Long ago, in the early 1960's everyone knew that mass-produced musical media (LP's and prerecorded tapes) were unavoidably inferior to the original master tapes that were recorded in the studio or concert hall. During the 1970's the quality of sound available from LP and tapes improved dramatically . But around that time there were still large difference in sound quality between master tapes and the LP's made from them.So when the compact disc was introduced a dozen years ago, its greatest promise for audiophiles was that we would finally have access to mass-produced discs that fully duplicated the sound of the master tape without compromise. Many CD's have achieved that goal-especially when the master tape was an excellent digital recording transferred directly to the disc master. But in some cases the sound of the CD has fallen short of its promise. Many of these disappointing examples have involved analog tapes whose digital transfer was poorly handled. During the 1980's this was often the fault of design flaws in the equipment. For example, the designers of the early analog-to-digital (A/D) convertors did not understand the distortion-removing function of "dither", a low- level randomizing signal that must be added to the input before quantization. In recent years, many recording engineers have upgraded their digital recorders with new A/D converters. Instead of producing 16-bit digital tapes, whose codes can be copied straight to CD, most of the latest converters produce 20-bit recordings. If these are transferred directly to CD, the bottom 4 bits in each digital word are simply cut off, producing the same quantizing distortion as in a undithered recording. The challenge of transferring 20-bit recording to CD has led to a fresh appreciation of the role of dither. The distortion that truncation would introduce can be prevented by simply redithering the signal when it is reduced from 20-bit to 16-bit format, but several manufacturers have gone a step farther by designing processors that accept a 20-bit signals and apply "noise-shaping" while recording the signal into 16-bit words that will fit onto a CD. Examples include the Apogee Electronics UV-22, Meridian 618, Sony's Super Bit Mapping (SBM), and Deutsche-Grammophon's 4D process.
