Photo of transmitter facility courtesy of Geerling Engineering
Not long ago I was awakened at six in the morning by a strange alien voice emanating from my bedside radio.
Yes, how quaint to have a bedside radio! This one happens to be a Tivoli Audio model that came out around 20 years ago. It has an analog AM/FM tuner, line input, and a remote speaker for stereo. Ironically, all it’s used for now is playing white noise for sleeping (via a Sonos Connect). The voice came right over the white noise.
What I heard seemed partly human, but the sounds were garbled and chopped, and kind of filtered. Not the sort of distortion you’d get from simple audio equipment. Two phrases repeated over and over. It was eerie and unsettling, especially at 6am. The voice stopped and I tried going back to sleep. Then it started again, but turning down the volume did not stop it! The only way to kill the sound was to turn the radio off.
Demonic possession? If you listen to the audio file, here, you might think so, but it was actually my neighbor, who is an amateur radio (HAM) operator. This had happened once before and is similar to what I remember from my days playing music, when radio chatter from taxis or delivery trucks would blurt out of guitar amplifiers.
This phenomenon is the result of high-power transmissions that can overload certain components in the output amplifier of a device, causing them to act as AM radio “detectors.” The result is that the audio portion of the signal is demodulated and amplified.
Another odd phenomenon linked to my neighbor’s HAM operation was the occasional tripping of GFCI circuit breakers for no obvious reason. By luck I discovered that the ARRL (association for amateur radio in the U.S.) had noted this behavior with a particular brand and model of breaker— the very ones I had. Fortunately, ARRL had alerted the manufacturer and I got free improved replacements. Really weird, and really hard to troubleshoot!
RF interference can occur in professional equipment when the conditions are just right. It’s rare, but something to be aware of if strange symptoms crop up in an AV system.
Spectrum and Energy
I am not an RF engineer, nor any huge expert on the topic, but I’ve learned enough to be aware of many aspects that are useful in work and everyday life. Much of it is no more exotic than high school physics. Number one is that all wireless devices, from Bluetooth to Wifi to FM radio, television, satellites and cellular phones operate in some portion of the electromagnetic spectrum. The term comes up in the news occasionally when the FCC makes rules about what entities are allowed to use different frequencies. This can be a big deal for broadcast television and professional wireless audio as cellular and internet carriers vie for more spectrum.
We tend to think of “radio waves” as invisible, and they are to us, but that’s because what we know as visible light is in a different part of the spectrum. Some animals and insects can “see” in other areas, such as ultraviolet. From a physics standpoint it’s all a continuum and our environment is full of RF energy around us all the time.
And energy is a lot of what this is about. Some readers may recall occasional controversies about whether cell phones pose a health risk because they are transmitting next to our heads. The scientific position on this is still open, but RF energy can definitely be dangerous. Apart from ionizing radiation such as x-rays and gamma rays, that can cause damage at the cellular level, the energy in broadcast transmitters can cause burns if not contained. Another example is welding, where the RF energy (in visible and non-visible spectra) can cause eye damage without protection.
A second key principle is the importance of a signal’s wavelength. Higher frequencies mean shorter wavelengths (just as with audio waves in the acoustical realm). Lower frequencies, with longer wavelengths, tend to travel farther and are more easily reflected. Transmissions bouncing off part of the earth’s atmosphere is one reason that amateur radio operators can communicate around the globe.
Wavelength is a critical factor in transmission and reception of many useful signals because antennas and receiver circuits perform best when they operate at fractions of the desired signal’s wavelength. For example, wireless mics operating at 500MHz have a wavelength of just over one-half meter (23.6 inches). Often the specified antennas will be ½ or ¼ wavelength, such as the short whips that come with a wireless receiver. Antennas can also be wound or folded. Antenna designs are specialized for different purposes and it’s important to use the right one!
Wavelength and frequency are connected to resonance, the tendency for an object, substance, or circuit to respond sympathetically to vibrations. If you sing in the shower, you may find that certain notes sound louder than others—that’s acoustic resonance. In addition to antennas and the tuning circuits in many types of receivers, resonant response to electromagnetic waves is the principle behind microwave ovens and MRIs.
Propagation and Interference
When people talk about problems with relatively new wireless technology, like Wi-Fi, I may remind them that Wi-Fi is not some magical thing, it’s just radio. In other words, it’s a signal in the RF spectrum that follows the same physical laws as everything else.
For starters, the transmitted power that reaches a receiving device changes as the square of the distance. So doubling the distance to the receiver means the power is ¼ as strong. On the other hand, too much power can overload receivers causing distortion and errors.
RF energy is reflected by some surfaces and objects, and absorbed by others, which means that signal propagation is always affected by the environment—including people moving around (one reason wireless mics might behave differently in empty rooms vs. full). Reflected signals will reach the receiver later than direct signals, which can cause interference, sometimes referred to as multipath. Receivers must be able to filter out and otherwise compensate for these unwanted signals.
Wi-Fi can also be a good example of spectrum crowding. Open the Wi-Fi menu on a laptop or phone just about anywhere in an urban location and the number of individual Wi-Fi signals may be shocking. Every business, house or apartment has its own Wi-Fi router or access point, sometimes several, all broadcasting simultaneously.
Consumer equipment tends to be delivered with the same default settings, so many will be using the same Wi-Fi channel, at the same power level. For a given receiving circuit, the task is picking out the one desired signal from a cloud of noise. The reverse path, say from a cell phone to the Wi-Fi access point, is even tougher because the power level is likely to be lower, and the phone may be moving.
A common solution to “bad Wi-Fi” is to add boosters or more access points. Seems obvious, but what is the ultimate result if everyone in a shared RF space does that?
In the world of wireless audio (mics, intercoms, in-ear monitors), particularly in the busy TV UHF band (470 – 596MHz), using lots of devices has historically been a delicate dance of choosing frequencies that don’t interfere. Not only will two devices using the same frequency be a problem, but devices transmitting on different frequencies can create intermodulation products—new signals at other frequencies. When wireless manufacturers plan out the frequency groups for their devices, and when using their channel selection tools, these factors are taken into account.
It doesn’t matter whether the payload is digital or analog, the transmission process is still analog radio, and the rules of physics apply. Fortunately, clever engineering is finding ways to use different parts of the spectrum, as well as getting more signals into the same spectrum. Among other things, as of this writing the FCC just approved the use of WMAS (Wireless Multichannel Audio System), a new method to provide increased signal density.
But of course there are tradeoffs to every decision. Higher frequencies mean shorter wavelengths, so usable distance may be reduced and signals more readily absorbed. Plus, the spectrum areas available for unlicensed users, such as the Wi-Fi bands, are popular for many other devices and possibly already crowded. Specifying or adding wireless audio devices to a system should not be done casually!
A spectrum analyzer can be used to visualize radio signals at various frequencies, which may be helpful when setting up or troubleshooting RF systems. The TinySA analyzer covers the radio and television bands and is very inexpensive (see TinySA Review at www.svconline.com/author/eric-wenocur).
Containing RF
The term “RF” tends to suggest radio waves in the air, but it also applies to those same frequencies in wires and transmission line. Conventional “cable” TV distribution uses the same frequencies as over-the-air broadcast. The cables for wireless antenna distribution are carrying the same RF signal to the antenna, as are those for cellular, Wi-Fi, etc.
Signal propagation in these cases can be peculiar. First off, the signals no longer travel at the speed of light, as they do in free space (ideally in a vacuum). Different frequencies and cable characteristics will produce different amounts of propagation delay. This used to be an important consideration in analog television because cable length affected whether video signals reached a production switcher sufficiently close in time for a clean switch (the reason for the concept of “video timing”).
Fortunately, cable propagation delay is of no concern in typical AV and production situations and is not the cause of lip-sync errors and other common delay problems (see Technology Mythology at www.svconline.com/author/eric-wenocur). For that matter, when it comes to signals down in the kilohertz range, like audio, propagation delay is effectively non-existent regardless of cable length. It’s something to know about, not worry about.
Another RF propagation phenomenon is skin effect, a term sometimes tossed around by hucksters selling “audiophile” cables. The implication is that the audio signals travel on the surface of the wires, not in the middle.
This is effectively nonsense for audio, but is important at RF frequencies, which is why broadcast transmitter buildings are full of what looks like copper plumbing pipe. Solid conductors are not used because the high frequency, high power RF wants to travel on the surface.
The infrastructure of broadcast transmission is a specialty unto itself which includes transmitters, waveguides, towers, antennas, pressurized transmission line, and power systems (all of which are potentially dangerous). If this sounds interesting, the broadcast world needs more RF engineers!
Some handy links:
(1) https://phys.libretexts.org/Bookshelves/College_Physics/College_Physics_1e_(OpenStax)/24%3A_Electromagnetic_Waves/24.03%3A_The_Electromagnetic_Spectrum
(2) https://www.ntia.gov/sites/default/files/publications/january_2016_spectrum_wall_chart_0.pdf
(3) http://www.arrl.org/
