Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

in the Broadcast/Netcast Facility 33
In summary, the path to highest analog quality is that which is closest to a straight
wire. More is not better; every device removed from the audio path will yield an improvement
in clarity, transparency, and fidelity. Use only the minimum number of
amplifiers, capacitors, and transformers. For example, never leave a line amplifier or
compressor on-line in “test” mode because it seems too much trouble to remove it.
Small stations often sound dramatically superior to their “big time” rivals because
the small station has a simple audio path, while the big-budget station has put everything
but the kitchen sink on-line. The more equipment the station has (or can
afford), the more restraint and self-discipline it needs. Keep the audio path simple
and clean! Every amplifier, resistor, capacitor, transformer, switch contact, patch-bay
contact, etc., is a potential source of audio degradation. Corrosion of patch-bay contacts
and switches can be especially troublesome, and the distortion caused by these
problems is by no means subtle.
In digital signal processing devices, the lowest number of bits per word necessary
to achieve professional quality is 24 bits. This is because there are a number of
common DSP operations (like infinite-impulse-response filtering) that substantially
increase the digital noise floor, and 24 bits allows enough headroom to accommodate
this without audibly losing quality. (This assumes that the designer is sophisticated
enough to use appropriate measures to control noise when particularly difficult
filters are used.) If floating-point arithmetic is used, the lowest acceptable word
length for professional quality is 32 bits (24-bit mantissa and 8-bit exponent; sometimes
called “single-precision”).
In digital distribution systems, 20-bit words (120dB dynamic range) are usually
adequate to represent the signal accurately. 20 bits can retain the full quality of a
16-bit source even after as much as 24dB attenuation by a mixer. There are almost
no A/D converters that can achieve more than 20 bits of real accuracy, and many
“24-bit” converters have accuracy considerably below the 20-bit level. “Marketing
bits” in A/D converters are outrageously abused to deceive customers, and, if these
A/D converters were consumer products, these bogus claims would be actionable by
the Federal Trade Commission.
There is considerable disagreement about the audible benefits (if any) of raising
the sample rate above 44.1 kHz. An extensive double-blind test 8 using 554 trials
showed that inserting a CD-quality A/D/A loop into the output of a high-resolution
(SACD) player was undetectable at normal-to-loud listening levels by any of the subjects,
on any of four playback systems. The noise of the CD-quality loop was audible
only at very elevated levels.
Moreover, there has been at least one rigorous test comparing 48 kHz and 96 kHz
sample rates 9 . This test concluded that there is no audible difference between these
two sample rates if the 48 kHz rate’s anti-aliasing filter is appropriately designed.
8 Meyer, E. Brad; Moran, David R., “Audibility of a CD-Standard A/DA/A Loop Inserted into
High-Resolution Audio Playback” JAES Volume 55 Issue 9 pp. 775-779; September 2007
9 Katz, Bob: Mastering Audio: the art and the science. Oxford, Focal Press, 2002, p. 223