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 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.
