Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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Part 1: Recording Media
Compact Disc
Maintaining Audio Quality
The compact disc (CD) is currently the primary source of most recorded music for
broadcasting. With 16-bit resolution and 44.1 kHz sample rate, it represents the reference
standard source quality for radio, although it may be superseded in the future
by Blu-ray, which is capable of 24-bit resolution and sample rates up to
192 kHz. DVD-Audio, which also offers 24-bit resolution and 96 kHz sample rate,
was a commercially unsuccessful and now obsolete attempt to raise the quality of
consumer media, while SACD, which uses “bitstream” coding (DSD Direct-Stream
Digital) instead of the CD’s PCM (Pulse Code Modulation), is as of this writing hanging
on as a niche format, primarily for classical music. Most prognosticators believe
that the future of audiophile-quality music lies in downloadable high-resolution files
that use lossless compression. These files are already available from several sources.
Because most audio is still sourced at a 44.1 kHz sample rate, upsampling to 48 kHz
does not improve audio quality. Further, many broadcast digital sources have received
various forms of lossy data compression. While we had expected the black
vinyl disk to be obsolete by this revision, it is still used in specialized applications like
live “club-style” D.J. mixing and re-mastering when the original recordings are no
longer available or are in poor condition.
Although digital-to-analog conversion technology is constantly improving, we believe
that some general observations could be useful. In attempting to reproduce
CDs with the highest possible quality in the analog domain, the industry has settled
into technology using “delta-sigma” digital-to-analog converters (DACs) with extreme
oversampling. These converters use pulse width modulation or pulse-duration
modulation techniques to achieve high accuracy. Instead of being dependent on the
precise switching of voltages or currents to achieve accurate conversion, the new
designs depend on precise timing, which is far easier to achieve in production.
Oversampling simultaneously increases the theoretical signal-to-noise ratio and produces
(prior to the reconstruction filter within the CD player) a signal that has no
significant out-of-band power near the audio range. A simple, phase-linear analog
filter can readily remove this power, ensuring the most accurate phase response
through the system. We recommend that CD players used in broadcast employ technology
of at least this quality when connected to the broadcast facility via analog
connections. However, the engineer should be aware that these units might emit
substantial amounts of supersonic noise, so the low-pass filtering in the transmission
audio processor must be sufficient to reject this to prevent aliasing in digital transmission
processors or STLs.
The broadcast environment demands ruggedness, reliability, and quick cueing from
audio source equipment. The CD player must also be chosen for its ability to track
even dirty or scratched CDs with minimum audible artifacts, and on its ability to resist
external vibration. There are dramatic differences between players in these areas!
We suggest careful comparative tests between players using imperfect CDs to