Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

8
Maintaining Audio Quality
tive versions. Sometimes the original performance has been remixed for CD release,
which often improves the quality. However, beware of remixes so radical that they
no longer sound like the hit version as remembered by audiences. Furthermore,
many of these original performances may be available on many different albums,
possibly from the same label but from different sources with varying quality. A valuable
on-line resource for this information is Top 40 Music on Compact Disc:
http://www.top40musiconcd.com/
Some of the more recent remasterings may contain additional signal processing beyond
simple click and pop elimination. Newly remastered tracks should be validated
very carefully, as the newer tracks may suffer from excessive digital limiting that reduces
transient impact and punch. Therefore, the older, less-processed sources may
stand up better to Optimod transmission processing. Some computer audio editing
software, such as Adobe Audition, Steinberg WaveLab, iZotope RX, and Diamond
Cut DC8, contains restoration tools that attempt to undo clipping. This is a challenging
task, so these tools should be qualified and used very carefully. One size does
not fit all.
Unfortunately, there is no reliable formula for choosing old or new CDs. For example,
some original CD releases were simply transfers of a vinyl pressing. The limitations
of vinyl are usually audible and subsequent remasters were a dramatic improvement,
either because the original master tapes were discovered and used or
because improved vinyl restoration software and techniques were employed. On the
other hand, many remasters were subject to additional dynamic range compression
and peak limiting and do not sound as good as their original releases even though
the newer remasters may claim higher resolution.
We believe that the best-sounding CDs are probably those mastered from about
1990 to 1995. Before 1990, many mastering engineers used the Sony PCM1610’s
converters because the standard medium for transmitting a mastered CD to a replication
house was then a 3/4" “U-Matic” video recorder and the Sony formatted the
audio so that it could be recorded as video on such a machine. However, the
PCM1610’s converters were widely criticized for their sound. There were too many
low-slewrate opamps and electrolytic capacitors in the signal path, not to mention
high-order analog anti-aliasing filters.
Things changed around 1990 with the introduction of analog to digital converters
based on the then-new Analog Devices AD1879 delta/sigma A/D converter chip. This
chip uses oversampling to eliminate the need for high-order analog anti-aliasing
filters and has very good low-level linearity. The resulting signal path has very little
group delay distortion. If properly designed, converters based on the AD1879 sound
very good. Meanwhile, the loudness wars were still at least five years in the future.
Starting around 1995, average levels on CDs started to increase. The availability of
early digital look-ahead limiters like the WAVES L1 enabled CD mastering engineers
to limit peaks without obvious side effects. However, like most anything else, the
availability of the tools led to their abuse. Digital limiting, which was a bit like crack
to some mastering engineers, started to suck the life and punch out of material for
the sake of loudness. Certain engineers developed a reputation as “go-to guys” if
you wanted a “loud” CD. Look-ahead limiting by itself was no longer enough —