Re: garage remote jamming

[nobody@xxxxxxxxxxxx (Dave Houston)]

"Jeff Volp" <JeffVolp@xxxxxxx> wrote:

>"Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote in message
>news:WoSdnfla97CuGXXYnZ2dnUVZ_tLinZ2d@xxxxxxxxxx
>> "Jeff Volp" <JeffVolp@xxxxxxx> wrote in message
>>
>>> There is a technique that uses spread-spectrum and correlation between a
>>> received signal and a pseudo-random pattern.
>>
>> 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.
>
>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.

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.

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