Between the brands of 2.4 transmitters of on the market, which uses the most channels for transmitting? I was just told that spectrum uses only 2, can this be correct?

Also, how does the receiver know which channel the transmitter will use next? I assume that the channel sequence is totally random is this correct?

Gary

There are 4 major brands of radios - Futaba, Airtronics, JR/Spektrum, and Hitec. Of those the JR/Spectrum use DSM2 (Digital Spectrum Modulation) and the rest use FHSS (Frequency Hopping Spread Spektrum).

The DSM technology examines the spectrum of available frequencies when it is turned on and then chooses two of those frequencies for transmission on, the receiver gets these from the transmitter and those are what are used while in flight. If one frequency gets blocked then it can still transmit on the other frequency

The FHSS technology starts on a given frequency and then both the transmitter and receiver start "hopping" to another frequency every 40 milliseconds. If that frequency that is hopped to is being used it won't hurt the radio as it will move to the next frequency in 40 milliseconds. The time it is on the interfered frequency is so short you will never notice it.

Ken

Ken is correct...and you understand correct Specktrum/JR uses only 2 of the 80 channels available

Waht system does Hitec use? Is it compatible with Futaba receivers?

Gary

Hitec is also a FHSS system. While Futaba, Airtronics, and Hitec all use FHSS their receivers are not compatible with each other. As of right now all of the 2.4 systems are proprietary and you can only use the receiver with the same brand transmitter.

Ken</p>

You really need to put the ideas of freqencies and channels behind you because 2.4 spreads the signal out over its available bandwidth.

Unless it's JR/Spektrum - Then it only uses two


And what happens if the other frequency gets blocked?

Baracuda,
You are right that it is spread across a wide spectrum, but in fact some systems only use a few of the channels when in use, and this can be a problem. If you power up your Tx and it chooses two channels in the 2.4 spectrum that are near to each other, and then during flight someone powers up a strong 2.4 system (like for Video broadcast, FPV or something like that), and is sitting over your channels, it may overpower your signal and cause a glitch or crash. If the system frequency hops, then the liklihood of this is decreased. Also, some hopping systems choose one of 11 frequencies in the band, while others may grab more (maybe 80 like FASST). There are differences. I would recommend reading the following post by Bruce on RCModelReviews.com

http://www.rcmodelreviews.com/spreadspectrum01.shtml There are a lot more reviews and articles on his home page that I would recommend reading. It's very informative.
Curtis

Simple math kicks in here. 2 minus 2 =???

Ken

According to the Spektrum die-hards, 2 minus 2 = "The user did something wrong"

That is not quite an accurate view.... and it reflects the old 72mhz "channel" thinking.

In DSM/DSS the "channels" represent a range of frequencies, unlike 72mhz which is centered around a specific frequency.

Even if one frequency is blocked, adjacent bandwidth is available for the DIGITAL transmission and reception.

Only a very wide broadband signal has any hope of blanketing this 2.4gHz "channel", and that is relatively difficult to do.
As a safeguard there is the secondary frequency range also available.

If you start throwing other signals on one range, the receiver simply does not pay attention to them because of the encyption.

The link you posted also goes into this...



Frequency hopping works well if both the TX and RX are in sync with each other. I'm not familiar how Futaba's tech deal with the RX determining the next frequency though. I don't know if the TX tells the RX before the "hop" where to go to next.

I'd imagine there would be a delay in reaquisition if the target frequency is in use, but since our equipement functions so quickly now, it would be impossible to notice.

Spektrum, to my knowlege, doesn't use a fail over channel, it sequences packets between both "channels" in use.

OPJose made the point, you're still thinking channels, and while that terminology is thrown around, it's just not used the same way with DSS

beau0090_99

very informative web page, it cleared up alot of questions.

Thanks,

Gary

Channels are still the terminology used on 2.4ghz. Look at your Wi-Fi network. It still uses channels. I am using channel 11, which happens to be 2.462 ghz, or 2462 Mhz. How would the system know which frequency to look for? Because there are allocated channels within each system to assign. This is not much different from 72 mhz, and if you look at the spec for 2.4, you'll see that they are centered on specific frequencies. I don't have it at hand, but I believe the width of each channel is 1 mhz wide and spaced 5 mhz apart.

I agree that you don't choose a channel like you did with 72, but you leave it up to your system to do that. Also, if you are using DSS, vs. FHSS, you are going to use those two channels for the duration of when your Tx is powered up. Take a look at this article too. http://www.rcmodelreviews.com/dsm2flaw.shtml

In the case of Assan V2, it picks 2 channels, one at the bottom of the spectrum and one in the range. Then if the one in the range loses packets, it switches reception. Problem is that no one knows if the bottom channel is the same for all receivers.

One good thing I like is the fact that it is digital, so that even if you are on the same frequency, if the packet is not assigned to your receiver, the receiver will not do anything with it. Problem becomes when you get blanketed and you can't get your receiver to listen to your Tx.

I know this is getting way offbase, but there are some things to think about when choosing a system.
Curtis

No that is not accurate. DSS/DSM 2.4gHz is not directly analagous to WiFi 2.4ghz even though the same frequency range is used.

Channels in 2.4gHz DSS/DSM refers to the center range of a set of broader frequencies that will be scanned.

If you are using 2.462gHz, your receiver and TX can jump ( or rather focus on, to be more correct ) to adjacent frequencies under DSS/DSM.

Even if the center frequency is blocked the broad band nature of the TX/RX lets the signal get through on the adjacent frequencies.


As it says in the link you posted:


"Distributed Spread Spectrum radio can be likened to a multi-lane freeway where your car seems to appear at random in different lanes. In fact, it appears and disappears so quickly that it almost appears to exist in all lanes at the same time.

In radio terms, the transmitter uses a wide spread of frequencies to send data to the receiver, rather than the very narrow band of frequencies used by the older narrowband RC sets we've seen up until now. "

So by analogy, if your LANE is blocked, your TX/RX "see" traffic coming down another lane.

The problem with DSS, as is the case with the Assan and Spektrum/JR implementation is that it is simply using 2 lanes, rather than 3 or more. In this case it is scanning two frequencies, not more than that. If those two lanes are blocked, then you crash.
C

No again that analogy reflects the "old school" thinking.

It's NOT "two lanes", it's TWO MULTILANE HIGHWAYS instead.

The lane can be blocked with no ill effect what-so-ever.

You just cannot block the entire highway with a broadband emmision... something that is quite hard to do...

e.g. "Distributed Spread Spectrum radio can be likened to a multi-lane freeway where your car seems to appear at random in different lanes"

No,
It is one highway (the 2.4 spectrum), with multiple lanes. Where is this information coming from? This is completely backward from what I have read up to this point. If you look at a spectrum analysis of the 2.4 ghz band with a DSS system, you will see that it picks two channels or lanes at power up of the Tx, and will not move from those two lanes as long as you leave the Tx up. The last link I posted showed that at some times the Spektrum/JR Tx will pick two channels that are directly adjacent to each other in the spectrum. Thus, if the high powered 2.4 Tx that is dumb (doesn't look out for other things on the band) it can blanket these two adjacent bands and cause a lock out. Bruce showed this on his site. It is not like it will now move over to another lane or channel.

It's not like we have 2 dimensions to play in here, we don't even have two spectrums. Once a pair of channels or frequencies is chosen, they are that way until you power cycle the Tx.
C

No... I think you misunderstood what was said.

It is TWO highways with multiple lanes in each.... for DSM2, etc.

Web site you cited talks about the multiple lanes ( frequencies ) with respect to one center frequency that is designated the "channel".

Later DSS implementations also incorporate a second backup channel ( like another different highway ) that also works to back up the first.

That is why the "channel" analogy is poor. DSS is a BROADBAND transmission accross multiple effective frequencies as displayed in the graphs.


Tt will not move from those "highways"... the lanes are still available centered around the channel frequency.

"Bands" is correct, however "BAND" refers to a RANGE of frequencies.

It is a broadband tranmission not a narrow band one...

BTW: I discussed all of this with the JR people while in Toledo.

My understanding of it was that you had these lanes (I thought 80) within the spectrum. DSS chooses 2 and they were about a Mhz wide. What keeps you from interfering with others in that same channel was the luck that you didn't send your packet at the same time as the next guy. Being that the information that we send to our planes or helis is rather small, it doesn't take long to get it out. Like packets in an ethernet network. The problem begins when you have a system that is powerful (which generally means it oversteps lane boundaries), and it is Packet intesive(meaning that the time required to send the information is long, and the time between transmissions is very short). This would keep you from getting a word in edgewise.

I could be completely wrong about this. The reason for the above thinking is that the Receiver would not have any way of knowing which mini-lane to move (left or right) to in order to receive it's next packet. In FHSS the Rx is told where to go next, but with DSS, this is not the case. Therefor, it doesn't have the ability to veer around upcoming traffic in it's lane.

I liken the thought of spreading out the information as having a light freeway loading, where there is a lot of space between cars. The problem is that your Tx can't see the next car coming, but has to just jump in there and hope that he doesn't get hit.

My understanding is founded on the belief that there are assigned channels in 2.4. I think the way Spektrum/JR uses it. It takes the whole range and divides it into these channels, then picks them at random on powerup.

Also, I don't think we can really consider this transmission "broadband" as it may be wider than what we are used to, but because we can't limit the transmission to only one receiver (like that of a cable broadband), we can't use the entire radio wave spectrum, like cable broadband could use the entire useful electrical spectrum.
C

BTW, Opjose, I mean no disrespect in arguing with you, I want to know about this too and I want to make sure I understand it better.

No insult taken, you discuss and argue very well!

Part of the problem with all of this IMHO is that we are applying 72mhz "thinking" into our analogies, and the analogies do not exactly reflect what is actually going on....

-

In your example you mention "mini-lanes" and the need to move left or right.

This is effectively not necessary... the data is streaming to the receiver across multiple lanes at the same time. The transmitter does not need to move it's center frequency, rather the receiver listens to a band of frequencies only paying attention to valid data coming to it in a somewhat ( and I hope I don't get into hot water with this... ) parallelized manner... maybe redundant would be a better term.

As per the web site you cited said...

"In radio terms, a single (or even quite a few) other transmissions won't have much effect on your RC system because they'll only block a tiny amount of the signal being sent. In fact, unless the freeway is almost completely blocked, at least some of the signal from your transmitter will get through to deliver your control inputs to the receiver. "

So the TX/RX go looking for available "bands" around a free center channel at power up.

Due to the digital nature of the transmission it is indeed possible to have two adjacent "channels" in use, but under normal circumstances the receivers will not care as they will only listen to their assigned transmitters....

"Well because DSSS spreads your troops so thinly across the battlefield, there's plenty of room for another platoon from a totally different army to run between the ranks without the two colliding. This is why multiple DSSS systems can co-exist on the same channel without interfering. "

In the case of DSS/DSM it is a broad "band" of frequencies in use.


FHSS uses a BAND that the carrier signal frequency hops around in.


"The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same.

Whilst providing NO extra protection against wideband thermal noise, the frequency-hopping approach does reduce the degradation caused by narrowband interferers."


In other words, in FHSS technology you have a narrowband signal that is constantly changing carrier frequencies within a wide band.

Whereas in DSS you have a wideband signal that is transmitting in multiple frequencies within a wide band.

POTATO/PATATA

I guess I understand this, thanks, I guess. It gets kind of hard with two different ways of saying it.

I have the JR x9303 and as I understand it , it picks two highways ( many lanes ) and it can use either of them and any lane? It takes only one lane , to keep the TX talking to the RX?

Is that right?
sticks

Maybe this is a better analogy:

First, this highway has many lanes, not just a few.

Second, there are red, blue, green, yellow and many other colored cars driving bumper-to-bumper in every lane at the speed of light, so they would appear to just be a blur.

But your receiver only "sees" the blue cars.

[sm=spinnyeyes.gif]

Unless your name is Neo, then you can jump off the blue car and fly around the world before you save it!!!!

Ken