High bandwith Non LOS communication

Sniper King

If FSk 1200 baud communication is alternating bits between 2 frequencies, can the propeller split it up into say 25 frequencies?· I guess what I am asking is can we put 25 different frequencies on 25 ports and switch them on and off really fast?

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·- Ouch, thats not suppose to be hot!··


Michael King
Application Engineer
R&D
Digital Technology Group

Ken Peterson

For multiple outputs, or wired together? Seems it would be just as fast to change the FREQ value on one counter than to turn one on and another one off.

Also, why 25?

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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

- Bjarne Stroustrup

Sniper King

Let me redefine this.· Lets say 24 ports. We could send 3 byte per cycle.· Now a 9600 baud FSK modem uses 2 frequencies 1200 and 2200 HZ to communicate.· Multiply that by 12.· 12 x 9600=115200· Now our wireless over VHF packet could carry very low bandwidth video.

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·- Ouch, thats not suppose to be hot!··


Michael King
Application Engineer
R&D
Digital Technology Group

Timmoore

I wouldn't use fsk @ 1200/2200 for a 9600 modem, at 1200 baud, and 1200Hz tone you can 1 cycle of the 1200 tone for 1 bit. @ 9600 you get 1/8 of a cycle which is very hard to tell from a ~1/4 cycle of 2200. Theres also the problem of what the bandwidth of the radio link. the bandwidth must be > 2x the frequency you are sending so increasing the FSK frequencies is also hard because the radio will not transmit the higher frequencies correctly.
9600 modems use a different scheme called 16-QAM which the baud rate is 2400 baud but they send 4 bits per baud. They use ampltude and phase to get the 4 bits per baud. i.e. the frequency only changes at 2400baud, but you look at the phase of the frequency and its amplitude and you get 4 bits for every frequency bit that is sent.
Sending multiple 1200 baud streams has a different problem. You need to be able to separate the streams at the receive side. If you use the same frequencies then you need different radio frequencies to keep them separate. If you use different frequencies they need to be far enough apart to separate - means good expensive filters.

Phil Pilgrim (PhiPi)

Using 12 different carriers simultaneously? Why not just use one carrier and modulate it at 115.2 KBaud?

-Phil

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'Still some PropSTICK Kit bare PCBs left!

Sniper King

Thanks tim.· That pretty much answers my question.· I am really looking forward to your 1200FSK project.

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·- Ouch, thats not suppose to be hot!··


Michael King
Application Engineer
R&D
Digital Technology Group