[aprssig] RF modulations techniques

Stephen H. Smith wa8lmf2 at aol.com
Sun Nov 12 18:54:34 CST 2006

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 

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 <mailto:vk4tec at tech-software.net>
> web: http://www.tech-software.net
> Brisbane AUSTRALIA
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