Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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Maintaining Audio Quality
Most older broadcast electronic equipment (whether tube or transistor) is measurably
and audibly inferior to modern equipment. This is primarily due to a design philosophy
that stressed ruggedness and RFI immunity over distortion and noise, and to
the excessive use of poor transformers. Frequency response was purposely rolled off
at the extremes of the audio range to make the equipment more resistant to RFI.
Cascading such equipment tends to increase both distortion and audible frequency
response rolloffs to unacceptable levels.
Modern analog design practice emphasizes the use of high slew rate, low-noise,
low-cost IC operational amplifiers such as the National LM4562 family, the Signetics
NE5534 family, the National LF351 family and the Texas Instruments TL070 family.
When the highest quality is required, designers will choose premium-priced opamps
from Analog Devices, Linear Technology and Burr Brown, or will use discrete class-A
amplifiers. However, the 5532 and 5534 can provide excellent performance when
used properly, and it is hard to justify the use of more expensive amplifiers except in
specialized applications like microphone preamps, active filters, and composite line
drivers. While some designers insist that only discrete designs can provide ultimate
quality, the performance of the best of current ICs is so good that discrete designs
are just not cost effective for broadcast/netcast applications—especially when the
basic FM, DAB, audio codec quality limitations are considered.
Some have claimed that capacitors have a subtle, but discernible effect upon sonic
quality. Polar capacitors such as tantalums and aluminum electrolytics behave very
differently from ideal capacitors. In particular, their very high dissipation factor and
dielectric absorption can cause significant deterioration of complex musical waveforms.
Ceramic capacitors have problems of similar severity. Polyester film capacitors
can cause a similar, although less severe, effect when audio is passed through them.
Accordingly, DC-coupling between stages is best (and easy with opamps operated
from dual-positive and negative power supplies). Coupling capacitors should be used
only when necessary (for example, to keep DC offsets out of faders to prevent
“scratchiness”). If capacitors must be used, polystyrene, polypropylene, or polycarbonate
film capacitors are preferred. If electrolytic capacitors are used, it is wise to
use them with DC bias so that AC audio signals can never reverse-bias them.
To eliminate DC offsets, the best audio designs use servos instead of coupling capacitors.
However, if it is impractical to eliminate capacitors or to change capacitor
types, do not be too concerned: it is probable that other quality-limiting factors will
mask the capacitor-induced degradations.
Of course, the number of transformers in the audio path should be kept to an absolute
minimum. However, transformers are sometimes the only practical way to
break ground loops and/or eliminate RFI. If a transformer is necessary, use a
high-quality device like those manufactured by Jensen 6 or Lundahl 7 .
6 Jensen Transformer, Inc., Chatsworth, California, USA (Phone +1 818 374-5857, or Fax +1 818
374-5856)
7 Lundahl Transformers AB, Tibeliusgatan 7 SE-761 50, Norrtälje SWEDEN (Phone: +46 176 139
30 Fax: +46 176 139 35)