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[aprssig] APRS Bandwidth

G0JXN Jim g0jxn.jim at ntlworld.com
Fri Jun 26 21:28:02 UTC 2009


Hi Guys

Dave

I am trying to break in a new computer so have not had the time to go
through the web site you have suggested. I will do so in due course but am
always a bit nervous of treading where the emergency guys hang out.

> Message: 1
> Date: Thu, 25 Jun 2009 14:23:14 +0100
> From: "Dave Baxter" <dave at uk-ar.co.uk>
> Subject: Re: [aprssig] APRS-SIG (... proposal for a world wide APRS
> net on 20m?)
> To: <g0jxn.jim at ntlworld.com>, <aprssig at tapr.org>
> Message-ID:
> <FAD0F39D8FA7F440861941A74B1AE5602FCF75 at sbsserver.AREMV.local>
> Content-Type: text/plain; charset="US-ASCII"
>
> Hi Jim (and others)...
>
> There is already ALE or HFN.  A world wide automated HF digital
> messaging system, primarily for emergency use, it's there and working.
> With the ability to negotiate a link for voice too (SSB)
>
> http://www.hflink.com/  It also has some APRS like position reporting
> facilities.
>
> However, our licence limitations don't make that mode easy to deploy
> either, hence the lack of G (or M) stations active in this mode.
>
>
> 20m is congested enough as it is on most weekends (contests etc.)  I
> suspect a proliferation of APRS would not exactly be welcome, even if a
> compatible slot for it could be found.

In Europe 30m is a lost cause it's not stations that digi in band it's those
stations crossbanding from 2m. Apart having the view that Igates are not
real radio I don't have enough years to make any changes to 30m in Europe.

> I understand your concerns re 30m APRS, and it is useful on occasions.
> However, the problem with digi's is they dramatically increase the
> loading on a single channel of course, and at 300bd, that's already 4
> times as long (timewise) for a typical packet as at VHF.  Unless there
> could be some (internet based?) liaison between any such digis so that
> one and only one digi'd what was heard.
>
> But then, what happens when the i'net liaison link dies for whatever
> reason.
>
> 30m iGates:  If correctly setup, so as not to spew stuff out to RF, only
> messages to those in direct earshot, maybe, and only maybe that could
> work.  But again reliance on a service that we have no control of in the
> background.  The Inernet (not the APRS-IS) of course.
>
> Then there is that old chestnut, that FSK is not exactly spectrum
> efficient these days, compared to other choices of digital modulation
> available to us.
>
> Dave G0WBX.   Stirring it as usual.
>
> 73.
>
>
> ----- Original Message -----
> From: <aprssig-request at tapr.org>
> To: <aprssig at tapr.org>
> Sent: Thursday, June 25, 2009 1:00 PM
> Subject: aprssig Digest, Vol 60, Issue 25
>
>
> > That's the whole point of Carson's Rule -- it takes into account the
> > total energy in ALL of the sidebands. If it defines the effective
> > bandwidth as being where 98% of the energy is located, then only 2% of
> > the energy would remain in ALL of the higher order sidebands combined.
> >
> >
> >
> > > In any event the argument is flawed in that an FM signal only has one
> > > carrier that moves between the deviation limits not two as suggested.
> > >
Yes that was a crap statement.

> >
> > Nothing was said about "two carriers". With FSK, there is only ONE RF
> > signal moving +/- 100 Hz from a center frequency (i.e. "carrier
> > frequency") midway between the mark and space frequencies. Since you
> > never have "no modulation" (i.e. you are always sending either a "1" or
> > a "0"), the "unmodulated" center resting frequency of the carrier never
> > appears.You are always at either "peak deviation high" or "peak
> > deviation low".

Let us be clear about the carrier. Yes at any time there is only one but it
doesn't slide +/- 100Hz it switches between the 100Hz points and could
therefore be considered as two AM carriers.
>
> 73
>
> Jim, G0JXN
>
>
> Message: 4
> Date: Thu, 25 Jun 2009 11:42:02 -0700
> From: "Stephen H. Smith" <wa8lmf2 at aol.com>
> Subject: Re: [aprssig] aprssig Digest, Vol 60, Issue 25
> To: TAPR APRS Mailing List <aprssig at tapr.org>
> Message-ID: <4A43C4FA.1040804 at aol.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> 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"???
>
See above

> 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!
>
>
> Message: 5
> Date: Thu, 25 Jun 2009 23:13:53 +0300 (EEST)
> From: Pentti Gronlund <pentti.gronlund at tut.fi>
> Subject: Re: [aprssig] APRS Bandwidth
> To: TAPR APRS Mailing List <aprssig at tapr.org>
> Message-ID: <20090625201353.7F4D0C1BF58 at www.ele.tut.fi>
> Content-Type: text/plain; charset="US-ASCII"
>
> G0JXN Jim writes:
>
> > 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?
>
> Jim,
>
> Can you tell us how exactly APRS has two carriers where FSK has
> only one? Some test equipment plots would be a nice surprise.
>
> Benjamin OH3BK
> --
> Live Reports from the Taxman's Paradise!
>
Please see above
>
>
> ------------------------------
73

Jim, G0JXN





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