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[aprssig] RF modulations techniques

Wes Johnston, AI4PX wes at kd4rdb.com
Mon Nov 13 01:07:26 UTC 2006


You can run packet thru a voice repeater (check your TNC manual for AXhang
paramater), but noise is a problem.  There are also such things as bit
regenerating repeaters.

Wes


On 11/12/06, Stephen H. Smith <wa8lmf2 at aol.com> wrote:
>
> vk4tec at tech-software.net wrote:
>
> Gudday
>
> I was driving along as you do and pondered this thought.
>
> The existing packet radio systems turn data into tones and then modulate
> up to vhf and then do the reverse.
>
> A digi comes back down to audio and the does the reverse.
>
> I asked myself why don't we just modulate at RF levels and use
> transponders as digis ?
>
>
>
> 1)       Probably the main reason is that when packet first appeared on
> the ham radio scene some 25 year ago, the common terminal devices were 110
> baud ASCII Teletype machines, so there was no benefit in going much faster.
> 1200 baud wireline modems had well-established the Bell 202 1200/2200 audio
> tone pair. Modem chips for this standard were widely available and cheap
> when the first TNCs were designed.  Further, it was far easier to just jam
> audio tones into the mic jack of existing FM transceivers.   Direct FSK, as
> used on 9600 baud modes, requires cutting into the radio for direct
> DC-coupled access to the TX modulator and RX discriminator.
>
>
> Note that  we DO use direct FSK on 9600 baud VHF-FM packet. And that
> 300-baud audio tones used through an SSB transceiver on HF packet do result
> in the net effect of direct FSK on the air.
>
>
> 2)    The process of demodulating and then re-modulating the data at each
> digipeater allows the data stream to be completely regenerated free of
> noise.  Passing through a linear transponder will just add the uplink noise
> (if the signal is weak) and the downlink noise (as heard by the next digi or
> user) to the desired signal.   Not to mention that without demodulating down
> to baseband data, you can't inspect the data, process paths, decrement n-N,
> etc.
>
>
>
> How does D-STARS fit into the APRS equation ?
>
>
> It's not APRS but it is purely digital no matter what you are sending
> (voice, data, etc).
>
>
> Anybody ever played with PulsePositionModulation on APRS ? Planes use it
> at 1Mb/s
>
>
>
> A single one-megabit/sec signal on two meters would occupy nearly the
> entire band!
>
>
>
>
>  They can send GPS co-ords in 112us
>
>
>
>
> It's easy if you are not limited by the bandwidth of FM voice radios, and
> existing channel plans that expect to place more than one channel in the
> band.....
>
>
>  Cheers
>
> Andy
>
> -----------------------------------------
> Andrew Rich
> Amateur radio callsign VK4TEC
> email: vk4tec at tech-software.net
> web: http://www.tech-software.net
> Brisbane AUSTRALIA
>
>
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>
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>
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-- 
Where there's silence, there is no Hope.
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