisc (DVD), which was able to store up to 4.7 gigabytes of data, such as high-definition digital video files. A DVD has the same dimensions as a standard CD but cannot be read by a standard CD player, although a DVD player can read standard CDs. DVD players use a higher-power red laser (0.65 micrometre) that enables smaller pits (0.4 micrometre) and separation tracks (0.74 micrometre) to be used.

By 2002 two competing (and incompatible) technologies were presented for storing video in high definition on a CD-ROM-sized disc. The HD DVD, proposed by Toshiba and the NEC Corporation, and the Blu-ray disc, proposed by a group led by Sony, used higher-wavelength blue-violet lasers, which allowed even smaller pits to be traced on even more closely spaced tracks than on the DVD. This advancement greatly expanded the storage capacity of the medium, enabling single-sided, single-layer HD DVDs and Blu-ray discs (which would later come to dominate the consumer market, outcompeting HD DVDs) to hold up to 15 gigabytes and 25 gigabytes of data respectively.