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<a class="moz-txt-link-abbreviated" href="mailto:vk4tec@tech-software.net">vk4tec@tech-software.net</a> wrote:
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
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<div><span> Gudday </span></div>
<div><span> </span> </div>
<div><span> I was driving along as you do and pondered this thought. </span></div>
<div><span> </span> </div>
<div><span> The existing packet radio systems turn data into tones
and then modulate up to vhf and then do the reverse. </span></div>
<div><span> </span> </div>
<div><span> A digi comes back down to audio and the does the reverse.
</span></div>
<div><span> </span> </div>
<div><span> I asked myself why don't we just modulate at RF levels
and use transponders as digis ?</span></div>
</blockquote>
<br>
<br>
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. <br>
<br>
<br>
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. <br>
<br>
<br>
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.<br>
<br>
<br>
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
<div><span> </span></div>
<div><span> </span> </div>
<div><span> How does D-STARS fit into the APRS equation ?</span></div>
</blockquote>
<br>
It's not APRS but it is purely digital no matter what you are sending
(voice, data, etc). <br>
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
<div><span> </span></div>
<div><span> </span> </div>
<div><span> Anybody ever played with PulsePositionModulation on APRS
? Planes use it at 1Mb/s</span></div>
</blockquote>
<br>
<br>
A single one-megabit/sec signal on two meters would occupy nearly the
entire band!<br>
<br>
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
<div><span> </span></div>
<div><span> </span><br>
</div>
</blockquote>
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
<div><span> They can send GPS co-ords in 112us </span></div>
<div><span> </span> </div>
</blockquote>
<br>
<br>
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.....<br>
<br>
<br>
<blockquote cite="midLGEKIKIAKJPNFGILGJBBEECDDEAA.vk4tec@tech-software.net" type="cite">
<div><span> Cheers </span></div>
<div><span> </span> </div>
<div><span> Andy </span></div>
<p> -----------------------------------------<br>
Andrew Rich<br>
Amateur radio callsign VK4TEC<br>
email: <a href="mailto:vk4tec@tech-software.net">
vk4tec@tech-software.net </a><br>
web: <a class="moz-txt-link-freetext" href="http://www.tech-software.net">http://www.tech-software.net</a><br>
Brisbane AUSTRALIA </p>
<div> </div>
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