On Sat, May 1, 2010 at 2:05 PM, Brian Lloyd <[email protected]> wrote:
Uh, no. What you have described is the convention used by amateur > radio operators, not what really happens physically. Look, you can describe "what happens physically" as an offset from green light if you want to. The point is, what's the USEFUL thing to do? What makes SENSE and makes operation EASIER and SIMPLER without compromising anything, and this is especially so in the case of emergency ops. > We have 50 stations operation PSK on 20m. All have their radios tuned > to 14,070 kHz. None of them are interfering with each other. By your > reckoning, they are all on the same frequency because they have tuned > the big knob on their transceiver until the number on the display > reads 14,070 kHz. OTOH, if each of them uses a frequency counter to > measure the carrier frequency they are radiating, they are each going > to get a different number, something between 14,070 and 14,073 kHz. > So what you're doing is sliding the modem over the passband with the radio at one frequency, and what you've lost the ability to do is tell the other station where to tune their radio and reliably receive your signal; instead they've got THREE tuning steps to start: (1) tune the radio, (2) tune the modem, (3) tune the IF-shift or put up with AGC pumping. To re-tune, (1) tune the modem, (2) tune the IF-shift. OTOH, do it my way, there's ONE re-tuning step to start. Tune the radio. IF shift remains constant - when the modem freq is constant, so is the IF-shift. Passband width is the same either way. There's also ONE tuning step to re-tune: Tune the radio. There's minimal AGC pumping because the IF bandwidth is properly set the moment the signal is in the right place on the waterfall. Don't see the benefit of your way at all. Doesn't give you more space to tune around in; makes external filtering hardware you might want to use have to re-tune; adds tuning steps (or else commits you to use an open IF bandwidth so that everything in the passband gets through, which isn't the best idea I've heard recently, unless you like getting your AGC pumped by that strong signal down-passband... entertainment value? challenge? :) That is the problem of the receiver, not what goes out over the air. > To me all the following are the same: > The thing is, to an AF filter, to an IF shift, to the AGC circuitry, they aren't. And it isn't your sensibility about the "appropriateness" of an RF frequency that controls an these things. It's the actual AF frequency. And - just offhand - the rate of change of a lower frequency AF signal isn't as fast as a higher one. That's why most AFSK signals are designed to reside at the higher frequency side of the passband - e.g., SSTV isn't 300 Hz sync and 500 Hz to 1300 Hz, it's 1200 Hz and 1500 Hz to 2300 Hz. If you move stuff to the lower end, you're not doing the frequency response of the channel any favors that I'm aware of. > Even fldigi pays homage to this concept. That doesn't mean it's the right thing to do. It just means it was something that could be done, and was. No one has yet to put a *benefit* of this approach on the table. I've pointed out that it adds unnecessary tuning, compromises optimum IF settings requiring resetting (or expose you to AGC pumping), can lower the signaling rate, and compromises external filters requiring resetting. Whereas all of that is avoided if one treats the signal as an AFSK constant with known targets. > >> Fldigi does calculate and display the > > Au contraire! What fldigi reports is that actual carrier frequency, > the frequency one would measure with a frequency counter connected to > the transmitter output! > And, I ask again, what is the benefit? What step does this save? What advantage is gained? Why must the operator "know"? By adhering to this practice, you compromise external filtering gear; you increase the number of tuning actions without gaining any performance benefits; you have to re-adjust IF shift to compensate whereas if you were using a locked/defined set of AFSK tones... one tuning process; the IF shift and bandwidth remain constant; and external hardware knows what to do; the operator doesn't need to "know" anything other than (a) is the signal where it belongs in the waterfall and (b) what they want to send. > You actually believe what you wrote there, right? Of course. Please don't act like that. This is a technical discussion of the merits. If I am wrong, point out how I am wrong, don't attempt to impugn my mental processes. > Please tell me the proper modem design point for PSK31, Olivia, DominoEX, > JT65, etc. > Generally speaking, the sensible place for the (high[est] tone) is 2295 Hz, same as RTTY 170, 450, 800. Space and any intermediate frequencies are down from there. If one needs to re-tune, it should be done with the radio, so that IF shift doesn't need re-adjustment, and so that it is one operation. Such a setting lets you use external AF filtering and other hardware, if available, and also sets a convention so that the radio may be tuned, just one step, instead of radio, modem, if shift, external hardware - up to four or more steps. > One of the things I used to do was set up VSATs. The frequency we were > assigned was the center of the transmitted signal, not the frequency > of the LO injection. As far as I can discern, the only place where > "dial frequency" is "transmit frequency" is amateur radio. Everyone > else uses the center of the transmission. > That's because we almost always use AFSK. Not FSK. We don't have the same kind of gear; do you know of a transceiver that lets you tune to 14.. Consequently, the most efficient way to operate is tied to what we actually have -- not how commercial / military stations operate. Not my fault. Talk to the FCC. When you do, you might tell 'em I have zero respect for the shoddy job they've done WRT their mandate: To preserve the RF spectrum for the most benefit of the citizens of this country. But that's another thread. :) > And I agree with Rick, K9AO, 100% on one thing: this is an education > issue. Operators need to know precisely where their signal is going. > Ideally, it's going to the proper place relative to the radio's tuned frequency. Once you try to add sliding modems, you've added problems with filters and IF shift (or AGC pumping) you didn't have before. You're just making it more difficult for them. There's no benefit to be had. People keep saying "they need to know", and I repeat: WHY do they need to know? If they already know: 1) They're tuned to the correct frequency on the radio (because the signal is landing on the right spot on the waterfall), and 2) the received AF frequencies are always the same, so they can adjust hardware filters ONE TIME and then, for bandwidth and shift and external AF goodies, leave them be, leaving only concerns about notching and tuning the radio when signals change, 3) There is only one RF display, and one AF display ..then what's the benefit of trying to say to them "well, you're currently sending signals from 14.231200 to 14.232300 Mhz"? They're not going to be tuning any hardware filters to those frequencies. The numbers they NEED are 1200 Hz, and 1500...2300 Hz. And 14.230 MHz. These numbers give you (1) the correct radio receive tuning, (2) constant setting for filters, (3) the correct radio transmit tuning, (4) constant AF conditions so that IF shift remains constant if a signal comes up on another frequency. > Here is another way to look at it. When I see a 500Hz blank spot in my > spectrum display, I want to be able to tell my buddy to transmit in > the middle of that blank spot, not have to calculate a bunch of > offsets in my head. > Say you're on 14.230 Mhz. You see the blank spot. The Waterfall reads out from 0 Hz to 3 Khz (for example.) You're on USB. The blank spot begins at 2000 Hz. So, you tell you buddy to "tune up 2 KHz." No calculations involved. If you *want* to do so, you can say 14.232 KHz, but we're all used to moving a few hundred or a few KHz... aren't we? I know I am. "Meet you up (or down) x KHz" is an unbelievably common AR form of message, I have trouble believing anyone would balk at saying such a thing. In this case, it's "meet you up 2 Khz." No big deal. --Ben AA7AS
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