Suppose that you are a high ranking U.S. naval officer just before World War II. Germany has successfully controlled the seas with superior numbers of submarines, Winston Churchill is very nervous about it, and the U.S. is feeling threatened. Along comes someone with an idea that could provide an edge in the undeclared war to keep shipping lanes open, thus insuring the survival of the Allied countries. The only problem is that the idea comes from a glamorous Hollywood actress and an avant-garde concert pianist/composer. To make matters worse, this new technology relies on a punched paper roll similar to that found in a player piano.
This isn't a fictitious Hollywood script but a real life scenario. In the 1940s, Hedy Lamarr learned that in addition to being a film composer, her neighbor George Antheil also wrote an advice column in Esquire magazine, and another in the Chicago Sun about endocrinology – the study of human glands. She sought his advice about enlarging her breasts, and as you might expect, talk turned from breast enhancement to technological advancement. One thing led to another, and before you could say "endocrinology," they were discussing how they could control torpedoes by radio in a way that would avoid detection by the enemy.
Frequency-Hopping
They came up with a version of frequency-hopping, where the frequency of the control signal would constantly change to avoid detection. This would also lessen the impact of interference and lessen the chance of communications being intercepted and jammed.
Today, this technology is commonly used in cordless telephones, WiFi hot spots, and wireless DMX transmitters and receivers. It has been improved significantly over the years and is much more robust. As a result, it is being used more for wireless DMX transmission and reception. To understand how wireless DMX technology works, it helps to understand a little bit about frequency-hopping and radio communication.
Mad Science
Imagine that, in addition to your job as a naval officer, you also moonlight as a mad scientist. One of your first mad projects is to build a Jacob's Ladder, which is a high voltage arc gap generator that makes an arc rise between two diverging lengths of bare copper wire – because no self-respecting mad scientist would be without one. When you first turn it on, you notice that the arc discharge produces a large amount of electromagnetic radiation, and you think that is s-o-o-o-o cool. So you decide to try to harness its power and use it to communicate a message to the entire world.
If you could somehow connect your voice to the flow of the current in the high-voltage generator, you think you can make the electromagnetic radiation pulse with the voice signal, thus radiating an electromagnetic version of your message. You have no problem converting your voice to an electronic signal with a microphone, and after much trial and error, you finally succeed in connecting the output of the microphone to the voltage generator. You have an uncontrollable desire to toss your head back, fling your arms towards the heavens and exclaim, "It's alive!" But upon further examination, you decide that such celebration is unwarranted because the signal can only travel a very limited distance. In order to be a world-class mad scientist, that distance has to be more impressive. So back to the drawing board you go.
"Carrier" Signals
What you eventually discover is that your voice has a limited range of frequencies. If you try really hard to deepen your voice, you might be able to get it down into the hundreds of Hertz, and if you inhale helium gas – which, by the way, is very unbecoming for a mad scientist – then you might be able to squeak out tens of thousands of Hertz. But through a series of mad experiments, you discover that the higher the frequency, the farther the travel of the electromagnetic radiation. You reason that if tens of thousands of Hertz can travel a very limited distance, then a few million Hertz should carry it a very respectable distance. So you decide to use the higher frequency as the "carrier" signal. Now all you have to do is to figure out how to make the message ride on top of that high frequency carrier signal.
Late one night in the laboratory, you're feeling pretty good because there is a full moon and you've been inspired by reading Mary Shelley. You're in the zone and, almost by accident, you realize that if you changed the frequency of the carrier signal at the same rate as the frequency of your voice, you can encode your message onto the carrier signal. Then all you have to do is to build a receiver that strips out the carrier signal, and you will be left with the original voice message. For a world-class mad scientist like yourself, it was a no-brainer. You figured out how to "modulate" the frequency of the carrier signal, which is called "frequency modulation" or FM for short. (Years later, a rock band named Steely Dan would write a song about it.)
In the process, you also learn that you can modulate the voltage level, or amplitude, while keeping the frequency constant. That's what is known as "amplitude modulation," or AM. Alternatively, you can modulate the phase, or the starting time, of the carrier signal, which is "phase modulation." Radio communications typically use one of these modulation techniques, except instead of using a Jacob's Ladder, a very tall and powerful antenna works, too.
The Coyote Scenario
Now when you take off your mad scientist hat and put on your naval officer hat, you have the problem that when you try to communicate with your people using the radio, the enemy has just as much access to the radio waves as you do. So they can easily tune in and pick up your broadcast, thus spoiling the surprise you worked so hard to spring on them. And if that surprise happens to be a radio-guided torpedo heading for their U-boat, then all they have to do is to send their own radio broadcast at the same carrier frequency in order to jam the transmission. What you end up with is a Coyote-Roadrunner cartoon scene, where the torpedo turns around and chases the coyote. There has to be a better way.
One better way is to change the carrier frequency several times during the broadcast to throw off anyone who might be trying to listen in. That's what Lamarr and Antheil figured out, and eventually patented, in 1942. Their scheme was to change the carrier frequency periodically and synch the receiver at the same time using long rolls of paper with rows of perforations, which would change the tuning frequency. They proposed using 88 rows of perforations, just like a player piano, which would allow them to make use of 88 different carrier frequencies. They would synchronize the changes in the carrier frequency with the changes in the tuning frequency by using "calibrated constant-speed spring motors, such as are employed for driving clocks and chronometers." And to insure further accuracy, they suggested the use of a synchronizing pulse, which would be transmitted periodically to signal the receiver when to start the clock.
The use of frequency-hopping would effectively spread the carrier signal among a wider spectrum of electromagnetic radiation. This a technique known as frequency-hopping spread spectrum (FHSS) transmission. It differs from fixed-frequency transmission in that the transmitter and receiver are not set to a single frequency, as is sometimes the case with wireless microphones. Instead, the carrier signal ranges from 2.4 GHz to 2.4835 GHz, which is known as the ISM band (industrial, scientific, and medical), or from 5.47 GHz to 5.725 GHz, which is known as the U-NII band (unlicensed national information infrastructure).
Refresh Rates
The bandwidth of transmission is less than 1 MHz, so several hops can be made inside each range of frequencies. Depending on the manufacturer, the number of hops can vary from tens to thousands of hops each second. The maximum refresh rate of DMX is 43 Hz, which means that most wireless DMX systems are easily capable of transmitting the maximum number of data packets with plenty of room to spare. If there is any loss of data or data corruption, the data will be refreshed quickly enough that the error will most likely go undetected by the user.
If any of the frequencies in the transmission band have interference, then some FHSS technologies will adapt by hopping over those frequencies. This is a technique known as adaptive frequency-hopping. This helps improve the reliability of transmission. For example, if someone in the production office decides to pop some popcorn in the microwave oven while you're trying to control the lighting system, then the wireless transmitter will detect interference at a certain frequency and hop over that frequency in the course of sending a data transmission.
Microwave ovens, cordless telephones and Bluetooth devices all operate in the ISM band, which can potentially cause interference. For that reason, many of the new wireless DMX devices operate in both the 2.4 GHz and the 5 GHz bands. When you see the term "dual band," that's what it means.
FHSS offers a variety of advantages over fixed-frequency radio transmission. It helps make it more immune to interference by spreading the energy over a range of frequencies so that any narrowband interference has less of an impact on the entire transmission signal. It also allows a transmission to coexist with other devices in the same transmission band and still operate effectively.
Wireless Advances
Wireless DMX transmission has been around for several years, and it keeps getting better by taking advantage of improvements in wireless technology. In the last five years, it has become more reliable, easier to implement, and less nerve-wracking than ever before. In my experience, those who argue that wireless DMX is unreliable are typically reflecting on a bad experience they had with it several years ago. If you give the current technology a try, chances are you will not be disappointed.
These advances owe a debt of gratitude to a number of people who have contributed to the technology dating back to the early 20th century, including Hedy Lamarr and Geore Antheil. Lamarr and Antheil were never successful in selling their idea to the Navy. The mad scientist in the Naval officer who reviewed the patent might have been impressed with the resourcefulness of the duo, but the officer in him likely choked on the idea of using a low-tech solution in their high-tech, high-dollar submarines. FHSS technology was eventually used by the Navy, but not until the Cuban missile crisis in 1962, three years or so after the Lamarr-Antheil patent had expired.
