Spread Spectrum Radios, PCM Radios, FM Radios, AM Radios – what are the differences and which is better?
2.4 GHz spread spectrum radios have taken the RC world by storm. In a few years time, I predict this will be the only technology that will be offered on everything but the cheapest toy grade RC radios. So what is spread spectrum radio modulation and why is it so good? To answer this question, let’s first look at the other ”narrow band” modulation methods – AM, FM, PCM. AM, FM, and PCM all use narrow band radio transmission. This simply means they transmit a signal on a specific frequency within the radio spectrum. This is exactly the same way AM and FM radio stations broadcast and is why you have to tune into a specific frequency (radio station) to get reception. The RC world uses radio frequencies in the 27 mega hertz (MHz) to 75 MHz range. Most hobby grade RC aircraft used the 72 & 75 MHz band range of the radio spectrum. For your RC radio system to work, the receiver must be tuned into the exact same frequency as the RC radio, this allows several models to be flying at once provided they are all on different frequencies within the allowed band range. Over the years, technology has made it possible to “cram” more frequencies into this specific 72 & 75MHz range, but space is limited, and there lies the problem. With only a few dozen channels or so available, it is very important that only one person be transmitting on a specific channel at a time. This is what frequency boards and pins at RC flying clubs are used for. When it is your turn to fly, you take the frequency pin off the board and this lets others know that frequency is in use. Simple in principle, but with more and more people flying RC and computerized radios that need to be programmed, there is always the possibility that another RC radio on the same frequency will accidentally be turn on while you are flying – interference and crashing is the result. Spread spectrum radios as the name suggest are not bound by narrow band frequencies; they spread their radio signals out over a large range of the radio spectrum. They also use a much higher frequency range 2.4 GHz. That is a frequency of 2.4 billion cycles per second. This is well beyond the range of most RF (radio frequency) generated noise that occurs below 300 MHz making 2.4 GHz much more immune to interference issues. Before I get going on exactly how a spread spectrum radio works, let’s first look at how narrow band RC radios transmit their signals. AM RC Radios AM stands for amplitude modulation. This is the most basic and very first method used for controlling RC models. AM RC radios send information to the model by changing the amplitudes of a base carrier wave at a specific frequency. It is then a simple mater for the receiver to filter the highs and lows of the changing amplitudes of the carrier wave into usable information. The problem is it is really easy for those amplitude highs and lows to be affected by almost any electrical noise generating device. Any type of electrical or metal on metal noise from lighting to car ignition systems will result in interference (just listen to an AM radio station while an electric motor in your house is running or an electrical storm is approaching – big time noise). It is all these sources of interference that will cause loss of control issues on your RC model. This holds especially true for all types of RC helicopters with the many metal on metal contact points, electronic speed controllers and motors in electric RC helis , and high voltage ignition systems on gas powered RC helis . Bottom line – stay away from AM RC radios for RC helicopters at all costs. I guarantee you will have interference issues. Yes I know this from experience. FM RC Radios FM stands for frequency modulation. It is also referred to as PPM – Pulse Position Modulation and was introduced commercially to the RC aircraft world in the early 80’s. FM sends information by changing the frequency of the radio wave instead of the amplitude. Because the receiver is now looking for changing frequencies instead of changes in amplitude, any electrical noise source that changes the amplitude of the carrier wave simply won’t be seen by the receiver. I started out using a FM RC radio on my first nitro helicopter and it worked OK most of the time. However, there are many rotating parts than can sometimes send out electrical noise that can be interpreted by the receiver as a legitimate signal and cause a “glitch”. So every now and then the heli would twitch. If the glitch was bad enough or lasted more than a few seconds... well you get the "ugly" picture. Yup - I moved up to PCM 15 years ago and have never looked back. PCM RC Radios PCM stands for Pulse Code Modulation and works by embedding a digital signal within the basic FM radio wave. A digital processor chip inside the RC radio will encode a digital transmission and send it out on a standard narrow band FM carrier wave. The receiver also has a processor chip that decodes this digital data back into a usable analog signal for the servos. Think of this like hearing a bunch of people talking to you in different languages but you only understand one of those languages. Our brains have the ability to filter out all the other noise and only respond to the information from the language we know and understand. This method all but eliminates any glitching caused by electrical noise because unless the receiver “hears” a digital command that it understands, it won’t respond. It is this ability to ignore outside interference that makes PCM so perfect for all kinds of RC control – especially for helicopters. This brings us to the topic of “Fail Safe”. Like I just mentioned, a PCM RC radio receiver can ride out interference because it doesn’t understand it and simply ignores it. This makes it possible to add a secondary feature to that ability. Fail Safe is a safety function that allows you to “tell” or “teach” the receiver what to do if it no longer sees or understands the radio signals it receives. No, this doesn’t mean the receiver is capable of flying and then landing your helicopter if there is radio signal corruption, but it will move the servos to a predetermined value. For safety reasons that usually means throttle off and all other control functions at neutral. These fail safe servo settings are programmable, but the idea is to make sure if you do loose radio communication with your RC helicopter, to have it come down in a way that is least likely to cause excessive damage to your heli or hurt people. This doesn’t mean that the helicopter will absolutely crash if the radio signal is lost – the receiver will continue listening for the digital data and if reacquired, control will be regained. So does all that mean that PCM RC radio systems are immune to interference? I am afraid not. If another PCM radio is transmitting on the same frequency, you will certainly get interference – your receiver will see conflicting signals. Even a standard FM (PPM) RC radio will cause loss of control issues if it’s on the same frequency. Your PCM receiver won’t understand this FM noise of course, but it also won’t be able to “hear” your PCM radio’s digital voice over all the noise – it goes into fail safe mode and if the interference doesn’t subside - once again, by-by-birdie. Let’s go back to the illustration of all those people talking to us in different languages. If there are too many and it is too loud, you can’t hear the person talking to you in your own language right? Communication is lost. Spread Spectrum RC Radios. Welcome to the 2.4GHz spread spectrum radio REVOLUTION! No other advancement in RC radio technology has changed our hobby in such a profound and positive way. As big as PCM was – it doesn’t come close to the freedoms that all spread spectrum radios have. Interference issues are all gone! No more frequency conflicts! No more Worries! Actually, the RC radio world has been rather slow to adopt 2.4GHz spread spectrum technology considering it has been commercially available since the 90’s with cordless phones, cell phones, and later wireless computer technologies such as Wi-Fi and now Bluetooth. In fact, spread spectrum was invented during WWII to prevent radio signal jamming of torpedoes. Here is a very informative video of who actually invented it and how it came to be – you may be surprised. The main idea behind spread spectrum is to spread the radio transmission out over a wide range of the radio spectrum” - thus the name SPREAD SPECTRUM. This makes a spread spectrum signal much less likely to run into interference or jamming issues that are common with all narrow band radio transmissions. Even many spread spectrum radios all transmitting at the same time are very unlikely to interfere with each other as the spread of radio signals are random, changing, or coded. In most cases any signal conflict would happen for such a brief moment - you would never notice it. How does it work? RC spread spectrum radios use the same type of digital signal that is used in PCM giving all the same advantages that PCM has. The difference is how that digital signal is transmitted and received. There are essentially two different types of spread spectrum modulation methods that have been developed – FHSS and DSSS. FHSS was invented first and is exactly how the first WWII spread spectrum system worked. Frequency hopping, as the name suggests, transmits on a narrow band frequency, but changes the frequency of the transmission hundreds of times a second. For FHSS to work, the receiver has to know the frequency changing pattern so it can hop to the different frequencies in the same order and time frame as the transmitter does. DSSS was invented later and is harder to achieve. Unlike frequency hopping, direct sequence as the name suggests uses a special code sequence and spreads data over a wide band width on a select frequency. DSSS is said to provide somewhat faster data transmission and shorter delays because the transmitters and receivers don't have to spend time switching to different frequencies. With the high speed micro processors of today, this is really not the issue it once was. The speed of this data transfer and processing equates to latency. Latency is the elapsed time between your movement of the transmitter controls and the servo response on the model. Reduced latency means faster response times. These are very basic explanations of both spread spectrum methods and for most of us, it is simply enough knowing that the systems work and work well. If you want a more technical explanation of how both FHSS and DSSS work – here is a link to a very good article I found on the subject. So which is better? It depends who you ask and what spread spectrum radio manufactures web site you visit. You can read arguments for and against each method of spread spectrum radio control. I am not going to start a debate here – I have a few videos below that show both the Futaba system and the Spektrum system work very well at riding out interference. Futaba’s spread spectrum radios use frequency hopping (FHSS) technology. Futaba’s trademark name for their system is F.A.S.S.T - Futaba Advanced Spread Spectrum Technology. The FASST system hops or shifts to a different frequency every 2 milliseconds. The receiver has to be programmed to learn this unique hopping pattern so it can shift to the correct frequencies. This process is called “binding”. Once bound, it won’t accept any other pattern until it is re-programmed to a different radio. Futaba claims the frequency hop method is better at overcoming signal conflicts or interruptions than DSSS (in theory this is true) and further improves reliability by seamlessly selecting the best reception between two separate receiver antennas.
Spektrum/JR’s spread spectrum radios use direct sequence (DSSS) technology. Their system is called DSM and now DSM2 which stands for Digital Spectrum Modulation. It works by dividing the 2.4 GHz band into 80 individual channels (frequencies) and codes the direct sequence modulation with an imbedded GUID (Globally Unique Identifier) code for each radio. The receiver must then be programmed (bound) to this unique code so it only understands data from that specific radio. Spektrum claims direct sequence modulation is more costly and harder to develop than the frequency hopping method (this is certainly true) and because the gain rate is higher, the range is improved. Spektrum/JR unlike other DS systems on the market selects and transmits on two different frequencies to avoid the possibility of blocked or corrupted signals. Spektrum/JR also has a range of dual receivers to provide better path diversity – our next topic.
2.4GHz Limitations You should know by now almost nothing in this world is perfect and spread spectrum radios are no exception. As I mentioned earlier, transmitting and receiving in the 2.4GHz high frequency range certainly gets you out of most human generated and natural occurring radio frequency (RF) noise that occurs below 300 MHz. This is the reason modern spread spectrum radio equipment transmits in this range. Spread spectrum radio technology can be used with any radio wave length, but the GHz range is much less prone to noise. You have probably also noticed that all 2.4GHz devices from cordless phones to spread spectrum radios have very short antennas. This is because the frequency or wave length is so short, a short antenna is all that is required to transmit and receive it. This has made the long unsightly and usually difficult to route RC receiver antenna wire a thing of the past. It has also eliminated the long telescoping antenna that would always get bent on the RC radio. So what’s the issue? Blocked and reflected signals. Unlike the longer wave lengths used in 27-75 MHz RC radios that pass through almost anything, 2.4GHz short wave lengths are easy absorbed or reflected by many objects just like a light wave. Absorption and reflection of the 2.4GHz signal by parts of the model aircraft could lead to fail safe lock out of control if the signal is not strong enough for the receiver to hear or identify from shielding or reflecting. This is not so much of an issue with fiberglass or balsa fuselages, but it can be a problem for RC helicopters that use carbon fiber or aluminum side frames. The Solution… Diversity As was previously mentioned, Spektrum/JR solves this problem by utilizing two or more receivers giving improved path diversity. By mounting several antennas in different locations and orientations in the model, even a couple inches apart, pretty much ensures at least one of these antennas will always “see” a clear signal. Futaba deals with diversity by using 2 slightly longer antennas exiting one receiver and uses both antennas to scan for the strongest radio signal. In theory Spektrum/JR’s method of multiple receivers/antennas is better, but in practicality Futaba’s system seems just as capable. Either way, the result is the same – redundancy. In the real aviation world redundancy is the goal of all aeronautical engineers... if one system fails; there is always a back up. Obviously this is why both Futaba and Spektrum/JR are the two clear “head and shoulders above the rest” leaders in the RC spread spectrum radio world. As mentioned on the RC radios brand page you won’t go wrong with either Futaba or Spektrum/JR spectrum radios if you are looking for the best in RC spread spectrum radio technology. I think after reading this page, you now realize just how much better spread spectrum radios are over conventional narrow band RC radios and why these are really the only radio systems you should consider - especially if you are just starting out in this hobby.
|
