[NSRCA-discussion] SPECKTRUM Modules for Futaba and JR

[Jay Marshall]

At last, someone who understands SS. Jim, I believe that if the "chirp" is
fast enough, any potential interference will be momentary and can be
processed out by the RX. It's been a while since I worked on these systems
but I do remember how highly immune they were to any kind of interference.

 

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of J.Oddino
Sent: Friday, February 23, 2007 1:41 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] SPECKTRUM Modules for Futaba and JR

 

I'm having a real problem understanding what an "unused frequency" is.
First of all nothing that transmits information transmits on one frequency
as it must use a band of frequencies.  SS intentionally uses a wide band of
frequencies and the wider this band is relative to the information bandwidth
the more processing gain you get and that is good.  The Direct Sequence
system "chips" the baseband information at some high rate with a pseudo
random code that should be unique to your transmitter in order to spread the
bandwidth.  With different codes it should therefore be possible to operate
many RC systems simultaneously using the same band of frequencies. 

 

Now getting back to "unused frequencies" , which we will interpret as unused
channels about one megahertz wide, what happens if a system (not necessarily
an RC system) that uses Frequency Hopping turns on and spreads its info over
more than one channel?  Or just decides to use the same  channel that you
are using?  Maybe the FCC rules prohibit this but I'd hate to depend on the
government to make sure my airplane doesn't get shot down.  I've got to
believe that "unused" really means that the ambient noise level is below
some acceptable value and I'd like to think we can really operate a lot more
than 40 users in an 80 MHz band.

 

It sure would be nice to get an understandable, but more technical
description of this system and its limitations.

 

Jim O

----- Original Message ----- 

From: JonLowe at aol.com 

To: nsrca-discussion at lists.nsrca.org 

Sent: Thursday, February 22, 2007 7:26 PM

Subject: Re: [NSRCA-discussion] SPECKTRUM Modules for Futaba and JR

 

In a message dated 2/22/2007 6:52:04 PM Central Standard Time,
patterndude at tx.rr.com writes:

What does Spectral capacity - 40 systems mean?  Is this the number of
tranmitters that can operate in range of each other, or is this 40 channels
or what?

--Lance

 

This is grossly simplified, but the 2.4 ghz band has about 80 frequencies.
Each Spektrum unit picks two unused frequencies and transmits on both.
80/2=40 transmitters that can operate at once.  It is a requirement of the
FCC that things on this band don't interfere with each other, and
transmitter power is strictly regulated, therefore anything using the band
has to check before transmitting.  You have no idea which two frequencies
you are on (nor do you care), as they change each time the transmitter turns
on.

 

I've flown my Dad's Arresti III with the DX7 extensively.  Absolutely solid
link, with no fear of getting shot down.  I have been waiting for something
like the modules so I could use a decent transmitter for things like
throttle curves, conditional mixes, etc.

 

I don't want to be at a field that has 40 airplanes in the air at once!

 

Jon Lowe






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