[NLRS] Left Field (or maybe even outside the ball park) thought on VHF Field Day modes

[Dr. Gerald N. Johnson]

Using audio tones on FM 2m rigs was how ham radio RTTY began. Its 
perfectly valid as is make break CW or other digital modes, so long as 
the pitches of the tones don't run into the roll off of the audio 
stages. That's how 1200 baud packet as used on APRS still works. There's 
been much fiddling in packet TNCs with audio equalization sometimes 
good, more often good for only the rig it was developed with and taking 
out the audio filter makes for better reception. Then 9600 baud tries to 
be band limited FSK using the FM rig's modulator for the FSK keyer and 
the discriminator for the data detector and requires getting data to the 
modulator and from the detector avoiding the FM premphasis and 
deemphasis circuits. FLDIGI is nice in that it doesn't demand using 2125 
and 2975 tones for 850 shift and so whatever the high tone is gets to 
the sound card better. But that means users on FM have to agree on new 
tones, or use FLDIGI's on screen tuning to match the received tones of 
whoever was on first. One old standard for "low" tones was for Collins 
receivers, 2125 for the high tone and 1275 for the low tone wide shift 
or 1950 for the low tone narrow shift. The hazard with the low towns was 
that the second harmonic of the low tone could be in the transmitter 
pass band and be spotted with an FCC spectrum analyzer.

Three impediments with acoustical connections. One is ambient noise, the 
second is the equalization inherent in FM. One of the reasons FM has so 
little hiss noise in the receiver audio is that the receiver audio 
response is reduced at higher frequencies, linear with frequency. The to 
compensate HF audio is emphasized at the transmitter making it probable 
that the high pitched tone for RTTY will over modulate while the low 
tone will under modulate. Over modulation means distortion in the 
receiver and under modulation means a poorer S/N at the audio output. 
Avoiding the preemphasis and deemphasis circuits is what modern radios 
do on their data connection and so get better results. The third is 
control of modulation to improve received S/N.

Not to say acoustical connections won't work, just that hardwired 
connections work more reliably because they avoid those three problems.

Many a time I've suggested to new comers that running audio tones on FM 
repeaters for CW code practice was a good use of the repeater, 
especially when I was president (for 21 years) of the club owning the 
repeater, but I've rarely detected it happening.

I don't know if the rules interpreters at ARRL would consider CW 
acoustically coupled to an FM or SSB rig as a valic contest CW Q or not, 
but it should be.

73, Jerry, K0CQ


On 6/17/2013 1:35 PM, John (JK) Kalenowsky, K9JK wrote:
>
>
> For more "local" contacts, what about using "Audio Over FM" ("AOFM"?)
> for digital modes? Perhaps even CW?
>
> When I was watching an FLDIGI demo sometime back, I had my laptop "on",
> running FLDIGI and IN the receive mode, the transmission that was being
> 'copied' in the demo started appearing on MY laptop, just as it was on
> the computer screen that was showing in the demo, with just audio
> coupling to the actual microphone (NOT the mic input) on my laptop.
>
> For transmit, just hold the mic up to the laptop speakers.
>
> I think that would be in the *spirit* of the rules and legitimate to
> call it a 'digital mode' contact (or a CW contact if CW sending were
> used and regardless of how the CW were copied as long as it actually WAS
> copied accurately by a person or by a computer program), even if not
> specifically allowed (or prohibited).
>
> Or would that be considered too 'outside' of the envelope?
>
> Just an idea, that I've not really TRIED (as it was 'happenstance' on
> the receive part and I definitely have NOT tried the transmit part) and
> I really don't for sure that it would work but it seems possible.
>
> I'm a little bit far away from the Twin Cities to try it with anyone
> there but offer the idea for anyone (anytwo?) who might want to
> experiment. I'd just appreciate that if anyone does (or anytwo _do_),
> that you share the results of your attempts at this with me as a minimum
> or I suspect it might be of interest to the reflector.
>
> 73, JK
>