|Feb 24, 2009, 03:39 PM|
Panorama City, CA
Joined Sep 2004
Difference between FHSS and DSSS
The following explanation was picked up from internet. Posted here because it is easy to understand.
"In theory, FHSS is a bit more reliable (less prone to "jamming"), but slower (and I mean in theory). FHSS was a military development, as it was more difficult to eavesdrop a transmission changing frequencies all the time, but it was not designed with very high bandwith, transmitting speed or model aiplanes in mind. DSSS is faster as there is no timeloss for frequency changing, but easier to "jam" for the same reason (in theory) "
|Feb 24, 2009, 04:08 PM|
There's only one problem with that simplistic explanation. It's completely
irrelevant to R/C SS systems. FHSS systems are no slower than DSSS
at getting relevant control inputs from the Tx to the Rx, and in fact
are faster than the average in actual measured latency.
Notice that all Futaba FASST and Airtronics FHSS gear are faster than the median.
|Feb 24, 2009, 04:18 PM|
That explanation sounds a bit over-simplified. The bandwidth needed for model control is well within the capabilities of FHSS after all Futaba use it in their system successfully enough.
The explanation I prefer for the difference between FHSS and DSSS is...
FH means Frequency Hopping which is a fairly easy concept but DS, Direct Sequence is actually means switching the phase of the carrier RF by 180° in time with the code.
These may seem like completely different techniques but I'm informed by the theoreticians that mathematically the results aren't that different. In both systems the RF is spread over a large bandwidth in a defined pattern that to the casual observer will look like slightly more noise in that part of the spectrum.
Applied to model control there is plenty capability in either technique to make a perfectly usable system. What really matters is the resources put into the design and the quality of the components as this has a far greater effect on the reliability of the system.
|Feb 24, 2009, 05:29 PM|
WirelessUSB™ LS Theory of Operation
read this PDF file...........
best regard tito
|Feb 24, 2009, 05:37 PM|
The theory stuff is really much ado about nothing -a far as the current 2.4 systems are concerned
the fastest SYSTEM is the setup such as the new DX7SE uses
can YOU tell the difference in this and the fastest system by anyone else?
then show me what systems are used by the heli flyers who really do the wild on the deck constant flight changing sequences
The DSM2 systems I have seen are the most commonly chosen for this stuff.
take a guess.
|Feb 24, 2009, 07:45 PM|
Joined Apr 2005
in fact, standard SERVO understand the signal consisting of one pulse, 1 to 2 miliseconds wide (1.5ms means center position), which repeats every 20 miliseconds.
Some systems are repeating this pulse a bit faster, for example FASST uses 16ms..
Some systems are configurable, but the best is if they are direct copying the TX.
Some servos fail if pulse is repeated too fast, for example HS-81 stop working at 14ms.
Ordinary receivers in MHz band, those without any internal processing of signal, were passing the incoming signal out to servos directly with zero delay.
My 2.4 GHz system has 2.5ms delay between input to system in tx, and output to servo in rx, specified in parameters.
Some transmitters are spending quite a lot of time by oversampling an filtering measured values of stick positions, and perhaps also by computing various mixes. There is the most of delay.. Another tx is wasting almost no time, and sends information out with extremely low delay. It is funny that both slowest and fastest TX measured in test mentioned above, are the same brand..
Of course, the pure analog ancient tx was not tested at all, as there is almost zero delay. Same as it was in pure analog MHz rx.
Funny point for today: DX7SE seem like it was tested without receiver...?
Well.. except a case of extremely slow tx, there is no system that could improve latency in noticeable amount, using ordinary (as for signal rate) servos. You need special digital servos such as those heli tail servos directly driven by gyro, using much faster frame rate, to start even talking about TX latency.
All the hype concerning speed of 2.4 systems, come from the fact that servos are not driven at the same time, but they are driven one by one, which casuse problems in CCPM of extreme 3d heli.. most of 2.4 systems are faster than PCM was..
There are receivers which are designed specificaly for CCPM, they sends out commands for 3 servos at the almost same moment (but still not exactly at the same moment, as it causes troubles with way too high momentary load current in helis, and consequent voltage drop at receiver)..
|Feb 25, 2009, 04:45 AM|
Futaba isn't FHSS. Its a multi channel frequency agile DHSS schema..
FHSS was the early way to do spread spectrum..it was easy to shift (narrow) channels and use a lot of narrow band channels. In a wide band of narrow channels..so to speak.
DHSS spreads the spectrum by modulating with a very high subcarrier frequency driven by a particular spreading code. That smears the data across about a 5Mhz band (generally referred to as a channel!!), and AFAIK all 2.4Ghz sets use this technology. The differences are that Spectrum uses twin transmitters on two 'channels', and Futaba uses all 'channels' sequentially. Most of the rest simply use a single channel, and rely on the different spreading codes, and digital codes in the data frames, to sort the tangle out ..
There are only half a dozen or so channels on 2.4Ghz. Not sure, so dont take my word for it.
The 'specifications' say nothing beyond the fact that any given transmitter shouldn't transmit more than a certain amount of average power per channel.
|Feb 26, 2009, 04:09 PM|
It is of no concern for our RC systems whether they are FHSS or DSSS latency-wise.
What counts, is the internal processing speed of the TX and RX electronics and the frametime.
The TX samples the stick positions, encodes them, sends a packet, the RX decodes it, and gives the pulses to the servos.
Whether the next packet is on the same frequency (DSSS) or on a different frequency (FHSS) does not really matter.
The electronics will work just at the same speed, regardless of the frequency in use.
The time required to switch frequencies is absolutely negligible, and as it can happen in the pauses between the bursts, it does not count one bit.
Besides, I don't know of any RC system being a pure FHSS system. They are all DSSS systems, which hop on top of that, and thus combine the advantages of both methods.
|Feb 27, 2009, 02:02 AM|
Airtronics claims they're pure FHSS. Anyone have any solid info
that would prove otherwise?
I can say this. The Airtronics FHSS signal *looks* very different when
stepping on top of an analog 2.4Ghz video channel. Attached are a couple
images showing the difference. This represents interference to the video
transmission in the time domain (each frame takes 33ms to transmit
including the blanking interval), so what you see with FASST is two
short DSSS packets transmitted inside of 20ms or so. If it's hopping
channels between the packets, it's doing so on two channels close
enough that they're both overlapped by the fatter video channel
(which spans about 1/8th of the whole 2.4Ghz band)
With Airtronics FHSS, you can see one much longer diffuse packet.
The odd thing I noticed is that FHSS steps on the video channel
almost constantly. I see its two packets in almost every frame
of the video, while Airtronics FHSS seems to spend more time
hopping on non-overlapping channels and only comes back to
step on the video's channel every couple seconds.
Videos and images of all the 2.4Ghz systems I've tested this way here:
Stepping through the vids a frame at a time with something like
VirtualDub is intesting (at least to me).
|Feb 27, 2009, 01:02 PM|
Think otherwise? Then show me where it is defined.
Meanwhile, look at 802.11 and their "channel" layout (not pre-defined by the band, THEIRS). It is very different from Spektrum's definition of a "channel", which is different from Futaba's, etc.
Each is free to define their own... not pre-defined... which makes the use of the word "channel" in 99.99 percent of these forum posts very misleading.
You are right about the smearing, it called "spreading" and is REQUIRED to be on 2.4 band. However, when coupled with each tech defining their own "channels" it means the smear is across a different width...
|Feb 27, 2009, 02:20 PM|
Actually the smearing is not required to operate on the 2.4Ghz band.
There's a lot of narrow band analog 2.4Ghz equipment, but it must operate
at lower power levels to be Part 15 compliant. And true FHSS doesn't
have to transmit any single packet over a fat DSSS style channel. It could
transmit a very narrow band signal, as long as it then hops away
somewhere else quickly. The net effect is that the accumulated
transmission over time is spread over the whole band, but at
any given moment it might be both narrow band and high power
As for channels.. yes, each application and each chipset defines
the size, shape and distribution of channels across the 2.4Ghz band.
XPS/Jeti has 16 channels (The fewer the channels, the fatter they are)
Spektrum has 80 channels
FASST has 31-32 channels (dunno about Airtronics FHSS)
analog 2.4Ghz video uses about 8, but each manufacturer often uses
different non-standard (as in there is no standard) center frequencies.
|Feb 27, 2009, 03:23 PM|
Panorama City, CA
Joined Sep 2004
Price will be a determining factor
All technical discussions above sound like mantra to horse's ear.
Pros and cons arguments will continue forever.
I tested both FHSS and DSSS systems. They all worked well.
The final factor in selecting the brand is the price, especially receivers.
If you need several extra receivers, $100 a piece is too expensive.
And here come Chinese-made receivers costing $20 each, either FHSS or DSSS.
There are several brands already on the market; Corona, Assan (both marketed by Hobby City), iMax, etc.
There is now a 9 ch. 2.4G radio priced $100 entering market soon. One brand is already being sold at $180.
That will force the price of competing established brands down.
Last edited by John Kim; Feb 27, 2009 at 03:28 PM.
|Mar 10, 2009, 07:43 AM|
Joined Jan 2009
Very interesting dialogue taking place. I'm interested in reading the contents of my DX7 frames (I'm a network specialist in the day so love this stuff).
Any suggestions on software / hardware that is required? Ideally I'd like to use my 2.4Ghz wifi card on my computer to do the scan if possible. I saw a brilliant post a while back at this below URL and essentially want to capture something similar.
Thanks in advance and keep up the great discussion!
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