[aprssig] Modem tones, the KPC3+, and Bell 202

Phil N6TCT phil_aprssig at lapsley.org
Wed Aug 11 01:03:45 CDT 2010

I've always thought that AX.25 AFSK modem tones were 1200 and 2200 Hz,
i.e., "Bell 202."  Tonight I was playing with a Kantronics KPC3+ and was
surprised to see the AFSK frequencies at 1300 and 2100 Hz.  After
scratching my head and checking my equipment I finally thought to check
google, which pointed me at the KPC3+ spec sheet
(http://www.kantronics.com/products/kpc3.html) which says that the KPC3+
is a V.23 modem, not a Bell 202 modem.  The V.23 spec per
http://www.itu.int/rec/T-REC-V.23-198811-I/en says that V.23 is 1300 Hz
and 2100 Hz, sure enough.

My question for the group: does this matter?  The practical part of me
thinks, "Nah, it can't; the KPC3+ is sort of the gold standard for
packet, so this must be just fine."  But the theoretical DSP/comms
engineer part of me says, "Why on earth would anyone build in a 100 Hz
'error' (i.e., offset) into things if you didn't need to?  That's gotta
hurt, not help."


Stated differently, if you had a choice between using a KPC3+ and
something else (i.e., a true Bell 202 modem) for APRS packet, would you
choose "something else?"


Phil, N6TCT

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