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

Stephen H. Smith wa8lmf2 at aol.com
Wed Aug 11 03:24:55 CDT 2010

  On 8/10/2010 11:03 PM, Phil N6TCT wrote:
> 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."
> Thoughts?
> 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?"
> Thanks!

You can't just use a 202 (or even a V.23) modem as a replacement for a KPC3+.   
A TNC is far more than just a modem.  It's a "PAD" (Packet 
assembler/disassembler) that buffers a block of data to send, and then adds 
sending and destination callsigns, routing information, error-detection 
checksums, etc to the payload data.    A regular "dumb" 202 modem would require 
a considerable amount of software on the host computer to emulate the functions 
of a TNC.

[The closest to this would be a TNC running in KISS mode, where the TNC does 
function as a simple AFSK modem,  with all the packet "heavy lifting" done in 
software on the attached computer. ]



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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