[aprssig] More efficient use of channel capicity through shorter packets

Mark Earle wa2mct at mearle.com
Thu Oct 12 08:03:31 CDT 2006

Ron Stordahl * wrote:
> I realize there is both a TXD (at the head of the transmission) and a 
> TXTail (at the end).  Between these is data as required by the 
> specification.  Part of that data is of fixed length and other 
> variable length.  For 

TXD is two things: Enough time for the transmitter to key up and make RF 
(the time between asserting PTT, and having full, stable output). Part 
two is, how long it takes the slowest, worst-case, RECEIVER in range, to 
open it's squelch, and not "chop off" part of the packet on the way to 
the attached TNC.

You can use crystal controlled, PIN DIODE switch transmitters for fast 
ptt to rf out, but the slowest RECEIVER hampers you. Especially true 
where folks use squelched audio, as they do at 1200 in many cases. Using 
discriminator audio eliminates the squelch system delay. All receivers, 
though, require SOME time to start acting on an incoming signal. It can 
be close to zero, but there is some delay.

So TXD is a compromise for both transmitters (which you can control at 
your site or station) and receivers, which you have no control over.

      ) )    de WA2MCT Mark
     ( (     Echolink 99190      Grid Square EL17HQ
      ) )    You will be assimilated... oooh, coffee!!
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