[aprssig] Modem tones, the KPC3+, and Bell 202
Phil N6TCT phil_aprssig at lapsley.orgWed Aug 11 06:03:45 UTC 2010
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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! Phil, N6TCT
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