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Spektrum 2.4ghz vs PCM 900mhz
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Glen
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Spektrum 2.4ghz vs PCM 900mhz

just got onto thinking that after looking around at some of the new age radio systems.

anybody think one is particularly better than the other in terms of range and interference and so forth..

the 900mhz systems like the futaba 9CAP definitely has more features over the spektrum which only has like 4 in its stock dx3 form... but i still cant help but wondering if the 9cap or spektrum would be better for getting rid of the interference..
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Post Sat Oct 29, 2005 10:35 pm 
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Spockie-Tech
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To get rid of the Interference, turn the Robot off - All Interference is now gone and nearly any radio works perfectly.. ! Razz

To effectively *cope* with the interference being unavoidably there is a different matter.

I havent tried a Spectrum System, but as far as Interference resistance goes, what I now believe makes the greatest improvement as far as Robot Use is concerned is a good error-detection and correction transmission system, not just the operating frequency

Remember, unlike other modellers, we are not operating at Extreme range. In fact, we arent even operating at "short range". Most R/C users would consider the <5 meters range we operate at practically Zero.

The challenge we have with robot use is dealing with extremely powerful bursts of radio noise very close to the receiving antenna from the wildly varying huge electrical currents in a robot, and having to somehow pickup a radio signal in amongst all that noise with a little fragment of an antenna gingerly poked out of our RF opaque armour.

We discovered that the American 72/75mhz radio' do improve things a lot over the Australian 36mhz ones.. - Probably because the amount of noise generated by our systems at the higher frequencies is less - It would be interesting for someone with a Spectrum Analyzer to do a noise-emitting-bands test on a robot and see where the noise is mainly generated to confirm this.

Given that the heavy currents are all down at 1-5Khz from the ESC's PWM output, and the arcing from the motors commutators, I would think that the higher frequencies should be less affected, but once you get up into the gigahertz bands, the difference in interference between 1ghz and 2.4ghz from motor noise might be less significant. Any radio gurus out there like to comment ?

As anyone who has suffered "the twitches" on a bot can tell you, its not a lack of range to start the 'bot moving, its *keeping* it moving through the noise once its motors have started.

Because of the way our receiver systems consider a transmission error to be a *bad*-thing and stop the robot immediately on an reception of an error.. what is needed is a system that ignores or corrects for errors in reception without significantly decreasing the safety of the system.

I used to be fairly against PCM radio systems as "decreasing the safety of the failsafe" since the receiver can continue to tell the robots controller that it has a good signal when in actual fact its currently receiving an invalid signal, and is waiting for the valid data stream to be resumed.

While this is strictly speaking true, having had a PCM radio system for a while now it doesnt seem to be the case in practice. I dont know the details of the PCM decoding scheme in the radio, but it has never falsely activated on me yet, or even twitched for that matter, so I suppose it has some system, where like a failsafe, it requires a certain number of valid signal in a row before the output activates.

This *might* just be a case of "I'm using it now, so it must be good" Wink but I'm trying to be objective and not biased.

The main change that I have observed is that if a drive/weapon is running and you turn off the radio, it takes maybe 1/2 to 3/4 of a second longer to stop than a FM system that stops instantly. This isnt a big-deal safety wise - The main thing we want to avoid in a robot is accidental activation, particularly with instant-on weapon channels like pneumatics. PCM systems dont seem to be any more succeptible to this than straight FM or AM despite what I thought would be the case. if anything, they seem to be even better to me..

I've found that some Aero modellers dont like PCM, since a lot of them work on the theory that "half a signal (a noisy one) is better than none" when trying to bring your plane safely to the ground, or operating at long range.. but to our robots, a bad-signal is just as bad as no-signal and they will stop, so the modellers criteria dont really apply to our short range application.

So..

Altering the frequency the message is sent at may help to avoid some of interference bands, but having a system where the message's validity can be positively verified and acted upon or discarded accordingly seems to help more, so I'd be comparing the 9CAP and Specktrums transmission/error control systems more than their frequency.

*what a waffle.. been a while since I did one of those.. Smile *
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Post Sun Oct 30, 2005 4:26 pm 
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Totaly_Recycled
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I have found with tinkering with my own radio systems since we went to electronic speed control with failafes is that your bot needs real good ears .Ie a good tuned aeiral system if your radio can hear the signal a lot louder than the noise around it then it doesnt realy matter what frequancy you are operating on .Heamorge and Sproing both have the best radio reception of all out robots as i had enough room to set up full ground plane type dipole arrials .
Both these robots perfom in the arena with out even haveing to extend the transmiter aerial .and both work equaly well on Arrons 72 _75? Mg radio as they do on our old 27mgs am radio . Most of our other robots have tuneing coils on the aerials to tune them to the signal and tune the noise out .both heamorage and sproing dont have supresion caps on any of their wheapon motors either does vertical limits . And i have even tried my radio reciever's in the metal case along with the ibc with good results but it makes it to hard to change crystals and servo leads ect so they are most times mounted on the out side of the ibc csae .
The science of tuneing an aerial (unles you have proper mesureing equipment ) is more like voddoo and some frequancy calculations along the way sometimes helps .I have spent up to two days sometimes to just get an aerail system to work on a particular robot .as the amount of metal and aerail mounting positon ect all determine how good it will work or not .

Some aerials will work extra well on one robot but when taken off comlete and put on another diferent style robot doesnt seem work at all and a completly diferent aproach is then needed. I have never tried any of the extra high frequancy radios or the coded signal types so i cant realy give any advise other than to say like Bret that the higher frequancie recievers probaly dont pick up as much of the low frequancy noise generated by out motors ect .

Post Sun Oct 30, 2005 6:22 pm 
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original_carnage
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....I'll be able to give a report on a robot with 2.4ghz specktrum radio gear v.soon. As far as general rc use, I can vouch that it is by far the best system I've ever used (and it has been a few years in the hobby). To date, I've had no interference AT ALL. The response of the vehicle is very accurate BUT I have noticed a long delay before the system will failsafe (maybe a little longer than Brett mentioned for the PCM gear).

As far as features of the Dx3 go, you must remember that spectrum was originally created as a upgrade for high-end frequency module equipped radios. The dx3 was brought out to grab the market that didn't want to spend $800 to $1000 (cost to buy module Tx, then upgrade to specktrum), but there is a digital-style spectrum set out now...
edit: the specktrumgear is still only 3 channels though...
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Post Mon Oct 31, 2005 10:28 am 
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Knightrous
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6-Channel Now Available

Well according the the Spektrum website, they have expanded there operations to 6 channels.

The DX-6
http://www.spektrumrc.com/DSM/Better/DX6-glance.html
http://www.spektrumrc.com/DSM/Better/DX6.html
http://www.spektrumrc.com/DSM/Better/DX6-2.html
http://www.spektrumrc.com/DSM/Better/DX6-3.html
http://www.spektrumrc.com/DSM/Better/DX6-4.html

It seems they have added some cool features, like a dual reciever module that has two aerials, plus a nifty feature of being able to dim down the transmitters output power, so you can range check at 30ft instead of walking 300m, might be useful for our bots since we are rarely more then 20ft away from them.

Unit price is USD$200 plus USD$60 for the reciever. I'm starting to consider the idea of selling my 72mhz 4ch to buy a Spektrum DSM.
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Post Mon Oct 31, 2005 11:27 am 
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Nick
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Hang on, Spektrum has a direct competitor that we have not heard of before, called Normadio . Their product is amazingly similar to Spektrum's and it comes standard with telemetry. I havn't found a comparison review yet, but the Normadio gear looks at least as interesting as Spektrum. The only negative comments have been about the plastic casing quality - not much or an issue to me.

Worth checking out: http://www.nomadio.net/default.asp
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Post Sun Nov 13, 2005 12:46 am 
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Nick
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Just emailed their local distributor: http://www.jpmracing.com.au/ , It would be great to get my handson one for a test...
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Post Sun Nov 13, 2005 12:56 am 
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Spockie-Tech
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Mmmm, Transceivers, Telemetry.. Lots of fun stuff possible with that Very Happy

Although, I question whether one would have enough time to look at the screen when racing or competing with a 'bot. The vibration feedback is a good idea, I wonder how long it will be before someone builds *servo's* with force feedback ?

Can you imagine how the model plane guys would love to feel the resistance of the wind over their planes ailerons/rudders ? It would give a "real airfraft" feel to the controls, variable force as the speed/G's changed.

Force feedback might be useful in some bot situations as well, edging closer to the "lost in space" style holographic robot interface there. Smile VR anyone ?

I dont really like wheel radio's, but I'll be interested to hear what price is.
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Post Sun Nov 13, 2005 8:20 am 
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Glen
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quote:
Force feedback might be useful in some bot situations as well


yeah you take a hit from jolt and it breaks your wrists Very Happy

could be good if you have a jammed motor or gearbox though, have it lock the sticks or whatever on the transmitter so you cant drive it anymore.
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Post Sun Nov 13, 2005 9:28 am 
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Nick
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If the handheld display could be be changed to a heads-up display mounted on a headband, that would be truely awesome! Spektrum are planning even more telemetry than this but it isn't expected until early next year. What I find amazing is that there is no direct comparison on-line!
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Post Sun Nov 13, 2005 10:00 am 
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Glen
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quote:
If the handheld display could be be changed to a heads-up display mounted on a headband


lol you would actually wear that at an event Razz

how does the normadio system compare to the spektrum in terms of cost?
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Post Sun Nov 13, 2005 4:57 pm 
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Nick
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I have a sneaky suspision they are built around some common components - its too similar in function and release timing to be a coincidence. The 3 chanel Spektrum is much cheaper than the Nomadio 4 ch (@ $519 US Shocked ) but Spektrum charges extra for the telemetry and they havn't released a price for that yet - I'm guessing it will still be a bit cheaper than Nomadio.

The Nomadio has some interesting fuetures over the Spektrum like detachable co-ax RX antenna, better ergonomics, left hand operation, PC interface, programable audio, while the spektrum has the much cheaper retrofit modules.
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Post Sun Nov 13, 2005 5:06 pm 
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original_carnage
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The specktrum telemetry gear is intended as info for pit crew (remaining fuel levels, motor/engine heat, battery levels, etc, etc)- not the driver.

I fully agree with Brett - if you have time to check out the telemetry info during a race (or fight) then you surely can't be going hard enough !

....It's about time someone cracked open a specktrum set and opened up the technology to $$$ competition. It may come down in price sooner than hoped. Smile
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Post Sun Nov 13, 2005 7:56 pm 
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Nick
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On the Nomadio set, it seems you can program audio warnings at preset events like a certain temp or battery voltage. It would be really handy to get an audio signal like "motors over temp" during a pushing match. Its also possible to hold the panel up higher so it can be read like a car dashboard - I don't have a problem reading the speedo while steering. A HUD would still be the ultimate though!

It would still be easy to have pit crew to interpret the figures & call advice - it isn't hard to find yourself some extra crew...

I am mystified as to why the major manufacturers have not capitalised on this technology, it might be due to the fact that 2.4HGz is not legal in many countries. The same thing could also work on 900 MHz and that frequency IS legal in many countries.

You would think that Futaba & Hitek would have radio modules on the market by now and that makes me a little nervous about pruchasing from the small guys - once the market leaders deliver competitive products, it will be hard for these two small companies and support might die off.
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Post Sun Nov 13, 2005 8:13 pm 
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Valen
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actually its the other way around
2.4Ghz is availablle world wide mainly because its considdered too crap for anything usefull.
900Mhz is only (apparently) availablle in US Aus and israiel.

speaking of 900Mhz
we have gotten some 900 mhz radio lurvin
yay me
phase 2
put it into robot
phase 3 underpants
excelent
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Post Sat Nov 19, 2005 2:07 pm 
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