All about radio control transmitters, FM, AM, PCM, and Spectrum.

July 26 2009 No Commented

2.4 GHz spread spectrum radios have taken the RC world by storm. In a few years time this could be the only technology that will be offered on everything but the cheapest toy grade RC radios. So why is spread spectrum modulation so good? To answer this question, let’s first look at the other “narrow band” modulation methods: AM, FM, PCM.


The difference between FM, AM, and PCMAM, FM, and PCM use what is called narrow band radio transmission. Simply put this means they transmit a signal on a specific frequency within the radio spectrum. The same way AM and FM radio stations broadcast their signal. The RC world uses radio frequencies in the 27 MHz to 75 MHz range. Most RC models used the 72 and 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. However, space is limited on this band, and there lies the problem.

With only a limited amount of channels available, it’s important that only one person be transmitting on a specific channel at a time. This is why you see frequency boards and pins at RC clubs. When it is your turn to drive or fly, you take the frequency pin off the board and this lets others know that frequency is in use.

Simple enough, but with more people getting in to the RC hobby, there is always the possibility that another RC radio on the same frequency will accidentally be turn on creating interference.

Spread spectrum radios are not bound by narrow band frequencies they spread their signals out over a large radio spectrum. They also use the 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 making 2.4 GHz much more immune to interference. Let’s now look at how narrow band RC radios transmit their signals.

Understanding AM signal vs FM signalAM RC Radios
AM stands for amplitude modulation. This is the first and most basic method used for controlling RC models. The problem is it is really easy for the AM signal 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 (if you ever listen to an AM station when it’s raining out you get the idea). It is all these sources of interference that will cause loss of control on your RC model.

FM RC Radios
FM stands for frequency modulation. It is also referred to as PPM – Pulse Position Modulation and was introduced to the RC world in the early 80’s.

FM receivers are less prone to electrical or metal noise from outside, but there are times when moving parts can send out electrical noise that can be interpreted by the receiver as a legitimate signal and cause a “glitch”. So every now and then your model might twitch. If the glitch was bad enough or lasted more than a few seconds your model could end up in a heap of broken parts.

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 encodes a digital transmission and sends 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.

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.

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 called “Fail Safe”. Fail Safe is a safety function that allows you to tell 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. This is a good feature to have to insure if you do loose radio communication with your RC helicopter or plane, as to cause excessive damage to your model or hurt people. This is not to say it will absolutely crash if the radio signal is lost, the receiver will continue listening for the signal and if reacquired, control will be regained. But again much like FM and AM PCM is still not immune to interference. If another PCM receiver transmitting on the same frequency you will still have problems.

In-depth view of spectrum signalSpread Spectrum RC Radios.
This brings us to the 2.4GHz spread spectrum radio. No other advancement in RC radio technology has changed the hobby in such a profound and positive way. Interference issues are all gone! No more frequency conflicts! No more Worries! 2.4GHz spread spectrum technology has been widely available since the 90’s with cordless phones, cell phones, and later wireless computer technologies such as Wi-Fi and now Blue-tooth.

The main idea behind spread spectrum is to spread the radio transmission out over a wide range of the radio spectrum. This makes a spread spectrum signal much less likely to run into interference or glitching issues that are common with all narrow band radio transmissions.Even with many spread spectrum radios all transmitting at the same time they are very unlikely to interfere with each. In most cases any signal conflict would happen for such a brief moment – you would never notice it.

So 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.

Frequency Hopping Spread Spectrum (FHSS)
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.

Direct Sequence Spread Spectrum (DSSS)
Invented later and is harder to achieve. Unlike frequency hopping, direct sequence 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. However, with the high speed micro processors of today, this is really not the issue it once was.

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. 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 to a different frequency every 2 milliseconds. Futaba claims the frequency hop method is better at overcoming signal conflicts or interruptions than DSSS .

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 embedded GUID (Globally Unique Identifier) code for each radio.

Spektrum claims direct sequence modulation is more costly and harder to develop than the frequency hopping method 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
2.4GHz limitations

What about the cons
You should know by now almost nothing in this world is perfect and spread spectrum radios are no exception. Unlike the longer wave lengths used in 27-75 MHz RC radios that pass through almost anything, 2.4GHz has short wave lengths are easy absorbed or reflected by many objects. 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. Different manufactures solve this issue in different ways.

So my advice would be to go visit your local hobby store, find out what others in your area are using and determine if you spread spectrum is right for you.

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