garage remote jamming

Except that X-10 also uses 418MHz for its Powermids.

It's his level of satisfaction and a mention that he left some test gear at a client that make me suspicious. ;)

Sounds like a new angle for the aluminum siding pitchmen: Protect your loved ones from the black-helicopter conspiracy with your very own 50,000 cu ft faraday cage.

There is a technique that uses spread-spectrum and correlation between a received signal and a pseudo-random pattern. It is possible to recover a weak signal even in the presence of jamming with this method. Bandwidth is limited, and it takes time to search until a pattern match is found. But it amazing how that signal pops out of the noise when correlation is achieved. GPS receivers use this technique to recover the weak satellite signals.

Jeff

And meanwhile it's nothing but unsubstantiated FUD claiming it's "possible". S*it, ANYTHING is possible. You want proof it can't be interferred with, well, show me one that has suffered from interference. Otherwise you're just spewing "chicken little" style nonsense. Meanwhile PLC continues to be fraught with perils, often not within it's realm of control, that will further hamper it's uptake into the residential market.

In the context of this thread I'm merely less than sympathetic to the "problem" of the garage door openers being interferred with because the vendors have known about this for YEARS.

What makes you anything, Dave, is an unfathomable mystery. I've no connection to any of this gear besides being an informed customer. Granted, quite a bit more informed than the average customer.

We trust that BobbyG understands that the limits of what his understanding happen to be does not constrain or alter the actual physics of the matter.

This summary of spread spectrum may be helpful:

... Marc Marc_F_Hult

I don't need no stinkin' Powermids to run X10 RF! I've had enough problems with them recently (too much RF on the 418MHz band, I wonder?) that I am going to retire them for a $100 system that Smarthome sells. The are at least 10, if not 20 years old, so I got my money's worth. I'm monitoring for some feedback at their site since some IR gear appears to get completely flummoxed by interference from plasma TV's. I'll try to remember to post the URL when I make up my next order.

I've been having to turn then Powermids on and off remotely via X-10 RF so that only the pair I am currently using is powered up, but that's a bit of a bear. IR is certainly something that can be handled via wire pretty easily. I'll bet someone here could even figure out a mod to make the Powermids work via wire only, but it's probably not even worth the effort since I can sell them intact on Ebay.

I just assumed that meant he had test gear (of some sort, not necessarily RA related), a client and an occasionally faulty memory, like a lot of us. I have test gear, and I often leave it at friends and clients alike, but I'm very obviously NOT a Radio RA dealer. I don't think Bill's a dealer, either, just a very satisfied customer who had been traumatized by X-10 and PLC demons and is very relieved to be free of them.

I've got to go search out ways to declack some TM751 or RR501's. For coverage reasons, I need two units in the basement. In monitoring the dogs at night, I can clearly see they awaken and get antsy every time they hear a relay clack. I don't need a single extra reason in the *world* to make one of the Devil's Four awaken in the middle of the night. I haven't had an uninterrupted night of sleep since last year. Maybe a declacked transceiver will help. I recall seeing a mod that used solid state relays . . .

-- Bobby G.

What Smarthome system? It probably uses 418MHz - almost all of the IR extenders do.

To confirm that they use 418MHz, here's a FAQ from a dealer...

Sorry, I was unclear. The Smarthome unit is a 4 or 6 drop hardwired system. I can't recall which at the moment.

-- Bobby G.

But, by definition "spread-spectrum" is not "on the same frequency".

I understand that Dave. I was just pointing out there are techniques to get around being jammed by a stronger signal. The ICs developed for GPS may make it feasible to use that approach for mid-price automation products. While not totally jam proof, such a system could be much more resistant to external interference.

Jeff

Sounds interesting. But as Dave notes, when you are talking about spread-spectrum techniques, you've strayed outside the limits of my original words "same frequency."

I am sure that by using multiple frequencies and high levels of redundancy that a delayed radio message can be sent intact through heavy interference. It will always take longer than a straight-up message sent on a single frequency because of the reconstitution required. It would be nice to know if Lutron Radio RA use such a method . . .

Would it work if more than one of those frequencies were deliberately jammed?

From your description it sounds like it relies more on the weak signal appearing just slightly more coherent in the face of random background noise of the same general signal level. My guess is that if there are spread spectrum transmitters there are spread spectrum jammers. And if the Pentagon is afraid that terrorists will use GPS info against them, you can be pretty sure they are already in the field. (-:

-- Bobby G.

This discussion didn't occur ex nihilo. It was a response to similar RF transmitters, apparently operating on the same frequencies as RA, being shut down by recent military tests. If it did come out of nowhere, you'd be right, it's just FUD. But it didn't come out of nowhere. There have been persistent, credible, nationwide reports of RF jamming of very similar equipment by military signals. Signals that obviously weren't being transmitted as strongly as they are now when RA was designed.

The question of RadioRA's resistance to a jamming signal is perfectly germane in light of the very recent experience of 100's and maybe 1,000's of garage door owners. As I've noted before, there are probably so few RA users *nationwide* that finding any that have had a problem is not going to be easy. But that doesn't mean that they don't exist. Nor does it mean that system is designed to reject such interference. In fact, it's becoming more and more clear that it can't, just on technical, and not anecdotal evidence. Why would they have a replacement frequency in the wings, as Marc suggested, if they were immune to interference, as you suggest, or if they used a spread spectrum design?

If you really wanted to convince me, a testimonial from an RA user in one of the known affected areas would be nice. A thousand units operating happily away in the absence of interference isn't any proof. There are tens of millions of garage door openers operating normally. Despite those millions, the ones near Elgin, AFB and Quantico (so far) don't work.

We haven't had any personal reports from any of the affected 100's or

What's the worst case scenario? Probably that you have to pull all your gear and return it for a re-fit. Not the end of the world, but a pain. I believe people considering LutronRA living next to military bases should be alerted to any potential problems. They also should find out what Lutron will do for them if they need to switch to a new frequency. That doesn't seem like FUD to me as much as getting the facts straight for someone that might be thinking of buying Lutron. Let's see, what's the inverse of FUD? HCS: Happiness, certainty and surety.

Or is there something wrong with potential Lutron RA purchasers knowing whether the system is immune to interference already plaguing some home control RF users? Is there anything wrong with knowing what Lutron will do for their customers should a jamming transmitter fire up in their neighborhood, either?

-- Bobby G.

Spread-spectrum not as complex as you may think. It does not involve multiple transmitters running on different frequencies. The pseudo-random digital pattern is used to modulate a single transmitter, causing its output to look a lot like noise. The receiver has a copy of the pseudo-random pattern, and uses that as an input to the correlator. When the patterns exactly line up, the correlator produces a valid output. All you get is noise if just one bit off. As I recall, the P-code bit rate is 10.23MHz, so each bit is only 98nS. Data is modulo-2 added to the pseudo-random code, and the result modulates the transmitter. So, when the codes are aligned at the receiver, the data pattern comes out the correlator.

Jeff

They have an 8-drop hard-wired system for $110.

Philips uses a simpler method with their RF-capable Prontos. They modulate the 418MHz carrier with both the bit pattern and with ~36kHz. The receivers use an Atmel U2538B chip to demodulate the 36kHz after the RF receiver demodulates the 418MHz. It's similar to how Consumer IR distinguishes a signal from ambient IR. But I suspect it, as well as the method you describe, could be jammed by a sufficiently strong RF signal just as Consumer IR fails in a strong ambient IR field (e.g. fluorescent lights).

AFAIK RadioRA doesn't do anything out of the ordinary. At least, they don't mention it in their literature or FAQs. I believe their repeaters repeat in (near) real-time (as opposed to the sequential repeaters used by Z-Wave). The slight smearing this causes would probably preclude using the method you describe.

JV>>Spread-spectrum not as complex as you may think. It does not involve JV> >multiple transmitters running on different frequencies. The pseudo-random JV> >digital pattern is used to modulate a single transmitter, causing its output JV> >to look a lot like noise. The receiver has a copy of the pseudo-random JV> >pattern, and uses that as an input to the correlator. When the patterns JV> >exactly line up, the correlator produces a valid output. All you get is JV> >noise if just one bit off. As I recall, the P-code bit rate is

There are a number of spread spectrum techniques. The one I am familiar with:

I was trying to find out some more information about the subject and came across the FCC rules about frequency hopping:

The most interesting tidbit I found at Wikipedia was this:

"The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and composer George Antheil, who in 1942 received patent number

DH> Philips uses a simpler method with their RF-capable Prontos. They modulate DH> the 418MHz carrier with both the bit pattern and with ~36kHz. The receivers DH> use an Atmel U2538B chip to demodulate the 36kHz after the RF receiver DH> demodulates the 418MHz. It's similar to how Consumer IR distinguishes a DH> signal from ambient IR. But I suspect it, as well as the method you DH> describe, could be jammed by a sufficiently strong RF signal just as DH> Consumer IR fails in a strong ambient IR field (e.g. fluorescent lights).

Which gets us back to the original question. Can RadioRA be jammed with the same sort of signal that is affecting garage and car door openers near Elgin AFB and Quantico? Its clear spread spectrum techniques may help make communications more immune to the standard "noise soup" in the modern RF environment. That could simply be a result of the FCC allowing SS transceivers to operate at higher powers than single frequency devices.

Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits, at least according to Wiki. But there's no telling whether the door openers in question had such enhancements and still failed or whether Lutron RA will fail if operating in the same geographical area.

An "The usefulness of spread spectrum clocking as a method of actually reducing interference is often debated, but it is probable that some electronic equipment with sensitivity to a narrowband of frequencies will experience less interference, while other equipment with broadband sensitivity will experience more interference."

DH > AFAIK RadioRA doesn't do anything out of the ordinary. At least, they don't DH> mention it in their literature or FAQs. I believe their repeaters repeat in DH> (near) real-time (as opposed to the sequential repeaters used by Z-Wave).

There's a good selling point - "we're *much* faster than Z-wave!" That's like MS's advertising of XP: "much more reliable than previous versions!"

DH> The slight smearing this causes would probably preclude using the method you DH> describe.

Part of the reason RA may(?) work so well is that they are very conservative about the radio range, saying that the units work for 30' in any direction a worst case scenario with lots of metal in the walls. If your transmitters and receivers are close, they are more immune to noise. But those low limits mean (according to them) a 10,000 sq. ft. home will require the maximum of 4 repeaters. Even so, there's still a maximum of 32 loads and that's just not enough for me so it doesn't really matter to me if it works during an EMP blast from an airburst neutron bomb. LutronRA comes up short in other places, at least for me, no matter how reliable it's claimed to be.

-- Bobby G.

There are two basic types of spread-spectrum transmitters. The spread-spectrum technique I described earlier is used by millions of people every day in their GPS receivers. Here is the reference:

The other technique uses a number of discrete frequencies, and the RF link hops between those frequencies based on some sequence. This is an easy system to impliment with digitally tuned synthesizers.

As our RF environment becomes more polluted every day, techniques like this will become more commonplace in our attempt to achieve reliable communication.

Jeff