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[aprssig] Careless AX.25 encoding in APRS messages

Stephen H. Smith wa8lmf2 at aol.com
Fri Oct 2 22:07:34 UTC 2009


Matti Aarnio wrote:
>
> Can you name any additional details ?  Like recommendation of what deviation
> an 2200 Hz tone, and 1200 Hz tone should make on NBFM ?   That would be an
> "gold standard" to tune our modems and transmitters/receivers to.
>
>   

This is an insoluble problem, due the FM pre-emphasis/de-emphasis 
issue.      Traditionally (in the U.S. at least), 1200-baud packet TNCs 
were connected to the speaker-out and mic-in jacks of unmodified FM 
radios.  (The big selling point of 1200 baud vs 9600 packet was that no 
radio modifications or connections to the radio's innards are required 
-- just  "stuff it in  the mic jack" and go.)


Using the mic jack instead of direct connect to the modulator results in 
the 2200 Hz high tone having nearly TWICE the deviation of the 1200 low 
tone, due to the speech channel preemphasis.   The conventional 
assumption has been to set the deviation of the high tone alone (using 
the TNC "CAL" mode) to around 3.5 to 3.75 KHz   which will result in the 
low tone having a deviation somewhere around 2 KHz.   (Assuming 
"classic" 5KHz peak deviation channels -- ultra-narrow 12.5 KHz 
bandwidth channel splitting has not yet reached the U.S. amateur bands.)


Then the Kenwood radios with built-in TNCs arrived. These radios' TNCs 
apply packet transmit audio directly to the  modulator and transmit with 
both tones at the same deviation; i.e. "flat".   Further, for the casual 
appliance operator, the deviation is non-adjustable. 


The result is that the user base and APRS infrastructure is a mish-mash 
"kluge" of pre-emphasized, flat, and sometimes compromise audio 
equalization settings "splitting the diffference" between pre-emphasized 
and "flat".


For example a digipeater composed of a old commercial mobile rig and a 
KPC3 TNC running standalone connected to mic and speaker jacks, will 
operate pre-emph/de-emph.   On the other hand, a digi made from a 
Kenwood D700 with it's TNC controlled by UIview will be 
transmitting/receiving "flat". 




------------------------------------------------------------------------

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Stephen H. Smith    wa8lmf (at) aol.com
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