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[aprssig] aprssig Digest, Vol 60, Issue 25

Stephen H. Smith wa8lmf2 at aol.com
Thu Jun 25 18:42:02 UTC 2009


G0JXN Jim wrote:
> Hi Guys
>
> Stephen
>
> You miss my point. Yes FSK only has one carrier as does FM but APRS
> effectively has two. So can Carson's rule be applied to a two carrier
> system?
>
>   

What do you mean by "APRS effectively has two carriers"???

APRS is an application of standard AX.25 packet.       In turn, 300 baud 
HF packet uses standard FSK (like classic RTTY). In fact, often the 
identical FSK keyer circuits or AFSK tone generators are used for both 
PACKET and RTTY. 

The only differences are that the customary shift on RTTY is 170 Hz vs 
200 for packet, and that the symbol rate is 300 bps for packet vs 45 bps 
for classic RTTY.     [The AEA PK-232 even "cheated" and used the 
identical 190 Hz "split-the-difference" shift for both modes....]

I suppose one way around your Offcomm's picky-picky rules about occupied 
bandwidth would be to reduce the FSK shift on packet from 200 Hz to the 
standard 170 used on classic RTTY.   

2 (150 bps + 85 peak dev) =  470 Hz bandwidth for 98% of the sideband 
energy.  This would virtually guarantee that less than 1% of the signal 
energy was outside the 500 Hz bandwidth, given that the 150 bps is worse 
case for 100% alternating "1"s and "0"s

.

Actually, if you are primarily talking about live-typed 
keyboard-to-keyboard chat rather than automated bulk data transfer, a 
more spectrally-efficient mode for transmitting APRS text strings would 
be to dispense with AX.25 packet format entirely, and use some form of 
PHASE-shift keying such as  QPSK31 or PSK62.  (In my opinion, standard 
PSK31 is just too impossibly slow to be practical for 50-100 character 
APRS strings.)

APRS is saddled with the AX.25 packet infrastructure, with it's 
simple-but-inefficient FSK modulation, mainly because at the time of 
it's birth around 20 years ago, packet was the most widely-used data 
mode among hams. Packet TNCs were widely available, especially after the 
Internet killed off the ham packet "craze" of the 1980s, starting in the 
early '90s.  Even in multi-mode "data controllers" like PK-232s and 
KAMS, all the modes offered (RTTY, PACKET, AMTOR,  PACTOR,  etc) were 
based on simple FSK transmission, just with varying bit rates and 
handshaking schemes.

APRS was conceived of as a way to implement a "one-to-many" data 
broadcast mode, using the beacon function of old packet TNCs.

In turn, the simple AFSK or FSK modulation used was based on the Bell 
201/202  300 and1200-baud wireline modem standards (and chipsets) of the 
1970s!   Remember, this was the era of dedicated HARDWARE, before the 
explosion of cheap computing power (and computer soundcards used as DSP 
systems) allowed more advanced and efficient modulation schemes to be 
easily implemented purely in software.

Some evidence of the type of computing power commonly present in ham 
shacks at the birth and heyday of packet remains in the KPC3+ 
instruction manual to this day.  Many pages in the manual are still 
devoted to cable diagrams and info to connect the TNC to Commodore 64s, 
Apple IIs and Radio Shack TRS-80s!





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

--

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

JavAPRS Filter Port 14580 Guide
  http://wa8lmf.net/aprs/JAVaprsFilters.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:





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