Maintaining Audio Quality In The
Broadcast Facility


Robert Orban and Greg Ogonowski

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.

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. ( 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.  

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.