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By
Robert Orban and Greg Ogonowski
Orban/CRL
Equipment
Following OPTIMOD
Some
of the equipment following OPTIMOD in the transmission path can also affect quality. The
STL, FM exciter, transmitter, and antenna can all have subtle, yet audible, effects.
STL
The
availability of uncompressed digital STLs using RF signal paths has removed one of the
major quality bottlenecks in the broadcast chain. These STLs use efficient modem- style
modulation techniques to pass digitized signals with bit-for-bit accuracy. Provided that
the user uses their digital inputs and outputs, and does not require them to do sample
rate conversion (which can introduce overshoot if it a downward conversion that filters
out signal energy), they are essentially transparent. Uncompressed digital STLs using
terrestrial lines (like T1s in the United
States)
also provide transparent quality and are equally recommended. An older digital STL
technology uses lossy compression. If the bit rate is sufficiently high, these can be
quite audibly transparent. However, all such STLs introduce overshoot and are therefore
unsuitable for passing processed audio that has been previously peak limited. Analog
microwave STLs provide far lower quality than either digital technology and are not
recommended when high audio quality is desired. They are sometimes appropriate for AM,
because receiver limitations will tend to mask quality limitations in the STL.
FM
Exciter
Exciter
technology has improved greatly in the last twenty years. The most important improvement
has been the introduction of digitally synthesized exciters from several manufacturers.
This technology uses no AFC loop, and can have frequency response to DC, if desired. It
therefore has no problems with bounce or tilt to cause overshoot. In conventional analog
exciter technology, the major improvements have been lowered non-linear distortion in the
modulated oscillator, and higher-performance Automatic Frequency Control (AFC) loops with
better transient response and lower low-frequency distortion. At this writing, the
state-of-the-art in analog modulated oscillator distortion is approximately 0.02% THD at
±75 kHz deviation. (Distortion in digital exciters is typically 10 times lower than
this.) In our opinion, if the THD of your exciter is less than 0.1%, it is probably
adequate. If it is poorer than this (as many of the older technology exciters are),
replacing your exciter will audibly improve sonic clarity and will also improve the
performance of any subcarriers. Even if the distortion of your modulated oscillator is
sufficient, the performance of the AFC loop may not be. A high-performance exciter must
have a dual time-constant AFC loop to achieve satisfactory low-frequency performance. If
the AFC uses a compromise single time-constant, stereo separation and distortion will be
compromised at low frequencies. Further, the exciter will probably not accurately
reproduce the shape of the carefully peak- controlled OPTIMOD-FM output, introducing
spurious peaks and reducing achievable loudness. Even dual time-constant AFC loops may
have problems. If the loop exhibits a peak in its frequency response at subsonic
frequencies, it is likely to “bounce” and cause loss of peak control. (Composite STLs
can have similar problems.)7
Digital
exciters have none of these problems. However, a properly
designed analog
exciter
can have good enough performance to limit overshoot due to tilt and bounce to less than 1%
modulation. Therefore, either technology can provide excellent results. 7
Co-author
Greg Oqonowski, Orban’s Vice President of New Product Development, originallybrought
this to the industry’s attention. (www.indexcom.com). Ogonowski has developed
modifications for several exciters and STLs that improve the transient response of their
AFCs.
FM
Transmitter
The
transmitter must be transparent to the modulated RF. If its amplifiers are narrowband
(< 500 kHz at the -3dB points), it can significantly truncate the Bessel sidebands
produced by the FM modulation process, introducing distortion. For best results, –3dB
bandwidth should be at least 1MHz. Narrowband amplifiers can also introduce synchronous
FM. This can cause audible problems quite similar to multipath distortion, and can
particularly damage SCAs. Synchronous FM should be at
least –35dB
below carrier level, with –40dB or better preferred.8
If
the transmitter’s group delay is not constant with frequency, it can also introduce
synchronous FM, even if the bandwidth is wide. Please note that the “Incidental FM”
reading on most FM modulation monitors is heavily smoothed and de-emphasized, and cannot
be used to measure synchronous FM accurately. At least one device has appeared to do this
accurately (Radio Design Labs’ Amplitude Component Monitor Model ACM-1).
FM
Antenna
Problems
with antenna bandwidth and group delay can also cause synchronous FM, as can excessive
VSWR, which causes reflections to occur between transmitter and antenna. Perhaps the most
severe antenna-induced problems relate to coverage pattern. Proper choice of the antenna
and its correct installation can dramatically affect the amount of multipath distortion
experienced by the listener. Multipath-induced degradations are far more severe than any
of
the other quality-degrading factors discussed in this paper. Minimization of received
multipath is the single most important thing that the broadcast engineer can do to ensure
high quality at the receiver.
AM
Transmitter
We
live in the golden age of AM transmitters. After 75 years of development, we finally have
AM transmitters (using digital modulation technology) that are audibly transparent, even
at high power levels. Previously, even the best high-power AM transmitters had a sound of
their own, and all audibly degraded the quality of their inputs. We recommend that any AM
station that is serious about quality upgrade to such a transmitter. By comparison to any
tube-type transmitter, not only is the quality au- 8
Geoff
Mendenhall of Harris has written an excellent practically-oriented paper on minimizing synchronous
FM: G. Mendenhall, “Techniques for Measuring Synchronous FM Noise in FM Transmitters,”
Proc.
1987 Broadcast Engineering Conf., National Assoc. of Broadcasters, Las Vegas, NV, pp.
43-52 (Available from NAB Member Services) dibly
better on typical consumer receivers, but the transmitter will pay for itself with lower
power bills.
AM
Antenna
The
benefits of a transmitter with a digital modulator will only be appreciated if it feeds an
antenna with wideband, symmetrical impedance. A narrowband antenna not only audibly
reduces the high frequency response heard at the receiver, but also can cause non-linear
distortion in radios’ envelope detectors if asymmetrical impedance has caused the upper
and lower sidebands to become asymmetrical. Such antennas will not work for any of the AM
IBOC systems proposed at this writing. Correcting antennas with these problems is
specialized work, usually requiring the services of a competent consulting engineer.
Summary
Maintaining
a high level of on-air audio quality is a very difficult task, requiring constant
dedication and a continuing cooperation between the programming and engineering
departments. With the constantly increasing quality of home receivers and stereo gear, the
radio audience more and more easily perceives the results of such dedication and
cooperation. One suspects that in the future, FM and DAR will have to deliver a
state-of-the-art signal in order to compete successfully with the many other program
sources vying for audience attention, including CD’s, DVD’s, videodiscs, digital
audio, subscription television, direct satellite broadcast, DTV, streaming programming on
the Internet, and who knows how many others! The human ear is astonishingly sensitive;
perceptive people are often amazed when they discover that they can detect rather subtle
audio chain improvements on an inexpensive car radio. Conversely, the FM
broadcast/reception system can exaggerate flaws in audio quality. Audio processors (even
OPTIMOD) are especially prone to exaggerating such flaws. In this discussion, we have
tried to touch upon the basic issues and techniques underlying audio quality in radio
operations, and to provide useful information for evaluating the cost-effectiveness of
equipment or techniques that are proposed to improve audio quality. In particular, we
concluded that today’s high-quality IC opamps are ideally suited for use as
amplification elements in broadcast, and that compromises in digital standards, computer
sound cards, disk playback, and tape quality are all likely to be audible on the air. The
all-digital signal path is probably the single most important quality improvement that a
station can make, but the installing engineer must be aware of issues such as lossy
compression (particularly when cascaded), word length, sample rate, headroom, jitter, and
dither. Following the suggestions presented here will result in better on-air audio
quality— and that is a most important weapon in attracting and maintaining an audience
that is routinely exposed to compact discs and other high-quality audio reproduction
media.
The future belongs to the quality-conscious.
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